Potential Applications for Additive Manufacturing Technology in Forensic Pathology Practice

Potential applications for additive manufacturing technology in forensic pathology practice

Thesis submitted for the degree of

Doctor of Medicine

at the University of Leicester

by Dr Michael James Peter Biggs MB ChB, MRCS, FRCPath, MFFLM

East Midlands Forensic Pathology Unit University of Leicester October 2020 Abstract

Potential applications for additive manufacturing technology in forensic pathology practice

by Dr Michael Biggs

Additive manufacturing (i.e. 3D printing) processes have been in development since the 1980s. Medical practitioners were among the first to appreciate the value of this technology, but early industrial devices were prohibitively large and expensive, limiting access to all but a few. More recently, smaller scale “desktop” printers have become available, bringing this capability within the realistic reach of many. Computed tomography (x-ray) scanning has increasingly become routine in forensic pathological investigations in Leicester since its implementation in 2002. The obtained scan data has previously been used to produce graphical 3D representations of pathological evidence to assist jury comprehension in court. With the advent of smaller scale 3D printing, the possibility of further enhancing this court evidence with 3D printed anatomical models was considered a viable option. This thesis explains the overall concept and specific processes of 3D printing, and explores potential applications for this technology in the context of forensic pathology practice. Following acquisition of suitable hardware and software, the initial testing, calibration and accuracy checks are detailed. Strategies developed to overcome hurdles encountered during the early stages are also described. After completion of the technical evaluation phase, several practical applications were trialled and the results are illustrated. Acceptance of such 3D printed models for use within court is then demonstrated using a number of real case examples. By confirming that the recently-available, “desktop” 3D printing devices can be acquired and operated within the modest setting of a working forensic pathology department, this project contributes to the knowledge in this small but emerging field of practice. In addition, establishment of the provision of this technology represents a genuine improvement to the forensic pathology service offered within the East Midlands region.

Acknowledgements

This project would not have been possible without the considerable advice, assistance and tolerance of a number of key individuals.

Professor Guy Rutty and Professor Bruno Morgan were the primary and secondary supervisors respectively. Throughout the project they have balanced guidance with the freedom to pursue any avenues that interested me, and for this I am truly grateful. The shortcomings of the project are mine alone, and are not a reflection of the advice received.

Ms Claire Robinson and her radiography colleagues at the University Hospitals of Leicester NHS Trust have performed many CT scans, often at inconvenient times and short notice, without which there simply would not have been the raw data needed by the project. Additional micro CT scanning was carried out for me by Mr Graham Clark at the Department of Engineering, University of Leicester.

My colleagues at the East Midlands Forensic Pathology Unit have long had to tolerate my obsession with all things 3D, and I apologise to them if this has been tedious to bear at times. On a similar note, my wife, Michelle, has frequently had to cope with this 3D printing enthusiasm spilling over into the home environment. As my affliction is unlikely to subside following the conclusion of this project, I thank them all in advance for their continued patience and ongoing support!

It is not feasible to list here every individual with whom I have had contact during the course of the project, but I would like to extend additional thanks to all colleagues and acquaintances from the various police, coronial, legal and academic teams encountered whilst attempting to turn this idea into reality.

Contents

Abstract 2

Acknowledgements 3

Contents 4

List of tables 9

List of figures 10

List of abbreviations 14

Chapter 1 Origins of the study 16

1.1 Introduction 16

1.2 Background 18

1.3 Thesis aims and structure 19

1.3.1 Technical aspects 20

1.3.2 Practical applications 20

1.3.3 CJS acceptability 21

1.3.4 Unexpected benefits 21

1.3.5 Conclusions 22

1.4 Ethical approval 22

Chapter 2 3D printing overview 23

2.1 History 23

2.2 The 3D printing process 24

2.2.1 The STL file 25

2.2.2 Creation of the 3D model 26

2.2.3 Editing the 3D model 27

2.2.4 Slicing 27

2.2.5 Supports and orientation 28

2.2.6 Printing 31

2.2.7 Post-processing 33

2.3 Types of 3D printer 34

2.3.1 Stereolithography 35

2.3.2 Selective laser sintering 36

2.3.3 Fused filament fabrication 37

2.3.4 Material jetting 39

2.3.5 Binder jetting 40

2.3.6 Laminated object manufacturing 41

2.4 Device type selection 41

Chapter 3 Putting theory into practice 45

3.1 The beginning 45

3.2 Initial testing and calibration 45

3.2.1 Test cubes 46

3.2.2 Variation between materials 48

3.2.3 Effect of layer height 49

3.2.4 Effect of size and wall thickness 50

3.2.5 Curing shrinkage 51

3.2.6 Anatomical models 52

3.3 Process refinement 58

3.3.1 Model extraction parameters 58

3.3.2 Resin conservation 60

3.4 Moving forwards 62

Chapter 4 Taking shape 63

4.1 First steps 63

4.2 Basic anatomical models 63

4.2.1 Bone 63

4.2.2 Skin 70

4.2.3 Blood vessels 71

4.3 More advanced models 72

4.3.1 Fractures 72

4.3.2 Multi-part models 75

4.3.3 Micro CT 82

Chapter 5 Potential uses for techniques being developed 85

5.1 Initial ideas 85

5.2 Skeletal trauma 86

5.2.1 Fractures 86

5.2.2 Tool mark analysis 87

5.3 Soft tissues 88

5.3.1 Skin wounds 88

5.3.2 Ballistic trajectories 88

5.3.3 Projectile recovery 89

5.4 Identification 89

5.5 Anthropology 90

5.6 Education 91

5.6.1 Anatomical teaching 92

5.6.2 Molecular teaching 92

5.7 Thinking outside the box 93

Chapter 6 Forensic case application 94

6.1 The best laid plans 94

6.2 Skull fractures 94

6.2.1 Assault with a weapon 94

6.2.2 Shod foot assault – living victim 96

6.2.3 Shod foot assault – fatal 97

6.3 Dental identification 98

6.4 Anthropology 99

6.5 Penetrating weapon injury 101

6.6 Dismemberment 103

6.7 Weapon reproduction 108

Chapter 7 Bonus capability 110

7.1 Unforeseen advantages 110

7.2 CAD prototyping 110

7.3 Examples 111

7.3.1 Research specimen custom container inserts 111

7.3.2 Enlarged inserts and associated items 113

7.3.3 Clinical engineering collaboration 116

7.3.4 Brain box 117

7.3.5 Corpse screw 118

7.3.6 Phantoms 119

7.3.7 Phone microscope adaptor 120

7.3.8 Micro centrifuge vial 122

7.3.9 Molecular models 123

7.3.10 Mathematical models 124

Chapter 8 Wider interpretation of “technology” 126

8.1 Software still counts as technology 126

8.2 3D file editing 127

8.2.1 Basic “cleaning” 127

8.2.2 Complex mesh editing 129

8.2.3 Joining different datasets 130

8.2.4 Creating custom parts 132

8.2.4.1 Medieval skeleton 133

8.2.4.2 Bronze Age artefact 136

8.3 Image rendering 137

8.3.1 Basic images 137

8.3.2 Enhanced representation 141

8.3.3 Texture wrapping 142

8.4 Animation 144

8.4.1 Movement 144

8.4.2 Mesh deformation 145

8.5 The whole is greater than the sum of its parts 146

Chapter 9 Moving forward 149

9.1 Where now, where next? 149

9.2 Continued research 149

9.2.1 Reconstruction aids 149

9.2.2 Skin models 150

9.2.3 Wound trajectories 150

9.2.4 Software applications 151

9.3 Dissemination of information 151

9.4 Collaborations 152

9.5 Consolidation and expansion of the forensic service 153

9.6 Limitations and weaknesses 154

Chapter 10 Conclusions 157

References 161

List of tables

Tab 2.1 Relative strengths of different technology types 42

Tab 2.2 Comparison of potential devices 43

Tab 3.1 Example cube measurements (solid 20 mm cube) 48

Tab 3.2 Relative percentage discrepancies for each layer height 50

Tab 3.3 Relative percentage discrepancies for grouped cubes 51

Tab 3.4 Bone, print and scan measurements in anatomical planes 55

Tab 3.5 Dimensional discrepancies in comparison to original specimen 56

Tab 3.6 Summary of all objects measured 56

Tab 4.1 Peg and hole options with initial cube trial 78

Tab 4.2 Fine-tuning of size discrepancy trial results 80

Tab 5.1 Summary of 3D printing characteristics applicable to forensic cases 85

Tab 9.1 Chief limitations of chosen devices 154

List of figures

Fig 1.1 NHS clinical CT scanner in action 16

Fig 1.2 Example grayscale axial image from CT scan data 17

Fig 1.3 Example 3D printed skull model 17

Fig 2.1 The concept of 3D printing 23

Fig 2.2 Overview of the 3D printing process 25

Fig 2.3 The STL file 26

Fig 2.4 “Slicing” an STL file 28

Fig 2.5 The need for supports 29

Fig 2.6 Water-soluble support demonstration 30

Fig 2.7 The relevance of orientation 31

Fig 2.8 The effect of varying layer heights and printing processes 33

Fig 2.9 The stereolithography process 36

Fig 2.10 The selective laser sintering process 37

Fig 2.11 The fused filament fabrication process 39

Fig 2.12 The material jetting process 40

Fig 3.1 Test cube measurement 47

Fig 3.2 Comparison of original bone with 3D printed models 54

Fig 3.3 Physical bone and print measurement comparison 54

Fig 3.4 Electronic scan measurement 55

Fig 3.5 The virtual and physical effects of STL decimation 60

Fig 3.6 The effect of model hollowing 61

Fig 4.1 The segmentation process 64

Fig 4.2 The surface rendering process 65

Fig 4.3 Segmentation and surface rendering comparison 66

Fig 4.4 STL example 66

Fig 4.5 STL artefacts 67

Fig 4.6 Comparison of large and small 3D prints 67

Fig 4.7 Stab wound skin model example 71

Fig 4.8 3D printed and virtual coronary angiogram models 72

Fig 4.9 Neck fracture example 74

Fig 4.10 Neck fracture example (continued) 75

Fig 4.11 Peg and hole method of joining model parts 76

Fig 4.12 Design for test cubes to establish crude-fit size discrepancy 77

Fig 4.13 3D printed initial peg and hole test cubes 77

Fig 4.14 Design for fine-tuning the friction fit size discrepancy 79

Fig 4.15 3D printed modified size testing apparatus 79

Fig 4.16 Magnetic inserts being incorporated into virtual model 81

Fig 4.17 Magnetic multi-part model skull 82

Fig 4.18 Comparison of virtual models from clinical and micro CT scanning 83

Fig 4.19 Magnified STL mesh to compare clinical and micro CT data 84

Fig 5.1 Virtual recovery of air weapon projectile 89

Fig 5.2 Virtual anthropology 91

Fig 6.1 3D printed skull injury – overview 95

Fig 6.2 3D printed skull injury – cropped area 95

Fig 6.3 3D printed skull injury – further example 97

Fig 6.4 3D printed skull injury – further example 98

Fig 6.5 3D printed teeth to assist with dental identification 99

Fig 6.6 3D printed bone to determine fragment origin 100

Fig 6.7 3D printed bone to determine fragment origin (continued) 101

Fig 6.8 Duplicate copies of 3D printed skull showing weapon damage 102

Fig 6.9 Internal / external feature comparison of 3D printed damaged skull 103

Fig 6.10 3D printed arm bone showing saw marks from dismemberment 104

Fig 6.11 3D printed leg bone showing saw marks from dismemberment 104

Fig 6.12 Comparison of 3D prints from clinical CT scan and micro CT scan 105

Fig 6.13 Life-size 3D printed model of spine at point of dismemberment 105

Fig 6.14 Cut surface view of 3D printed spine model 106

Fig 6.15 3D print from micro CT scan of dismembered bone 107

Fig 6.16 3D prints of dismembered bone and corresponding saw blade 107

Fig 6.17 Life-size 3D print of dismembered foot specimen 108

Fig 6.18 3D printed knife model 109

Fig 7.1 Early prototype research specimen container 113

Fig 7.2 Further refinements to research specimen container design 114

Fig 7.3 Final 3D printed research specimen container 114

Fig 7.4 Virtual design for microscope coverslip adaptor 115

Fig 7.5 Virtual design for specimen suspension apparatus 116

Fig 7.6 Virtual design for brain slicing aid 118

Fig 7.7 Basic handle adaptor for testing blades 119

Fig 7.8 3D printed adaptor for smartphone microscope photography 121

Fig 7.9 Virtual micro-vial and cutaway examples 122

Fig 7.10 Multi-part molecular model example 123

Fig 7.11 Complex molecular model example 124

Fig 7.12 Mathematical model example 125

Fig 8.1 Debris removal process 128

Fig 8.2 Cropping within Blender compared to standard medical software 129

Fig 8.3 Virtual repair of defects within thin bone 130

Fig 8.4 Metallic dental x-ray artefact removal 130

Fig 8.5 Rendered images of dismembered skeleton 131

Fig 8.6 Standard medical software image of dismembered skeleton 132

Fig 8.7 Virtual fusion of fragmented objects 133

Fig 8.8 Creation of custom intervertebral discs 134

Fig 8.9 Creation of custom intervertebral discs (continued) 134

Fig 8.10 Rendered image of upper cervical spine model 135

Fig 8.11 3D printed model of the spine of Richard III 135

Fig 8.12 Virtual casting of surface defect 137

Fig 8.13 Standard medical imaging compared with Blender rendering 138

Fig 8.14 Virtual anthropological assessment 140

Fig 8.15 Enhanced visualisation of fractured bone fragments 141

Fig 8.16 Virtual simulation of ballistic trajectory 142

Fig 8.17 Wrapping 2D textures around 3D surfaces 143

Fig 8.18 Leg fracture virtual reconstruction 144

Fig 8.19 Spinal fracture virtual reconstruction 145

Fig 8.20 Deformation of STL mesh 146

Fig 8.21 3D printed body parts and freezer compartment 147

Fig 8.22 Still frames of virtual dismembered body parts within freezer 148

Fig 9.1 Virtual reconstruction of fragmented skull 151

Fig 10.1 Micro CT skull example, rendered 159

Fig 10.2 Micro CT skull example, life-size print 160

List of abbreviations

2D 2 Dimensional 3D 3 Dimensional ABS Acrylonitrile Butadiene Styrene AP Antero-Posterior C1 1st Cervical vertebra C2 2nd Cervical vertebra CAD Computer Aided Design CJS Criminal Justice System COP College Of Policing CT Computed Tomography DICOM Digital Imaging and COmmunications in Medicine DLP Digital (or Direct) Light Processing DNA DeoxyriboNucleic Acid DVI Disaster Victim Identification EMFPU East Midlands Forensic Pathology Unit FDM Fusion Deposition Modelling FFF Fused Filament Fabrication GB GigaByte GBH Grievous Bodily Harm GHz GigaHertz HIPS High Impact PolyStyrene HM Her Majesty’s HRA Health Research Authority IPA IsoPropyl Alcohol IRAS Integrated Research Application Scheme LED Light Emitting Diode LOM Laminated Object Manufacturing ML Medio-Lateral mm millimetre

NHS National Health Service nm nanometre NRES National Research Ethics Service PLA PolyLactic Acid PMCT Post-Mortem Computed Tomography PMCTA Post-Mortem Computed Tomography with Angiography PMOC Police Mortuary Operations Co-Ordinator PVA PolyVinyl Alcohol RAM Random Access Memory RAF Royal Air Force REC Research Ethics Committee ROI Region Of Interest SI Supero-Inferior SIM Senior Identification Manager SIODP Senior Investigating Officers’ Development Programme SLA StereoLithogrAphy SLS Selective Laser Sintering STL STereoLithography file format TB TeraByte UK United Kingdom USA United States of America

Chapter 1 – Origins of the study

1.1 Introduction

The East Midlands Forensic Pathology Unit (EMFPU) provides post-mortem and medico-legal opinion services to the coroners and police forces of the East Midlands region (Leicestershire, Nottinghamshire, Northamptonshire, Derbyshire and Lincolnshire). Since 2002, computed tomography (CT) scanning techniques have increasingly been used by the EMFPU to enhance, and in some cases replace, the traditional invasive post-mortem examination (autopsy). With continued advancements in the available hardware and software, conversion of two-dimensional (2D), grayscale axial “slices” of the body into multi-coloured, three-dimensional (3D) images has become a standard component of this type of examination. Within this context, the feasibility of transforming virtual 3D anatomy into physical models was considered. This project outlines the process by which the possibility was first explored as a concept, and then subsequently expanded to create a novel service, which has now been successfully used by police forces both within and outside the East Midlands region.

Fig 1.1 An NHS clinical CT scanner being used by the forensic pathology team (in this case, during the examination of the skull of Richard III).

Fig 1.2 An example of the type of grayscale axial image produced by the above scanning process. Whilst a wealth of useful information is available to the trained observer, non- medical viewers might not readily appreciate what is depicted.

Fig 1.3 A 3D printed skull model produced from a forensic post-mortem CT scan. Presenting the scan data in this manner provides non-medical observers with a more recognisable object that can be handled and examined intuitively to appreciate the location and extent of any fractures, for example.

1.2 Background

The principles and processes of 3D printing will be examined in detail in chapter 2, but it is worth establishing at this point that the concept is not a new one. Additive manufacturing or rapid prototyping technologies that would now be recognised as “3D printing” first emerged in the 1980s, with industrial and manufacturing applications being most immediately apparent.1 However, given the complex morphology of anatomical structures, medical practitioners were in fact some of the first to appreciate the potential benefits of this emerging technology soon after its development.2-6 Healthcare related applications continue to make up a significant proportion of the ongoing research and innovation in this field.7-13 Given the rapid growth in cross-sectional imaging as an adjunct (and sometimes replacement) to traditional autopsy in forensic pathology practice worldwide, the likelihood of 3D printing technology enhancing current and future forensic investigations is increasing. Production of 3D printed anatomical models to display injuries in a forensic context has previously been described,14-17 but has yet to be widely adopted.18 Early inertia may have stemmed from inaccessibility or prohibitive cost (whether perceived or actual), or existence of the technology may simply not have been known by the majority of forensic practitioners. The recent surge in mainstream awareness of 3D printing, as well as proliferation of affordable and available devices, has removed these barriers to a considerable degree. A project was therefore devised with the aim of evaluating the potential for more routine application of these technologies to forensic pathology practice, both in terms of post-mortem casework and in related research.

The initial project plan considered three broad phases to investigate. In the first instance, the technical considerations would need to be thoroughly assessed in order to determine if a particular type of technology or model of device would indeed be suitable for “in house” work, or whether this would not be feasible and such activity would instead be restricted to remote printing by an external provider. This first, “technical” phase would also need to prove the theoretical process of turning forensic CT scan data into 3D printed models to ensure viability in practice, and then to experiment with varied software and hardware parameters to ascertain real-world settings for reliable results and optimum quality using the selected device(s). Secondly, potential applications would need to be evaluated in turn to determine whether such theoretical uses would prove to be truly achievable in practice. It was initially predicted that this series of casework type

18 examples would comprise the bulk of the experimental work of the project. Thirdly, as an aspirational target, an attempt would be made to establish whether the application of this technology to forensic pathology practice would be deemed acceptable for use within the Criminal Justice System (CJS). For the avoidance of doubt, “3D bio-printing” (for example, where living tissues are constructed from constituent parts or living cells are embedded within synthetic scaffolds) is considered a separate discipline and will not be discussed further within this thesis.19-21

1.3 Thesis aims and structure

As discussed, the literature review conducted prior to embarking on the project confirmed that the process of 3D printing from medical scan data in forensic casework was indeed feasible as a concept. However, use of more recently available “desktop” devices to achieve this in more routine practice had yet to be demonstrated. Therefore, in order to help bridge this knowledge gap, the ultimate aim of this project was to demonstrate the feasibility of introducing 3D printed models from forensic CT scan data into the courtroom environment, using desktop-scale devices.

In order to achieve this main aim, several smaller objectives had to be completed. First and foremost, the 3D printing process and available devices had to be researched, along with relevant terminology and everyday practicalities so that a working understanding could be used to select and acquire appropriate hardware and software. Practical familiarity with the chosen systems then had to be gained to enable models to be produced reliably and to an acceptable standard. Once reliable “in house” model production capability had been established, demonstration to potential “end users” (i.e. police detectives and criminal barristers) had to be carried out to ensure that suitable cases were subsequently referred for initial court case inclusion. Identification of suitable cases, where production of a 3D printed model might genuinely be useful rather than simply a gimmick, then had to take place before finally introducing physical models into live criminal trials to demonstrate benefit in practice.

The following chapters will outline the process by which an initial review of technological variants, available devices and necessary software led to the establishment of a viable office-based process to enable “in house” 3D printing, rather than outsourcing model production to an external provider. Evolution of this process in order to overcome

19 various technical hurdles will be demonstrated, and several cases where 3D printing has been used during actual forensic investigations will be illustrated.

1.3.1 Technical aspects

Chapter 2 will commence with an overview of 3D printing, including its origins as well as explanations of the key terminology. Particular attention will be paid to the 3D software virtual environment, which must first be navigated if the physical 3D printing process is to be accomplished successfully. The main technological categories of 3D printing process will be explored, together with their relative merits and drawbacks, and the chapter will conclude with a discussion of parameters considered when selecting hardware and software for use in this project.

Chapter 3 will outline the various testing and calibration experiments carried out initially following acquisition of a departmental 3D printer, in order to gain familiarity with each stage of the entire process. It should be noted that extremely precise systems are available for detailed 3D measurement and comparison, and that in a project aimed at validating the reproduction fidelity of 3D printing it would be necessary to employ such technology. From the outset, this project accepted that many years of research and development in engineering and manufacturing had already produced sophisticated devices and abundant data detailing their accuracy and tolerances, and so no attempt was made to produce data to emulate findings that were already readily available. Instead, only rudimentary testing was conducted to confirm satisfactory operation of the acquired hardware within the scope of the project, and to optimise device performance for the various project requirements.

Chapter 4 will chart the progress through initial simple anatomical models to more complicated and challenging printing projects, whilst developing the skills and experience necessary to enable future forensic cases to be approached in a predictable and reliable manner.

1.3.2 Practical applications

Chapter 5 details the systematic approach adopted to assess potential uses in turn, with the selection of suitable examples and production of concept-proving 3D prints. During the initial proposal stages of this project, the list of perceived experiments was considered

20 lengthy, although not exhaustive. In practice, once some of the more technical prints had been successfully achieved, duplicating the experiment to confirm that a similar, but slightly different, model could also be produced was deemed unnecessarily repetitive. In addition, once knowledge of the project and its ultimate aim became known to local police forces, requests began to be received to produce 3D prints for court use in live investigations, circumventing the need to ascertain that, in theory, such a print would have been possible if needed in the future.

1.3.3 CJS acceptability

As mentioned in the previous paragraph, requests for courtroom exhibit 3D models began to be received during the project. Chapter 6 provides illustrative examples of some of these cases, indicating the diversity of subject matter already benefitting from the local availability of this technology, as well as confirming acceptance of 3D printing into the court system. Initial concerns about whether there would be genuine added value, or if 3D printing might instead be considered a “gimmick”, as well as whether model production costs would be considered reasonable have therefore been allayed in practice.

1.3.4 Unexpected benefits

During the course of the project, additional benefits of this technology over and above the target aim of producing 3D anatomical models from forensic CT scans became apparent. Chapter 7 will provide examples of how acquisition of “in house” ability to design and manufacture custom items has enabled departmental research projects to take advantage of equipment optimised for specific requirements. Details will also be given of collaborations with various other departments, including multiple examples of computer-aided design (CAD), prototype production and small-scale manufacturing now made possible by the introduction of 3D printing technology into the department.

As mentioned in section 1.3.1 above, departmental implementation of the 3D printing process required acquisition of new 3D software skills. Whilst the familiarity acquired with these software packages was initially targeted solely at refinement of 3D printing ability, the wider functionality of this technology quickly became apparent and further potential applications were explored. Chapter 8 delves deeper into the fundamental

21 stages of 3D software manipulation, and how these processes can be used to assist with forensic casework in ways other than production of physical 3D models.

1.3.5 Conclusions

Having reached the end of the research phase envisaged at the start of the project, chapter 9 outlines the proposed plans for continuing to build upon the lessons learned to date, and details steps being taken to disseminate the acquired knowledge and expand the fledgling service created. The limitations and weaknesses identified throughout the project are also discussed. Finally, chapter 10 briefly summarises the project’s key learning points and confirms achievement of the overall objectives.

1.4 Ethical approval

Prior to commencing any manipulation of scan data or 3D printing, ethical approval to carry out the research project was sought from the NHS Health Research Authority (HRA) National Research Ethics Service (NRES) via the Integrated Research Application Scheme (IRAS). The IRAS project reference was 191181 and a Letter of Favourable Opinion was received from the North West – Greater Manchester East Research Ethics Committee (REC) on 10th May 2016 (REC reference 16/NW/0401).

Chapter 2 – 3D printing overview

2.1 History

Several different technological approaches have been developed with the common the aim of creating complete, three-dimensional objects during a single manufacturing process.22 Examples of the most established technologies will be discussed later in this chapter. Collectively these processes have been described as additive manufacturing or rapid prototyping technologies.23 The descriptor “additive” refers to the method of starting with nothing and gradually “adding” material to the model to build the final shape, in contrast to “subtractive” manufacturing, which starts with a solid mass of material and gradually removes (subtracts) this material to leave behind the finished shape.24 In much the same way as a conventional computer printer gradually builds up text or images line-by-line on a sheet of paper, a 3D printer works in layers to build up a solid structure. As an example, a series of identical flat circle shapes placed directly on top of one another in a stack will effectively create a cylinder once fused together (figure 2.1). As the 3D shape is built using a layer-by-layer approach, the term “layered” manufacturing is also sometimes applied.

Fig 2.1 a Virtual model, b Model digitally “sliced” into layers, c Layers printed sequentially on top of one another, d Completed physical model composed of fused layers.

Strictly speaking, the term “three-dimensional printing” refers only to one specific branch of this group of technologies.25,26 However, as knowledge of the existence of these techniques has become increasingly prominent within the mainstream media, terms such as “3D printed” and “3D printer” have increasingly been more loosely applied and are now commonly associated with any such manufacturing process or device.27 Although the first successful additive manufacturing process was patented by Charles Hull in 1986,28 this technology and its alternatives largely escaped widespread public attention during the early years of their development. Despite some early adoption of the technology by pioneering medical specialists, it is only relatively recently that knowledge and availability of 3D printing has allowed largescale proliferation outside the confines of manufacturing, technology and engineering specialists. Before embarking on detailed descriptions of individual printing processes, it is useful to examine the generic workflow pattern common to all methods.

2.2 The 3D printing process

First, a virtual 3D model is required. This could have been created de novo inside a CAD software package, or digitised from a physical object using surface capture modalities such as laser scanning or photogrammetry.29-34 When radiological scanning has been undertaken (as in this project), anatomical shapes can be extracted directly from the clinical imaging data. 3D printers cannot discern the shape of an entire object, so first the virtual model must be digitally “sliced” into a series of consecutive 2D layers that can be interpreted individually. Prior to slicing, there exists the option of using intermediate software to edit the 3D model for particular purposes. Once sliced, printing can take place, followed by any post-processing (curing, cleaning, etc.) steps necessary to finish the physical model. Figure 2.2 briefly outlines the entire process workflow, and will be followed by detailed explanations of the various technical terms used.

Fig 2.2 a Medical scan imaging data, b Surface scanning or photogrammetry of physical object, c Extraction of virtual 3D model (from either a or b), d Preparation of 3D printing file (including orientation and support structure placement) in “slicer” software, e Optional editing of 3D model “mesh” prior to slicing, f 3D printing process, g Post- processing (in this case curing with a combination of heat and 405 nm light).

2.2.1 The STL file

In the early days of additive manufacturing using stereolithography, a standardised file format (the STL file) was developed to encode 3D shapes.35 Whilst in many 3D software applications this older format has been superseded by more advanced file structures that can incorporate additional surface data such as colour and texture, the STL file remains ideally suited to 3D printing processes on account of its simplicity, transferability across software platforms and hardware operating systems and its reliability during the conversion of 3D models into the series of instructions that the printer will eventually follow whilst manufacturing each layer of the print.

The STL file essentially consists of a list of points (called “vertices”) aligned in 3D space using x, y and z Cartesian co-ordinate axes. As illustrated in figure 2.3, the vertices are joined together by edges to form a network (known as a “mesh”) of triangular faces. The

STL mesh is a digital representation of a 3D shape, that can be edited, sliced and transferred to a 3D printer to enable production of a physical model.

Fig 2.3 a Representation of a 3D cube (grey) using x (red arrow), y (green arrow) and z (blue arrow) axes, b An individual vertex (white), c An edge connecting two vertices (white), d A triangular face connecting three vertices, e Close view of an odontoid process (small neck bone) giving an indication of the complex geometry that can be represented by the seemingly simple “mesh” of triangular faces in an STL file.

2.2.2 Creation of the 3D model

Many medical practitioners will already be familiar with the Digital Imaging and Communications in Medicine (DICOM) image data sets produced by medical imaging equipment. Forensic pathologists within the EMFPU had already been using the popular OsiriX 36 DICOM viewing software to interpret and manipulate post-mortem CT scans for a number of years. This software package contains a 3D surface rendering module, which employs an established “marching cubes” mathematical algorithm37 to extract an iso-surface from the stacked 2D image slices. This surface can then be directly exported as a 3D model in a variety of file formats (including STL). As will be discussed specifically in chapter 4, parameters can be adjusted to isolate structures of interest (e.g. bone or skin), and the resolution altered depending on the preferred level of detail. Higher resolution meshes contain more triangular faces, and therefore generate increased memory and processor burdens. “Decimation” is a method of reducing the number of triangles within the mesh, and its effect is illustrated in more detail in chapter 3.

An alternative method of anatomy isolation is to use so-called “segmentation” tools to determine a region of interest (ROI). Segmentation is the standard term for the process of separating specific structures of interest from background data, and is illustrated in chapter 4. The OsiriX program contains a number of powerful segmentation tools that allow additional anatomical structures to be extracted over and above those that can readily be isolated using the aforementioned surface rendering module. During the course of this project, the numerous options for segmenting anatomical structures were assessed, providing a variety of different strategies to refine segmentations if initial results obtained using default settings proved unsatisfactory. An alternative method of segmentation was also sought to facilitate the adoption of these techniques by individuals without access to OsiriX. Several open-source programs were assessed for suitability (discussed in more detail in chapter 4), and 3D Slicer 38,39 was found to be highly effective. (For the avoidance of potential confusion, despite its name this program is not a “slicer” in the context of preparation of files for 3D printing, as described below.)

2.2.3 Editing the 3D model

The 3D model that has been extracted could be imported immediately into a “slicer” program to create files for the 3D printer. However, forensic pathology cases contain highly complex anatomical morphology, and the generated 3D files in their raw state contain numerous anomalies (such as holes, scatter artefacts and loose fragments) likely to result in errors during printing. An additional step in the software process may therefore be required to address any such anomalies prior to printing, in order to eliminate or minimise subsequent printing errors. Powerful 3D editing software that enables sophisticated fine-tuning of anatomical data is freely available, and several examples of this type of software were evaluated for use during this project (discussed in more detail in chapter 4). One open-source program that is widely used across the 3D community is Blender,40 and this quickly became the software of choice for this purpose due to its comprehensive capability. Additional advantages to using this software were also discovered, and these will be revisited in chapter 8.

2.2.4 Slicing

Once a virtual 3D model has been extracted from the medical scan data (and possibly edited), it must be converted into a format that can be interpreted by a 3D printer. As

27 mentioned above, 3D printers themselves cannot discern an entire object, but instead must be given a series of layer-by-layer instructions. The 3D mesh object (STL) is therefore electronically “sliced” into a stack of 2D layers, which the 3D printer can then trace out in succession, each one fused to the preceding layer, eventually reconstructing a 3D object with contoured surfaces that recapitulate the form of the original triangular mesh. Some manufacturers provide bespoke software designed specifically to function optimally with their own printers, whereas others rely on more generic slicer software that can control a variety of different devices. The precise sequence and degree of automation or customisation varies depending on the hardware and software used. In general, the slicer program will allow a model (or models) to be imported into the build volume, arranged and orientated both to fit within the volume and to print optimally, have supports added where necessary (see below) and then parameters such as build material and resolution (see below) to be specified. The model is then “sliced” (i.e. prepared for printing), and the resulting print file is transferred to the printer. Figure 2.4 demonstrates the slicing process.

Fig 2.4 a The 3D model (STL file) exported from medical imaging software is now imported into the slicer software, b Slicing is performed, giving an indication of the actual layers to be constructed by the printer, c Scrolling through the slices gives a preview of each layer.

2.2.5 Supports and orientation

Depending on the shape of the 3D model, a temporary supporting scaffold structure is often necessary during printing to overcome the problem of gravity preventing material from being deposited successfully in mid-air. An example is illustrated in figure 2.5. (The nature of some 3D printing processes means that they do not require dedicated support structures, as will be discussed separately for each technology below.)

Fig 2.5 a Some overhanging features can be supported by lower layers of the model. A rule of thumb is that overhangs with an angle (white) greater than 45º (i.e. more vertical relative to the build platform) do not require any additional support (although this will vary in practice), b Where an excessive overhang or significant bridge is unsupported, gravity will cause the print to fail, c Temporary support can be provided at the time of printing by inclusion of additional material where needed. This material is either manually removed or dissolved after printing.

Overcoming the problem of support requirement remains one of the major challenges in 3D printing, especially in the context of complex anatomical shapes and surfaces. Modern slicing programs contain sophisticated algorithms to calculate support requirements and locations automatically, but a degree of manual editing is also often required. The placement of supports is critical in ensuring a successful print, but physical removal of non-soluble supports after printing results in some degradation of the surface finish. For this reason, support structure placement is tailored to each print depending on the desired end result, and the precise orientation of the model during printing is critical in determining not only the quality of the finished print, but also whether the print will be successful or fail partway through. Seemingly logical placement of supports does not always guarantee success, and fine-tuning this step can be more of an art than a science, relying heavily on knowledge gained from previous failures. Optimisation of supports for printing reliability can result in excessive support material usage (and therefore expense), as well as increased printing time. Conversely, time and material can be saved by conservative use of support structures at the expense of potential print failure. Valuable lessons were learned during the testing and calibration phase of this project (see chapter 3), such that the service now offered to police forces is relatively predictable and reliable.

Support structures can broadly be categorised as soluble or non-soluble. In devices that construct the supports out of the same material as the model, they remain physically attached to the model at the end of printing, requiring manual separation. Printers that

29 are capable of delivering more than one material during the printing process can take advantage of a dedicated support material to simplify post-print removal, as the support material can be designed to be soluble (either in water or a more specific solvent). Other non-soluble materials are available that bond sufficiently with the model during printing to function as a support, but that can relatively easily be separated after printing. The complicated organic form of an anatomical model means that it will usually require an intricate support structure to make printing possible. It is therefore of benefit if these supports can be removed afterwards without adversely affecting the surface quality of the print. Certain regions of a model (such as inside an intact skull) may be inaccessible after printing, and soluble supports may be the only viable option in such circumstances. Figure 2.6 demonstrates the use of water-soluble supports.

Fig 2.6 a Water-soluble (PVA) supporting scaffold surrounding the base of a model after printing, b After soaking in water, the supports have completely dissolved to reveal the entire surface of the model.

Model orientation can help minimise or even remove altogether the need for supports, as illustrated in figure 2.7. Aside from influencing support requirements and location, model orientation itself is a factor that must be taken into consideration along with the choice of process used. For example, the Formlabs 41 3D printer used in this project generates mechanical shearing forces every time the model is separated from the resin tank to be raised in preparation for the next layer (see discussion of stereolithography, below). Consequently, printing flat objects at an oblique angle reduces the cross- sectional area of each layer during printing, and therefore minimises potential distortion

30 of the model. Alternatively, the Ultimaker 42 printer used in this project produces models with mechanical properties that vary greatly depending on how the model is orientated with respect to the “z” plane, as it is inter-layer fusion that is least strong with this manufacturing technique (see discussion of fused filament fabrication, below). In addition, the visible contour lines on the surface of fused filament models reduce surface quality to an extent, and so the direction in which these contours will be laid down can be chosen to lessen their impact on the most important features of the model. Removal of support structures that are adherent to the surface of a model will degrade the surface finish, and so some models may need to be orientated in such a way as to preserve important details whilst potentially sacrificing others. The project’s initial trials investigated the effects of these individual variables, and models now produced for forensic casework benefit from pre-print fine-tuning as a result.

Fig 2.7 a This basic shape has a large area of unsupported overhang that would fail to print (arrow), b Addition of supports (red) would allow successful printing at the expense of increased print time, support material consumption and potential surface degradation at points of support attachment, c The need for supports is avoided altogether by simple re-orientation of the shape with respect to the build platform.

2.2.6 Printing

The precise mechanical process varies depending upon the model of printer being used (see below). The term rapid prototyping is potentially misleading. It is an industrial term indicating that this type of technology is appreciably faster than previous methods of producing individual prototypes. However, a large, complex print with numerous support structures can still take many hours, even days, to complete. Adjustments can be made to printing parameters at the slicing stage to reduce overall printing time, but invariably

31 at the expense of print quality. Initially, small prints were used for trial purposes so that the printers did not need to be left unsupervised. With improved confidence in printing reliability following initial testing, the devices could eventually be left unattended overnight, facilitating the production of large prints requiring continuous printing over several days.

It is worth noting at this point that the concept of “resolution” is not straightforward in the context of 3D printing and 3D printers. Data provided by manufacturers is likely to specify the highest resolution criterion achievable by a particular machine, but the printer may actually perform more satisfactorily at an “inferior” setting during real-world printing. The finite size of an extrusion nozzle or laser spot, for example, does not necessarily provide a reliable indication of the minimum feature detail that will be acceptably discernible on the surface of a finished model. It was therefore necessary during the early phase of the project to inspect test prints made using varied parameters in order to learn the optimal settings to use for different printing applications.

3D printing involves production by sequential layers, and the most useful equivalent of resolution is therefore the “layer height” (sometimes referred to as the “z” height, as this is conventionally the vertical axis). Smaller (thinner) layer heights will generally produce finer details, but require longer print times and increase the chance of print failure. The effects of layer height on print quality, as well as different technological processes using the same layer height, are illustrated in figure 2.8.

Fig 2.8 a The effect of different layer heights at the time of software slicing, b After printing, c Comparison of the same layer height (0.1 mm) printed on different devices: an Ultimaker S5 fused filament fabrication printer (left) and a Formlabs Form 2 stereolithography printer (right).

2.2.7 Post-processing

Different printing methods require varying degrees of post-processing. Models produced by the Ultimaker printer used in this project are essentially ready for use as soon as they are removed from the build platform, if no support structures were required during printing. When print supports have been necessary, these are soluble and so simple immersion in water for a period of time is sufficient to reveal the finished model. Blemishes and other surface imperfections particular to this method of printing often need

33 to be removed by rubbing with an abrasive surface. For project applications where surface finish of the model was paramount, the Formlabs printer was used instead due to the inherently superior surface quality delivered by stereolithography (see below). However, after printing with this type of printer all models have to be removed from the build plate and washed in a solvent (isopropyl alcohol, IPA) to remove uncured resin adherent to the surface. After drying, a period of extended curing with specific wavelength light in a heated chamber is also recommended for optimum results, adding to the overall time and complexity of the process. Supports are always required with this method of printing, and they cannot be dissolved so must be manually removed using tools, causing some minor surface degradation meaning that additional abrasive smoothing is also required. As experience was gained through initial testing, it became apparent that the advantages and disadvantages of each printer’s post-processing requirements would also have a bearing on the choice of printer to be used for different applications in future casework.

2.3 Types of 3D printer

As stated above, different technological approaches have been developed to facilitate additive manufacturing. There is no “perfect” 3D printer for every situation, and each model has relative advantages and disadvantages depending on the proposed application and available resources. Prior to making final decisions about the printers to acquire for this project, a comprehensive review was carried out of the various technology types as well as the available models of each. Visits were made in person to a number of specialist vendors stocking multiple examples of different devices, and sample prints were obtained from several of these to provide a true indication of the results obtainable. In order to achieve the aim of this project, a realistically affordable device that could be operated in an office setting would need to be sourced, prohibiting the consideration of larger, industrial scale machines. Other than cost, the most important influencing factors would be the total available build volume, support structure capability and overall print quality, as these were factors that would determine the types of anatomical model that could be printed. Whilst the project’s intention was to make use of a “desktop” grade device, the potential still existed for outsourcing of final model production to an external provider having access to industrial equipment, and so an awareness of the full range of technologies was deemed essential. What follows is a summary of the different

34 technology types investigated, to provide a rationale for the eventual equipment acquisition decision-making process. The list of specific processes outlined below is not exhaustive, and other systems also exist.

2.3.1 Stereolithography

Stereolithography (SLA) was the first successful additive manufacturing process,23 and works by guiding a laser spot around a defined path across the surface of a tank of photoreactive liquid resin. Where illuminated, light-sensitive initiators within the resin promote local cross-linking to polymerise (solidify) the liquid at that point. The model is created, one layer at a time, with the part being lowered slightly into the tank of resin before the next layer is drawn on its surface by the laser. The original (and contemporary) industrial strategy requires a large resin tank into which the build platform gradually submerges to allow the next layer to be added to the top of the model at the surface of the liquid. These industrial machines are well beyond the resources available for this particular project. More recently, “desktop” stereolithography has become possible, and this inverts the process so that the build platform is instead raised out of the resin at the end of each layer. By adding to the bottom of the model in this way, the requirement for a deep tank holding a large volume of resin is avoided, allowing for considerable miniaturisation of the system and substantial cost saving. A similar process known as digital (or direct) light processing (DLP) replaces the moving laser spot with a screen of illuminated pixels, so that an entire layer can be exposed at once, rather than gradually tracing around an outline. There are therefore speed and resolution differences between these two, otherwise similar, technologies. Both provide a relatively affordable, desktop- sized option for 3D printing, and so were considered further as potential candidates for a device to acquire. However, DLP devices tend to have considerably smaller build volumes, and so were soon determined to be unsuitable for the predicted size of anatomical models likely to be required.

Models manufactured by curing resin in this way must be washed in a solvent bath after printing to remove uncured excess resin, and an additional stage of post-print curing (using a combination of heat and specific wavelength light) is required for optimal mechanical performance and longevity, depending on the specific resin used. As discussed previously, any unsupported overhanging structures within the model must have a scaffolding structure added to allow successful printing, and this scaffolding must

35 be manually removed after printing and processing. This additional post-processing burden might be considered off-putting when deciding upon a device to acquire, but this must be weighed against the high-quality print surface finish for which the SLA / DLP processes are renowned. Figure 2.9 illustrates the “upside down” arrangement of desktop SLA.

Fig 2.9 a Precisely controlled mirrors direct the laser (violet) along the transparent base of the resin tank (orange). Where the laser contacts the liquid resin, local curing takes place to form that particular layer of the model, b The build platform (grey) rises vertically as new layers are added to the bottom of the model, which can be seen emerging from the resin tank. Note the oblique angle of the model relative to the plane of the build platform, and the presence of the vertical support structures.

2.3.2 Selective laser sintering

Selective laser sintering (SLS) is in some ways similar to stereolithography, but replaces the tank of light-sensitive liquid resin with a chamber of powdered solid material. As the laser spot moves across the surface of the powder, tracing the outline of that particular layer of the model, the powder is locally sintered (fused) into a solid. The build platform upon which the model sits is then lowered slightly, the chamber is covered with another layer of powder and the process is repeated. By the time of completion, the model will have become buried within the mass of unused powder filling the chamber, and must be removed and cleaned. A proportion of unused powder can be recovered and used again for the next print, and the surrounding powder acts as a support to the model throughout the printing process meaning that no additional supporting scaffold is necessary. The ability to create complex anatomical shapes without the necessary complication of

36 additional support structures is appealing, but unfortunately this type of technology remains restricted to large, industrial devices that are both prohibitively expensive and overly cumbersome for the proposed scale of this project. In the event of a final model having to be outsourced to an external provider, this type of technology does have potential advantages: the dimensional accuracy is considered to be very good, and the relatively low cost of consumables combined with the ability of the provider to “pack” the build volume with multiple projects during the same print run means that model production costs can remain lower than some competitor technologies. Figure 2.10 illustrates the SLS process.

Fig 2.10 a The laser scans across the surface of the powder (grey), resulting in localised thermal fusion to create each layer of the model (circular object), b As further layers are added, the build volume fills with powder. The unfused powder acts as a support beneath and around the model.

2.3.3 Fused filament fabrication

Fused filament fabrication (FFF) is one of several generic terms used to describe the technology originally trademarked by Stratasys43 as “Fusion Deposition Modelling” (FDM). However, many sources use the FDM abbreviation loosely to describe all devices of this type. These printers heat thermoplastic material to a temperature that enables extrusion of molten material through a fine nozzle, followed by rapid solidification by cooling after leaving the nozzle. The nozzle moves horizontally around the build volume, depositing one layer of molten plastic at a time. The build platform (“bed”) is then lowered vertically to allow the process to continue with the following layer. The temperature of the molten plastic being deposited is sufficient to allow it to fuse with the solidified plastic from the previous layer. High specification industrial

37 devices of this nature do exist, but this is the type of technology that has become most prolific as a “desktop” printer. Numerous manufacturers have released FFF printers, leading to a wide spectrum of price and quality available in this sector of the market, including self-build kits that incorporate 3D printed component parts. When considering which device to purchase during the initial phase of the project, much attention was focused on this particular technology, as the cost, size and sheer choice of devices made this a very sensible place to start when aiming for an office-based 3D printing solution.

An increasing variety of thermoplastic materials can be printed using these devices, with polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) being the most commonly used. ABS has a higher working temperature and emits fumes during use, and PLA is considered easier to print with (on account of better adhesion to the build platform and less warping), so many of the less expensive machines aimed at home use rely chiefly on PLA as the print material. However, much of the ongoing research and development with this particular technology is focused upon expanding the range of printing materials. New thermoplastics with different properties and advantages are being developed and released in order to cater for requirements such as strength or flexibility, and often a second material (such as wood fibre or metallic dust) is added to the raw plastic filament in order to impart desirable qualities to the surface finish of the end result. In general, the surface finish and fine detail reproduction of objects printed using FFF devices are regarded as inferior to other technologies, although the industrial grade machines are capable of producing prints of consistently high quality. With such a wide variety of devices, price points and print quality available in this sector of the market, real-world capability would need to be scrutinised in practice when considering this type of printer for use in the project.

As with SLA / DLP printing, supporting scaffold must be added to the model prior to printing if complex geometry and overhanging elements are to print successfully. Single extruder printers can produce these supports using the same material as the main model being printed, and these supports must be manually removed afterwards. This process results in considerable degradation of the supported surfaces, and it can be difficult or even impossible to remove physically attached supports from internal or otherwise difficult to access areas of the model. More advanced (“dual nozzle”) devices can extrude a water-soluble support structure (usually polyvinyl alcohol, PVA) at the same time as printing the model. PVA can be problematic as a printing material, and so an alternative

(high impact polystyrene, HIPS) is often used instead to create the support structures. HIPS is not water-soluble, though, and must be dissolved in limonene, introducing an additional complexity factor. Use of soluble support structures simplifies their post-print removal, but in some cases it may be preferable to use a support material specifically designed to separate easily (“break away”) from the model after printing. Given the morphological complexity of the anatomical models to be printed for forensic casework, only multiple extruder models could realistically be considered viable for this project. Figure 2.11 illustrates the fused filament fabrication process.

Fig 2.11 a The heated nozzle moves horizontally over the build platform (grey), depositing a layer of molten plastic (red), b The build platform lowers vertically to allow subsequent layers to be added on top of the first, gradually forming a solid mass in the required shape.

2.3.4 Material jetting

Material jetting works in a similar way to 2D inkjet paper printing, in that a “print head” moves across the build area depositing tiny droplets of material, which are immediately cured by ultraviolet light. Depending on the complexity of the device, different materials can be combined within the same print, and soluble support structures can be produced simultaneously whilst printing the model. Currently, this technology is limited to large scale, industrial type machinery with consequent cost and practical implications. However, the quality of the models produced is very high, and these devices make it feasible to produce multi-coloured medical models with transparent outer surfaces revealing underlying anatomical features.44 This technology has even been used recently to create a complex, multi-material model of a generic infant head to facilitate

39 experimental investigation of paediatric head injury mechanics.45 This type of printer would lend itself very well to the production of forensic anatomical models, but the price, size and complexity of the devices means that they could not realistically be considered viable for this particular project. Figure 2.12 illustrates the material jetting process.

Fig 2.12 a The print head moves horizontally over the build platform, ejecting minute volumes of the resin materials required. In this case a soluble material (yellow) is necessary to provide temporary support to the model (red) during production, b The liquid resins are immediately cured by ultraviolet light (purple) at the time of deposition.

2.3.5 Binder jetting

A process combining some features of both material jetting and laser sintering is so-called binder jetting. Instead of the moving print head directly ejecting material to create the model, droplets of binder are precisely deposited onto a bed of powdered material to produce local fusion where necessary to solidify a complete layer. As with laser sintering, the model is then lowered and covered with further powder ready for the addition of the next layer. This again means that the unused powder supports the model during printer, avoiding the need for dedicated support structures. Depending upon the device and material used, coloured dyes can be added by the print head during the jetting process, facilitating the production of full colour models. This technology is currently limited to expensive, large scale devices, and post-processing steps are needed to stabilise and protect the model after printing. Binder jetting printers were therefore not considered further as potential candidates for “in house” printing in this project, but they remain a potentially useful option for outsourcing model production if needed at a later date, as

40 despite the high device cost they conversely have some of the lowest consumables costs of all 3D printers.

2.3.6 Laminated object manufacturing

Laminated object manufacturing (LOM) works by cutting (either with a laser or sharp blade) the outline of a particular layer from a sheet of material (often paper) supplied from a roll. Each consecutive layer of the model is then secured on top of the stack of preceding layers using adhesive. Full colour inkjet printing of the outline at the time of cutting can be used to impart a photo-realistic surface to the finished model. This technology is best suited to displaying the external appearance of solid models, and such models tend to be used to provide an indication of how an object looks, rather than how it functions. Although predominantly a large scale, industrial process, this technology has recently been scaled down into a desktop-sized, full colour paper 3D printer.46 The output of the full-colour, paper-based variety of this technology was inspected during the initial information-gathering phase of the project, as the photo-realistic surface appearance was thought to have potential merit in forensic casework. However, the lack of internal complexity was considered unsuitable for the type of anatomical model envisaged, and the relative fragility of the models meant that they might not stand up to vigorous handling in a courtroom environment. This technology was therefore not considered further.

2.4 Device type selection

Industrial scale machines such as those supplied by Stratasys, 3D Systems47 or EOS48 are capable of producing large, high quality, multi-material models, but are beyond the realistic resources of most forensic pathology departments. The operation of these machines requires considerable space, technical support and a high throughput to justify running costs. In recent years, “desktop” 3D printers have emerged and their prices have been falling at the same time that quality and reliability have risen. This was the initial stimulus for the project’s goal of determining whether or not such a device could be incorporated locally into a forensic pathology department and used to produce models that were genuinely useful in a courtroom environment.

Device type Cost Build volume Print quality

SLA (industrial)  ✓ ✓

SLA (desktop) ✓ ✓ ✓

DLP ✓  ✓

SLS  ✓ ✓

FFF (industrial)  ✓ ✓

FFF (desktop) ✓ ✓ ✓ to 

Material jetting  ✓ ✓

Binder jetting  ✓ ✓

LOM  ✓ 

Tab 2.1 Relative strengths of different technology types.

As can be seen from table 2.1, the majority of the printing technologies were excluded from serious consideration for purchase during this project on account of cost alone (and in many cases this high purchase cost would also have correlated with size and running complexity burdens that would have precluded office-based operation). The DLP devices, although potentially attractive from a size, cost and print quality perspective, would not provide sufficient build volume capacity for the vast majority of the predicted anatomical model requirements of the project. The prints produced by LOM devices would also not have been appropriate for the predicted applications, and so the final decision was between desktop SLA and desktop FFF machines. At the commencement of the project, and during its early phase, only one desktop SLA device (the Formlabs Form 2) was available. Numerous small, cheap, single extruder FFF machines existed, but these were discounted on the grounds of prohibitively small build volume and / or lack of dual extrusion (for soluble support structure capability). Large, industrial-scale FFF devices were not considered on account of high initial purchase cost. This produced a short list of realistically viable machines worthy of further evaluation:

Device Technology Cost (£) Build volume (cm) Min. z height

Form 2 SLA 3,000 14.5 x 14.5 x 17.5 0.025 mm

Ultimaker 3 FFF 3,500 21.5 x 21.5 x 20 0.02 mm

Replicator 2x FFF 2,800 24.6 x 15.2 x 15.5 0.1 mm

Cube Pro Duo FFF 2,200 24.2 x 23 x 27 0.1 mm

Xeed FFF 6,500 28 x 22 x 23 0.1 mm

Tab 2.2 Comparison of potential devices available during the early phase of the project.

From this table it can be seen that the notable exception to the general price range of £2,000 - £4,000 was the Leapfrog Xeed, said to have been marketed toward the more serious industrial or small manufacturing buyer rather than the home user. A visit to a supplier of this device for a practical demonstration and sample print was promising, but the experience of the local hospital radiology department, who had acquired just such a device to experiment with clinical 3D printing, revealed that in practice the printer struggled to achieve successful prints when attempting complex anatomical models with soluble support structures. This device was therefore not considered further, given its considerably higher purchase price and the potential for ongoing issues with reliability.

The UK distributor of the Cube Pro Duo was visited for a demonstration, but the sample print quality was not considered suitable as the complex anatomical shapes were not printed successfully, leaving numerous holes and other anomalies. Remembering that the aim of the project was to produce models that could be used in a courtroom environment, any amateurish qualities apparent in the final prints could harm the confidence and jeopardise the willingness of the police and barristers to adopt this technology in real criminal cases. The Ultimaker and Replicator devices were considered to have build volumes that were potentially restrictive, given the aim of producing skull models. Having conducted a further visit to a supplier of the newly-released Formlabs Form 2 printer, the sample print quality and surface finish of the SLA process was found to appear noticeably smoother and subjectively “cleaner” than those of the FFF devices

43 already assessed. For this reason, despite the smaller build volume chamber, a decision was made to proceed with this device for the initial testing phase of the project.

As the project progressed, it became apparent that whilst the Form 2 performed admirably for many applications, its chief limitations were its modest maximum build volume and the practical difficulty of support structure removal from inaccessible parts of models. A decision was therefore subsequently made to continue searching for an additional printer, with the capability of producing larger models (e.g. a full-size human skull or pelvis) whilst accommodating the use of soluble support structures. By the later stages of the project, introduction of the Ultimaker S5 FFF printer had been announced. This device offered a large build volume (33 cm x 24 cm x 30 cm) and support structure versatility that had not been available at an affordable price point (circa £6,000) at the outset of the project. Prior to launch, a sample anatomical model was requested from the UK distributor, and the successful print displayed a surface detail level and finish quality notably superior to the earlier FFF device prints examined at the commencement of the project. An early production model was therefore acquired to augment the project’s 3D printing capability, and examples of prints made using this device are illustrated within this thesis.

It is worth stating at this point that both the Formlabs and Ultimaker devices purchased for this project are supported by dedicated slicing software produced by their respective manufacturers, and that there was no requirement to research and purchase additional software for the purpose of print file preparation. Whilst experimenting with the radiology department’s aforementioned Leapfrog Xeed device, it was necessary to learn how to use a different “slicer” software package as no such dedicated software was provided by the Leapfrog manufacturer. This software, Simplfy3D, is a generic slicer program that is capable of preparing print files for a large number of printers from a wide variety of manufacturers. This approach potentially has some advantages, in that a single piece of software can be used to operate several different machines, rather than having to learn a separate program for each individual device used. However, given the intuitive simplicity and reliable performance of the dedicated software produced for the Ultimaker and Formlabs devices, this proved not to be a particular burden in practice.

Chapter 3 – Putting theory into practice

3.1 The beginning

Following a comprehensive review of the available devices at the time of project commencement, one printer in particular stood out as being an ideal candidate with which to start developing the “in house” 3D printing process. The Formlabs Form 2 is a desktop stereolithography printer with a build volume capacity of 145 mm x 145 mm x 175 mm.41 Whilst not sufficient to produce, for example, an entire human skull in a single print, scaled-down or partial anatomical models would fit within this volume. The speed, accuracy and surface finish of stereolithography prints was considered superior to the other contemporary desktop-scale devices (see chapter 2), and the purchase and running costs were readily affordable within the scope of the project. A printer was therefore purchased, and initial test prints were created to demonstrate the viability of the entire workflow from scan data model extraction through software manipulation and printing followed by the various finishing steps necessary to render the printed model suitable for display, handling and potential use in a courtroom environment.

3.2 Initial testing and calibration

As was alluded to in chapter 1, this project’s intention was not to validate the accuracy of 3D printing from a metrology or engineering point of view. Such work has already been conducted on an industrial scale over many years within research, engineering and manufacturing sectors, and is far beyond the scope of this project. To attempt such validation to satisfactory standards, the use of extremely precise data acquisition systems such as laser, micro CT or nano CT scanning would be needed, and dedicated 3D software comparison tools required to compare printed models with original artefacts to quantify discrepancies. These facilities are not available locally, and the logistical and financial implications of adding this level of complexity to the technical section of the project were not considered to be justifiable, given the limited scope of the project’s ultimate aim. Having said that, making no attempt at all to investigate the fidelity of the printed parts would not be satisfactory, given that criminal court trials were the intended target user of the 3D printing project’s output. Repeated iterations were printed of initial calibration tests to give an indication of dimensional accuracy, albeit with acknowledged inbuilt errors of measurement and comparison (see below). This process also enabled a degree

45 of refinement of processes so that efficiency and reliability could be improved, and an element of print outcome predictability obtained.

3.2.1 Test cubes

Consideration was first given to the dimensional accuracy to be expected when producing anatomical models from CT scan data. The process of stereolithography inevitably results in a degree of dimensional and conformational change due to physical forces acting during printing, as well as resin shrinkage at the curing stage.49 Depending on the application, small deviations between the computer file and eventual model may still be deemed satisfactory.50 As a guide to what might be considered tolerable deviation, a comparison was made against the measurement accuracy currently accepted within forensic anthropology practice (the discipline most directly associated with detailed measurements of skeletal anatomy). It has been stated previously that measurements within 2 mm would be considered an acceptable margin of error in forensic anthropology.51 For the purpose of this project, such a marker of acceptability was therefore taken as a surrogate of what should be considered tolerable within forensic pathology practice. Previous research has shown that there is no statistically significant difference between measurements made from CT scan data and their respective anthropological bone measurements.52,53 Furthermore, virtual 3D models extracted from CT scan data have been compared to defleshed bones and found to be accurate to sub- millimetre precision.54 Comparison between dry skull and corresponding stereolithographic 3D printed skull model measurements has shown dimensional accuracy of 0.62 ± 0.35 mm (0.56 ± 0.39%).55 The PreForm56 software used to slice models and prepare print files for the Form 2 printer has been developed to compensate for resin shrinkage during curing, and adjusts for variations in resin colour and printing layer height to ensure that results are as consistent as possible. Considering all of these factors, if the measured dimensions of the test prints remained within 2 mm of those expected then the actual models printed during the project would be regarded as fit for their intended purpose of anatomical demonstration.

Using the CAD functionality of the aforementioned Blender software (see chapter 2), virtual models of solid 10 mm and 20 mm cubes were created. When exported as STL files, the linear dimensions of these cubes were defined in millimetre measurements (with theoretical micron accuracy at the default design stage). To begin with, solid cubes were

46 printed in clear, white and grey resin (with a common layer height) to assess any variation between the physical properties of these different build materials. Additional cubes were then printed using alternative layer heights to assess any difference in accuracy that may be introduced by this significant variable in the printing process. The cubes were then reprinted, but this time as hollow shells (and with varying wall thickness) to ascertain the effects to be expected during printing of non-solid objects. Measurements of the x, y and z dimensions of the printed cubes were taken immediately after printing, and then repeated after an hour of additional post-print curing under 405 nm wavelength LED illumination to assess the effect of any resin shrinkage caused by curing. The complete series of measurements allowed several comparisons to be made, as discussed below.

Fig 3.1 Example digital calliper measurement of a 20 mm solid cube, printed in grey resin at a layer height of 0.1 mm.

As can be seen from figure 3.1, the method of measurement chosen for these initial experiments was a simple digital calliper device. The device displayed measurements with a resolution of 0.01mm, a manufacturer’s quoted accuracy of 0.02 mm (for measurements less than 100 mm) and a repeatability of 0.01 mm. When coupled with

47 the potential for subjective adjustments by the observer during measurement, it can be seen that the measurements taken inherently contain an element of deviation from the “true” dimensions. This would be far from acceptable in the realm of metrology, but in forensic pathology practice, where the dimensions of features as important as fatal stab wounds are routinely measured simply by placing a mortuary ruler alongside the wound, the low cost and ready availability of this basic measuring device was considered adequate for the limited purpose of approximate comparison.

3.2.2 Variation between materials

Three separate resins were trialled during this phase of the project: “clear” (i.e. colourless), “white” and “grey”. Although the chemical composition of each of these so- called “standard” resins is essentially the same (methacrylic acid esters and a photoinitiator), pigment molecules are added to the grey and white resins in order to impart the desired aesthetic characteristics. The presence of these pigments very slightly alters the laser exposure time required during the printing process, and so in theory there may be a noticeable difference in curing shrinkage artefact after printing. For each cube, the mean discrepancy between measured and expected values was calculated and expressed as a percentage. For example, table 3.1 shows measurements obtained for the solid 20 mm clear cube (after curing):

Axis Expected Observed Difference

x 20.00 mm 20.07 mm + 0.07 mm

y 20.00 mm 20.01 mm + 0.01 mm

z 20.00 mm 19.95 mm - 0.05 mm

mean difference + 0.01 mm

percentage + 0.05%

Tab 3.1 Solid 20 mm clear cube measurements, post-curing.

Accepting the aforementioned limitations of the specified measurement technique, when the obtained values for all of the clear cubes were considered collectively, the overall mean difference was + 0.05%. The same calculations were performed for the white and grey cubes, giving mean differences of + 0.15% and - 0.01% respectively. Whilst it appears even with crude measuring techniques that the printing process is not perfect, in that there are observed differences between the intended and eventual dimensions, the measured discrepancies remain very small (in percentage terms) for all three colours of resin. The variation in mean percentage difference was not constant for each of the three resins used, and an attempt was made to determine whether this was a genuine finding or simply due to chance within this dataset. Paired, two-tailed t-tests were calculated to compare each resin colour’s actual observed measurements with the values expected from the virtual model. The t-statistics calculated for the clear and grey resins were 0.278 and 0.655 respectively, suggesting that their deviation from the expected values was not statistically significant. However, the white resin’s calculated t-statistic was 0.002, indicating that for this resin there could indeed be a genuine detrimental effect on dimensional accuracy when choosing this specific colour of print material. It must be stressed that the sample size for this experiment was extremely small (6 cubes per colour, giving a total of 18 x, y and z measurements for each resin), and so any inferences must be drawn with caution.

3.2.3 Effect of layer height

As discussed in chapter 2, the layer height is a principal determinant of both print quality and print speed. This parameter will therefore frequently be adjusted to suit the required application, and so the real-world effect of any variance was investigated. Three possible layer heights were obtainable using this particular hardware, software and consumable combination: 0.1 mm, 0.05 mm and 0.025 mm. As described above, additional cubes were printed at these different layer heights and their physical dimensions measured. Discrepancies between expected and measured values were again obtained, and mean differences were calculated as before to allow crude comparison in terms of percentage difference (table 3.2).

Layer height Mean difference

0.1 mm + 0.05%

0.05 mm + 0.46%

0.025 mm + 0.54%

Tab 3.2 Relative percentage discrepancies for each layer height.

This trend would appear to suggest that there may be a decrease in dimensional accuracy (increasing size discrepancy) with increasing “resolution” (decreasing layer height). The manufacturer’s guide to using the PreForm software suggests that, unless the higher resolution settings are specifically necessary for discrimination of small feature details, the default setting of 0.1 mm layer height should be used. As the anatomical models to be produced later on were predicted to be relatively large, the fastest available print time (largest layer height) was considered to be advantageous, and it was reassuring to discover that this would not necessarily incur an accuracy penalty, and may even be beneficial.

3.2.4 Effect of size and wall thickness

Any distortion resulting from physical forces during the printing process may also show different degrees of variation depending on the size and / or thickness of the structure being printed, as a more fragile feature may deform to a greater extent than a sturdy one. In addition, thinner-walled structures have the potential to absorb more IPA (the solvent used after printing to remove surface residues of uncured resin), possibly resulting in swelling and consequent further distortion. This was the rationale for printing cubes at different sizes (10 mm and 20 mm), and also for the inclusion not only of solid cubes but also hollow examples (with both thick and thin walls) of each. Arbitrarily-chosen “thin” walls of 0.5 mm for the 10 mm cubes and 1 mm for the 20 mm cubes were followed by “thick” walls of 2 mm for the 10 mm cubes and 4 mm for the 20 mm cubes. As before, calculations were made to determine the mean percentage difference between measurements of the printed cubes and the values expected (table 3.3).

Cube group Mean difference

All cubes + 0.24%

20 mm cubes + 0.20%

10 mm cubes + 0.27%

Solid cubes + 0.29%

Thin-walled cubes + 0.10%

Thick-walled cubes + 0.31%

Tab 3.3 Relative percentage discrepancies for the various grouped cubes.

These figures demonstrate slight variations in the physical discrepancy between virtual model and printed object when considered in discrete groups, with the thin-walled cubes exhibiting the smallest magnitude departure and the greatest difference being observed with the thick-walled cubes. However, these results must again be considered within the context of a small sample size and crude measurement technique. Notwithstanding these limitations, even the cube group showing the largest deviation from intended dimensions (thick-walled cubes) displayed a mean percentage difference that remained small (+ 0.31%).

3.2.5 Curing shrinkage

Whilst an additional period of “post-curing” is said not to be strictly necessary with Formlabs standard resins (i.e. the clear, white and grey resins used in this project), it is recommended for optimum long-term physical properties. For this reason, a curing chamber was constructed to facilitate this process during the initial phase of the project. A sturdy 30 cm cubed black cardboard box with a removable lid was lined with aluminium foil to provide a reflective surface on all internal faces. An aperture was cut into the lid, through which the output from a 405 nm LED light source could be directed. (This wavelength of light matches the laser output of the Formlabs printer, and achieves optimum polymerisation and cross-linking of any remaining uninitiated monomers

51 remaining within the model after printing.) A solar-powered turntable situated in the middle of the floor of the box provided constant rotation of models placed inside the illuminated chamber, to provide even exposure during curing. As some further shrinkage of the model might be expected with additional post-curing, the test cubes were all measured both before and after this supplementary curing stage. The mean measurement for the 20 mm cubes was 20.037 mm pre-cure and 20.044 mm post-cure (+ 0.03% difference). For the 10 mm cubes the mean difference was 10.017 mm to 10.037 mm (+ 0.2%). Although the sample size in this experiment was relatively small (30 cubes in total, giving a total of 90 x, y and z measurements both before and after curing), an attempt was made to determine whether this difference between pre-curing and post- curing was statistically significant. A two-tailed paired t-test was calculated for the entire dataset of pre-cure and post-cure measurements. This returned a probability of 0.0026, indicating that the observed differences between measurements made before and after curing, although small, are likely to represent genuine alteration in physical dimension due to the curing process. This observation can be borne in mind for any future dimension-critical applications, so that omission of the post-print curing stage can be considered if deemed necessary. More recently, automated washing and curing devices have been developed, which standardise these critical stages of 3D print finishing. Acquisition of these devices in the later stages of the project has therefore now simplified model production to a considerable degree whilst simultaneously improving consistency between prints.

3.2.6 Anatomical models

Having identified in simple terms that there are slight dimensional discrepancies between virtual CAD models and physical 3D printed models, but that the models remained well within the defined 2 mm margin of acceptability, similar experiments were performed to assess the fidelity of anatomical model prints when compared to the original specimen. A human second cervical vertebral body (“C2” or “axis”) was selected for this on account of its relatively small size (to decrease print times and conserve consumables), its anatomical intricacy (to test real-world performance) and its potential forensic importance (frequently injured with fatal consequences, but difficult to access and photograph in the mortuary). The dry bone specimen was scanned with both a clinical CT scanner (Toshiba Aquilion 64) and a micro CT scanner (Nikon Metrology XTH 225)

52 using scanning parameters that have been described previously.57,58 For clarity, the decision to include micro CT scanning at this stage of the project was based upon local availability of a particular device within the University of Leicester and a curiosity to investigate the potential ability of the acquired 3D printing hardware and software to cope with the type of data generated by such a system, rather than an expectation that this would form a significant component of the departmental 3D printing workload as the project developed. Although experimental 3D prints were made using micro CT data at this stage of the project, the exquisite resolution and accuracy of this modality means that any attempt to measure either the electronic images or 3D prints produced using the available apparatus would not be appropriate, nor would any comparison between CT and micro CT results, and so such data is not included here. It is sufficient to report only that 3D printing from micro CT data using the acquired system was possible, which was an unexpected bonus that turned out later to be highly useful (see chapter 6).

Virtual models were extracted from both data sets (using OsiriX software) and printed using the same resin (clear) and at the same print resolution (0.1 mm layer height). Digital calliper measurements were again made of the original bone and the 3D printed models, and these measurements were compared with electronic measurements carried out using the same OsiriX DICOM viewing software. With both calliper measurement of physical objects and linear measurement within electronic files there exists potential for observer- introduced error due to the subjective action of selection of points to measure between, in addition to the limitations of the measurement devices used. Repeating “the same” measurements is extremely unlikely to have yielded identical results, and so the following figures are considered approximations only for the purposes of broad comparison and general consideration. As discussed above, “gold standard” measurement and comparison facilities were beyond the scope of this project and were not the aim of this series of initial test prints.

Fig 3.2 Original human C2 vertebral body bone specimen (left) compared with 3D printed models from the CT scan (right) and micro CT scan (middle) datasets. These models were printed in clear resin at a layer height of 0.1 mm. The background gridlines represent 1 cm intervals.

Fig 3.3 Digital calliper measurement of original bone specimen (left) and 3D printed clear resin model from CT scan (right). This measurement is being made in the supero- inferior (SI) anatomical plane of the bone.

Fig 3.4 OsiriX software electronic “calliper” measurement. This demonstrates an equivalent measurement to the one being performed in the previous image (i.e. the SI plane) using the CT scan data that generated the 3D printed model shown above. The measurement in this case reads 37.0 mm.

Measurements were made in standard orthogonal anatomical planes, namely: antero- posterior (AP – front to back), medio-lateral (ML – side to side) and supero-inferior (SI – top to bottom).

Plane Bone CT scan CT print

SI 37.17 mm 37.0 mm 37.05 mm

ML 50.55 mm 50.0 mm 50.53 mm

AP 47.44 mm 46.5 mm 47.95 mm

Tab 3.4 Physical (bone and print) and electronic (scan) measurements in the anatomical planes indicated.

Considering the original bone specimen dimensions to be the most “correct”, within the limitations of the measurement techniques, the following discrepancies can be seen:

Plane Imaging Printed model

SI - 0.17 mm - 0.12 mm

- 0.46% - 0.32%

ML - 0.55 mm - 0.02 mm

- 1.09% - 0.04%

AP - 0.94 mm - 0.51 mm

- 1.98% - 1.08%

Tab 3.5 Actual and percentage dimensional discrepancy when compared with original bone specimen in in each anatomical plane.

Object Max discrepancy Within 2 mm limit?

20 mm solid grey cube 0.06 mm Yes

20 mm thick-walled grey cube 0.03 mm Yes

20 mm thin-walled grey cube 0.06 mm Yes

20 mm solid white cube 0.11 mm Yes

20 mm thick-walled white cube 0.1 mm Yes

20 mm thin-walled white cube 0.08 mm Yes

20 mm solid clear (100) cube 0.07 mm Yes

20 mm thick-walled clear (100) cube 0.1 mm Yes

20 mm thin-walled clear (100) cube 0.07 mm Yes

20 mm solid clear (50) cube 0.22 mm Yes

20 mm thick-walled clear (50) cube 0.14 mm Yes

20 mm thin-walled clear (50) cube 0.12 mm Yes

20 mm solid clear (25) cube 0.15 mm Yes

20 mm thick-walled clear (25) cube 0.12 mm Yes

20 mm thin-walled clear (25) cube 0.08 mm Yes

10 mm solid grey cube 0.05 mm Yes

10 mm thick-walled grey cube 0.04 mm Yes

10 mm thin-walled grey cube 0.11 mm Yes

10 mm solid white cube 0.04 mm Yes

10 mm thick-walled white cube 0.08 mm Yes

10 mm thin-walled white cube 0.03 mm Yes

10 mm solid clear (100) cube 0.05 mm Yes

10 mm thick-walled clear (100) cube 0.08 mm Yes

10 mm thin-walled clear (100) cube 0.05 mm Yes

10 mm solid clear (50) cube 0.13 mm Yes

10 mm thick-walled clear (50) cube 0.14 mm Yes

10 mm thin-walled clear (50) cube 0.06 mm Yes

10 mm solid clear (25) cube 0.12 mm Yes

10 mm thick-walled clear (25) cube 0.15 mm Yes

10 mm thin-walled clear (25) cube 0.1 mm Yes

C2 bone print (CT) 0.51 mm Yes

C2 bone print (micro-CT) 0.58 mm Yes

Tab 3.6 Summary of all objects measured. The number of objects is small (n=32), but each was measured in three planes, and the cubes were all measured both pre- and post- curing, so the total number of measurements was higher (n=186).

Table 3.6 shows that the greatest measured discrepancies were for the anatomical bone specimens, rather than any of the cubes, but that these were still well below the 2 mm chosen limit of acceptability. The greater discrepancy with the more complex anatomical model when compared with the simpler geometric shape of the cubes is likely a reflection of the subjectivity and variability inherent with the mechanical callipers used. As discussed previously, a margin of error will have been introduced at the measurement stage, whether using physical callipers with the bone specimen and 3D print or digital

57 measurement tools with scan viewing software, but this alone may not account for the entirety of the measurement differences. It must be remembered that the scan images created are not primary images, but are reconstructions based upon mathematical manipulations of the raw x-ray detector data.59-62 These reconstructions can be biased towards soft tissue contrast, edge detection and so on depending on the clinical requirement. As such, the final dimensions of the image may not exactly recapitulate the original specimen. Such minute variations are considered negligible for diagnostic and therapeutic clinical purposes. The cumulative error arising as a result of the multiple steps of scanning, 3D surface extraction and 3D printing can be seen to be of the order of fractions of a millimetre. Bearing in mind the likely applications proposed (i.e. demonstration of anatomical structures and relationships to aid comprehension in a lay audience, rather than precise microscopic measurement), this margin of error is unlikely to be sufficient to result in major objections being raised on the grounds of dimensional accuracy. The accuracy of the test prints demonstrated using these rudimentary measuring techniques is at least equivalent, if not superior, to measurement discrepancies already accepted in the discipline of forensic anthropology.51 However, it is not merely metric accuracy that is of importance, but also surface detail.63 By conducting this series of rudimentary exercises, much was learned about subtle variations in printing characteristics, and this information could subsequently be used to inform decisions about the workflow process in order to achieve the desired end result.

3.3 Process refinement

Once initial testing and calibration had been completed, and successful production of an anatomical model from CT scan data had been achieved, efforts were made to optimise the workflow process to reduce time and consumables expenditure where possible without sacrificing quality. Whilst being of direct benefit to the project, this was also an attempt to increase potential viability of any service that may be offered to police and other service users in the future.

3.3.1 Model extraction parameters

3D anatomical model extraction from medical scan data generates complex digital files (often consisting of several million vertices), that consequently can be slow to transfer, open and manipulate using standard computer hardware. The size and associated

58 processing and storage burdens are directly affected by the parameters used to generate the 3D model, with reductions in eventual file size being associated with noticeable decline in detail. The physical process of creating and finishing the 3D print inevitably leads to some loss of fidelity from the original file’s virtual surface structure. With this in mind, trials were conducted to assess how much detail could be omitted from the virtual model before any loss of quality was discernible in the final print. The same anatomical structure was therefore duplicated multiple times, but printed with stepwise alterations of a single variable incorporated across the duplications. After comparing the appearance of physical prints with the appearance of the original virtual models, the effect of variation of each adjustable parameter could be assessed. Although a highly subjective assessment, it was found that the resolution (i.e. surface detail) of the model could be reduced during creation by as much as 50% of the maximum theoretically achievable before degradation of the final print became easily noticeable. In addition to resolution, a process known as “decimation” can be applied to models to reduce the number of triangles in the mesh (see discussion of STL files in chapter 2). This process systematically replaces groups of adjacent small triangles with larger triangles having approximately matching geometry, thereby reducing the total number of triangles (i.e. overall file size) whilst largely maintaining the original shape of the model. Varying the decimation ratio has a clear effect on the virtual model, but this effect is not as pronounced in the finished print, as illustrated in figure 3.5. When producing anatomical models during this project, the default strategy was to achieve the finest possible detail during virtual model creation. Having performed these model extraction parameter trials, confirmation has been obtained that if necessary in future, excessively large file sizes could be safely reduced to a considerable degree without detrimental loss of subjective finished print quality.

Fig 3.5 Decimation trial. The digital file (with green highlights outlining the vertex mesh) shows marked variation with increasing degrees of decimation. Direct comparison with the physical model shows that the virtual differences are not as apparent after printing. The additional detail (and therefore increased file size burden) of an extremely detailed mesh is therefore not strictly necessary for an acceptable print. Further iterations of this and other test parameters allowed determination of optimal settings for a particular process and hardware setup.

3.3.2 Resin conservation

The photoreactive resin used in the printing process has a finite cost per millilitre, and so a method of reducing any wasted volume was sought. An immediately apparent strategy

60 was to print models as shell-like structures containing hollow spaces, rather than leaving them solid. The manufacturer of the Form 2 printer specifically advises against printing hollow structures, as print failures can be caused by air pockets forming during printing, and uncured resin can become trapped inside cavities. Given the potential benefits of reduced resin consumption and faster printing times, cautious experiments were undertaken to assess the viability of hollow object printing. Whilst the hollowing of virtual models is relatively straightforward for simple objects (such as a cube or sphere), complex anatomical forms proved to be much more challenging. However, with perseverance, techniques were established using the Blender software package (described in chapter 2) that have enabled considerable gains in resin usage economy whilst still achieving a successful print.

Fig 3.6 This slice preview image shows a comparison between a solid version (left) and hollow version (right) of the same model, with the blue shaded regions previewing how much resin will be required for the current layer. The hollow version in this case represents a total reduction in resin consumption of almost 60%.

Further reductions in resin expenditure can be made by judicious use of orientation and the other settings determining placement of any support structures. Unless using a powder-based printing technology, where the unfused powder fills voids acts to support the remainder of the model during printing (see chapter 2), placement of temporary scaffolding structures is essential when attempting to print the complex geometry of anatomical models. These technologies are not currently available at the more affordable, “desktop” end of the device spectrum, and so the vast majority of examples printed during

61 this project required the use of supports. Removal of temporary supports after printing often results in at least some degradation of the surface finish at points of attachment. As discussed previously, support structure placement is tailored to each print depending on the desired end result, and the precise orientation of the model during printing is also critical in determining not only the quality of the finished print, but also whether the print will be successful or a technical failure. Automated support placement is a feature of most slicing software applications, but manual adjustments are often required. “Logical” placement of supports does not always produce the “best” result, and this stage of the process involves a degree of subjective judgement, relying heavily on knowledge gained from previous successes and failures. Optimisation of supports for printing reliability can result in excessive resin usage (and therefore wastage). Conversely, resin can be saved by conservative use of support structures at the potential expense of print quality.

3.4 Moving forwards

Following preliminary subject matter research, and the acquisition of equipment and familiarity with the necessary software, the theoretical concept of producing 3D printed models “in house” had been achieved. By this stage of the project, sufficient experience of the basic 3D printing process had been acquired to enable models to be printed with a high degree of predictability (in terms of time taken and consumable usage). Technical print failures were all but non-existent, primarily due to the robust Formlabs hardware chosen, but also due in part to an understanding of the causes of print failure and an avoidance of these issues during the print setup (slicing) stage. The 3D printed models produced were accepted to meet the project’s modest accuracy target of less than 2 mm dimensional discrepancy from original, and a standard workflow process (as illustrated in figure 2.2) had become established. Following successful completion of these first steps, specific experiments could now be undertaken to determine whether this technology might usefully be applied to real forensic pathology casework and associated research.

Chapter 4 – Taking shape

4.1 First steps

This chapter outlines the transition from initial testing, calibration and process refinement to production of the first anatomical models. Starting simply, and evolving in complexity, these models enabled the acquisition of techniques considered necessary for perceived likely forensic casework applications. A more detailed appraisal of predicted potential applications is considered in chapter 5.

4.2 Basic anatomical models

During the testing phase described in chapter 3, an anatomical model of a single, small bone was created. This was of limited scale and complexity, and further expansion of the production process was now needed to ensure that models could be produced that would be of genuine benefit in forensic cases. As bone had already been successfully printed, this was the first tissue type selected for more advanced experimentation.

4.2.1 Bone

During early review of the available literature, as well as discussion amongst colleagues and prospective future police service users, the skull was considered to be the most likely anatomical structure to benefit from 3D printing for court trial purposes. Although large and complex, this was nevertheless focused upon as an early goal. Using knowledge and experience gained from the initial testing phase, a complete skull was extracted from the research CT scan imaging dataset.

As described already in chapter 2, segmentation is the standard term given to the process of separating structures of interest from the rest of a dataset. This term is not restricted to the extraction of anatomical structures from medical scan data, and an engineer delineating a small component part from a scanned motor vehicle engine, for example, would be equally familiar with the term.64 Multiple strategies exist for segmentation, and these may be either automated or manual. The manual option could be considered the more accurate, although it is laborious and not immune from operator error due to the subjective nature of some elements of the selection process.65 Essentially, the user will scroll through the dataset, slice by slice, highlighting areas of interest directly using a

63 range of drawing tools available in the software package. Given the labour-intensive and time-consuming nature of such a system, automated methods are generally preferred wherever possible.

Automated segmentation relies on differences in intensity values of adjacent pixels in order for them to be separated from one another using software algorithms. For example, bone is very dense (i.e. white on an x-ray image), and therefore easily separated from surrounding muscle (grey) or air (black). The cut-off level (“threshold”) can be adjusted to increase or decrease the inclusion of surrounding tissue until all of the required anatomical structure is selected. Once the user is satisfied with the anatomical separation achieved by a particular set of thresholding parameters on a single slice, these values can be used to propagate (“grow”) the segmentation though the adjacent dataset slices so that the entire 3D anatomical structure is highlighted. Fine-tuning of segmented areas can be carried out using the manual drawing tools described above, so that unwanted structures can be erased, or holes closed, for example. After segmentation, the separated anatomical structures can then be exported as a 3D shape for subsequent editing and printing.

Fig 4.1 The “segmentation” process. On the left is a single axial slice from the CT scan dataset. Using the segmentation tools most appropriate to the intended application, parameters are adjusted until the region of interest (in this case bone) is highlighted satisfactorily (green areas of middle image). After propagating (“growing”) this region through adjacent slices, the highlighted region can be retained whilst all other areas (e.g. soft tissues) are deleted. This leaves behind only the structures of interest (right hand image), which can then be exported as a 3D object for further processing.

An alternative method of defining anatomical regions of interest is to use the surface rendering module of the OsiriX software. For this method, the user determines the pixel

64 density of the material of interest (e.g. bone), and the software implements a “marching cubes” algorithm37 to progress through the slices of the dataset, joining adjacent areas of matching density from one slice to the next and generating a so-called “iso-surface” (similar to isobars, or contour lines on a map). The resulting surface can be exported as a 3D object and edited or printed in exactly the same manner as the segmented model discussed previously. The surface rendering approach is quick, convenient and produces smooth results, although it offers less control over the anatomical structures included and so is not suitable for every application.

Fig 4.2 The surface rendering method of producing a 3D model. By joining together pixels of matching values in adjacent slices, a 3D “contour map” (called an iso-surface) can quickly be computed throughout the dataset. The pixel density value at which this iso-surface is created can be adjusted as required, for example to produce a model of bone (as in this skull example) or skin, etc.

Fig 4.3 Although this is the same bone from the same CT scan dataset, the region of interest segmentation method (left) contains visible steps between the scan slices, whereas the surface rendering method (right) has produced a smoother overall surface. Both the method chosen to extract a 3D model from the CT data and the variables selected for each of the extraction parameters can result in subtle differences to the final model, and so it is worth experimenting in individual cases to determine the optimum result for the required application.

Fig 4.4 After extraction from the CT scan dataset, the 3D shape of the entire STL model can be appreciated. “Zooming in” reveals that the apparently smooth, contoured surfaces are in fact composed of multiple triangular facets (inset).

Fig 4.5 Despite careful cropping of the CT scan dataset prior to extraction, the STL file will usually contain small items of loose debris (highlighted yellow, left). In addition, areas of thin bone may not be dense enough to register as “solid”, leaving holes and unconnected bone fragments (yellow zone, right). Strategies for overcoming these artefacts and achieving successful prints have been developed, and are discussed in further detail in chapter 8.

Fig 4.6 Finished large and small prints, testing the maximum size achievable within the printer’s build volume (in this case 75% life-size, left) and also the ability to reproduce fine detail (adjacent to £1 coin for size comparison, right).

The vast majority of model segmentation operations carried out during the course of this project were accomplished using the aforementioned OsiriX software package. The

67 reason this software was used as the default choice was because the pathologists in the department had already gained significant experience using this software whilst analysing CT scans in everyday forensic practice. This medical imaging program certainly can be used for the purpose of 3D model creation, but this is not its intended primary purpose, and it would not be considered suitable for serious industrial applications where utmost accuracy is required. More advanced segmentation software does exist, and engineering and metrology applications are best served by dedicated software such as VGStudio Max or Avizo. However, such powerful analysis comes at a financial cost: by way of an example, a recent subscription rate was $4,200 for the “Lite” version of Avizo and $10,000 for the “Full” version. Software of this calibre that is aimed more at medical or life sciences applications also exists, for example Amira ($4,000 licence plus $800 per year subscription) or Materialise Mimics ($6,900 for the basic package with $5,175 for the medical-specific functionality and an annual subscription rate of $900). These software costs are justifiable in industrial or research applications that necessitate use of the best available data capture and processing systems. The scope of this project (i.e. occasional production of anatomical models from clinical scan datasets using office- based 3D printing hardware) did not justify the cost of superior software acquisition when the existing option was already sufficient for the intended purpose. OsiriX itself has an annual subscription cost of £64, and so is not entirely cost-free. For this reason, limited experimentation was carried out to identify potential costless alternatives, in order that methods described could be adopted by other practitioners elsewhere without access to the OsiriX platform. The open-source program InVesalius was trialled at first, and found to be easy to use, although somewhat limited in overall function. Instead, 3D Slicer was adopted as the open-source alternative to OsiriX for trials of segmentation methods, and this was found to be highly capable although with a less intuitive user interface that required more time and perseverance to achieve the desired results.

Having discussed the need for a suitable software package, it is also necessary to consider the required computer hardware specification to enable 3D model creation and manipulation. As discussed in chapter 3, the manipulation of large, complex 3D files is highly memory and processor-intensive. Standard laptop or even desktop computer equipment may not be capable of loading such files, or processing them in a way which is tolerable within a realistic work environment. Of particular importance are available memory (both in terms of hard drive capacity for storage and random access memory,

RAM, for live display and manipulation of files) and processor speed. As an example, the operating requirements for OsiriX state that a minimum of 6 GB of RAM is necessary to run the software, but in practice a much higher capacity than this is beneficial if the computer is to continue to run smoothly whilst working with large files and completing complex tasks. Multiple core processors allow simultaneous operations to take place, dramatically speeding up individual processor tasks as well as allowing more than one task to run smoothly at the same time. Standard graphics processors (graphics “cards”) that are not specified to handle large 3D files may mean that the computer system struggles to process and display the models as expected. The display itself is important, as a larger monitor with a higher resolution will enable a greater proportion of the detail of the model to be discernible on the screen at a particular instant, easing viewing and making editing simpler to complete.

Fortunately, the Apple Mac Pro computers already in use within the department at the commencement of this project contained 3.7 GHz quad-core processors supported by dual graphics processors (each with 2 GB RAM), and had been expanded to 32 GB RAM and 1 TB of internal storage, so were able to cope with the vast majority of data handling requirements contained within this project. The computers were linked to dual 27-inch 5K Retina displays, each having screen resolutions of 5120 x 2880 pixels, enhancing viewing and editing operations over more conventionally-sized display systems. These systems had been set up with the objective of CT scan interpretation only, and not specifically for 3D model creation and manipulation. Given that 3D capability is now considered an ongoing, albeit minor, component of the work carried out by the department, future computer system acquisition will have to consider the additional benefits of uprated components over and above “standard” specification, in order to keep pace with potential future 3D work requirements.

Regardless of the precise hardware and software available to the user, the greater the contrast between anatomical structures and their surrounding tissues, the easier they will be to segment from one another. This is equally true regardless of whether manual or automated methods are used, and is a reflection of the fact that it is more straightforward to discern (either by eye or by algorithm) where one region ends and the next begins if there is a well-defined border between the two. This is why bone is usually relatively easy to separate from surrounding soft tissues, but why individual muscles lying adjacent to one another are extremely difficult to separate during segmentation processes. Having

69 stated that bone is usually easy to separate from surrounding soft tissue, it must be remembered that not all bone is of equal density. Thin parts of the skull, for example, are sufficiently thin that their mathematically calculated x-ray density value overlaps with particularly dense soft tissue structures. Segmentation threshold values that include all bone within the skull will therefore include much of the attached soft tissue in the final 3D volume. Conversely, resetting the threshold values to ignore all soft tissue will inevitably leave holes in the skull where the bone is less dense than the deleted soft tissues. Segmentation procedures therefore require some compromises to be made, and ultimately this will be influenced by the desired outcome: some fine elements may have to be sacrificed for a “cleaner” model, or a degree of “noise” might need to be tolerated if all feature details are to be included.

During this phase of the project, several different skull models were produced. Familiarity with the steps necessary for segmentation of anatomical data and subsequent file editing to ensure printability, as well as the orientation, support and slicing stages, was acquired through production of these skull prints. As a result, requests by police forces to assist in court cases can now be met in a timely fashion, and with a high degree of confidence of successful outcome prior to printing. Further details of specific case examples will follow in chapter 6.

4.2.2 Skin

Following on from bone, the surface of the skin was considered the next most important anatomical structure to investigate, given the potential forensic significance of skin wounds. The stark density interface between skin and surrounding air means that the process of segmentation to extract a 3D skin surface should theoretically be straightforward. However, where the skin contacts clothing, or lies against the surface of the scan table, the skin-air interface is obscured. The level of detail discernible therefore varies tremendously, not only with the nature of the wound, but also with its site and associated features as well as its location and orientation at the time of CT scanning. Consequently, not all skin injuries that are apparent to the naked eye are demonstrable in the CT scan images, and the surface detail subsequently reproduced in extracted 3D surfaces is not always a useful representation of the pathological appearance or extent of the injury. An obvious stab wound to the back may be all but invisible on a 3D surface of the back of the body extracted from a CT scan dataset. Despite these potential

70 limitations, the principle and process of 3D printing skin and relevant skin defects was confirmed during the project (illustrated in figure 4.7), and also now exists as a possible service to offer in cases where such a model might be useful. An example of a case where the skin’s surface was included incidentally in a 3D printed model for a police force outside the East Midlands region is detailed later in chapter 6.

Fig 4.7 On the left are the CT scan appearances of a stab wound in the sagittal (top) and axial (bottom) planes. Whilst these show the skin defect caused by the knife as well as penetration of the sternum (breast bone), the images may not be immediately clear to a non-medical audience, especially as an emergency medical dressing can be seen partially covering the wound. In the centre is a virtual rendering of the skin (with medical dressing removed), cropped to exclude everything outside the field of immediate interest. The skin defect is readily appreciable, and the underlying cut through bone (rendered in a different colour) can also be seen. On the right is the 3D printed model, which can be picked up, passed around a group of observers (such as a jury) and viewed from any angle. The back of the model has been selected to coincide with the passage of the knife blade through solid bone, so that the reverse face of the model (inset) reveals the profile characteristics and dimensions of the causative blade, in actual size.

4.2.3 Blood vessels

Whilst not initially predicted to be of particular forensic value in terms of 3D printing for court trial purposes, blood vessels have intricate anatomical features that may be difficult to comprehend in conventional two-dimensional formats for the non-medical audience. The potential exists for the production of useful educational models, either to demonstrate normal anatomy or to reveal pathological features. Experiments were therefore conducted to confirm that vascular structures could indeed be successfully extracted and

71 printed. In clinical practice, a radio-opaque contrast medium is injected into blood vessels to delineate the vascular anatomy, and a similar process has been adopted in forensic practice in the form of post-mortem CT angiography (PMCTA).66-68 When this imaging investigation has been performed, the acquired scan image data contains very well demarcated vascular structures that can be segmented with relative ease, allowing surrounding structures to be excluded and the subsequent 3D printing of the vascular anatomy in isolation.

Fig 4.8 3D printed right coronary artery model (left), with corresponding post-mortem CT angiogram as viewed in medical imaging software (OsiriX, right).

4.3 More advanced models

As forensic cases requiring 3D printed models for court will involve items other than intact, normal anatomy, strategies had to be developed to overcome the pathological lesions likely to encountered.

4.3.1 Fractures

Having considered the skull most likely to present forensic opportunities for 3D printing, the ability to recreate skulls with fractures was deemed essential both to the project and any future 3D printing service. To begin with, relatively uncomplicated linear fractures were investigated, followed by increasingly complex fracture patterns involving depressed, comminuted and completely separated fragments. The initial stages of isolating and extracting the 3D anatomy had, by this phase of the project, become routine and largely successful. The secondary step of editing the STL mesh (see chapter 2)

72 rapidly expanded during these exercises, as greater complexity required extended periods of digital manipulation in order to remove artefacts and correct anomalies, enabling the far more complicated models to be successfully printed.

The two most significant hurdles proved to be developing methods of displaying internal fracture features hidden by the outside of the skull, and printing separated fracture fragments in such a way that their spatial relationship with the rest of the skull could be retained. A relatively simple solution to the first problem involved the cutting of an appropriately sized and shaped “window” in the virtual model prior to printing (which could either be left absent from the final model altogether, or printed separately to incorporate a removable section). The second issue could be resolved by creating artificial connections between the individual fragments in the virtual model prior to printing. The entire model can be printed as a single item, with connections present at the time of printing, or printed as a series of sub-units that could be assembled later using suitable adhesive to construct the finished model (illustrated in figures 4.9 and 4.10). The latter option enables the connecting rods to be printed using a contrasting colour to emphasise their non-organic nature. The various software techniques necessary for these steps will be discussed further below, in the section on multi-part models. Examples of skull fracture cases successfully printed for court purposes will be detailed in chapter 6.

Fig 4.9 A neck fracture is visible in the original CT scan imaging (top left), but the precise three-dimensional relationships may be difficult to discern for anyone unfamiliar with the interpretation of such images. Even a virtual 3D reproduction carried out by standard medical imaging software (OsiriX, top right) may not be entirely clear due to surrounding anatomy and other artefacts. The 3D printed model (bottom, left and right) displays the fractured bones in isolation and in a form that can be handled and manipulated at will to facilitate easier comprehension. Successful printing of this model required a number of discrete elements to be printed separately and then subsequently assembled.

Fig 4.10 On the left is an exploded view showing the individual components printed for this single example. An intervertebral disc and a supporting rod (both in grey) had to be fabricated to join parts together where direct contact did not already exist. Corresponding peg and hole additions were fashioned in areas of direct contact (upper inset) to ensure accurate orientation of parts and to strengthen adhesive joins. The grey rod maintains the spatial relationship between separated fracture fragments at the time of scanning (lower inset). Virtual previews of the model to be constructed are shown on the right side (compare with actual 3D print in figure 4.9).

4.3.2 Multi-part models

All 3D printers have a maximum total build volume, which places a limitation upon the largest structure that can be printed in a single build. For desktop printers the maximum volume is modest when compared to industrial equipment. As an example, the Formlabs Form 2 device used in this project has a build volume measuring 145 mm x 145 mm x 175 mm in total. Additional space taken up by mandatory support structures can further reduce the ability of large objects to fit within the permissible build space. To facilitate construction of models larger than the total build volume of the printer, another practical procedure to be tested early on in the project was the ability to split models into multiple parts that can be printed separately and then joined together afterwards.

The simplest solution was to bisect the anatomical object within the Blender software package, and then add locator pegs and corresponding holes on opposite faces of the join lines. After printing, the pegs can be slotted into their respective holes, unifying the model.

Fig 4.11 This 50% scale human adult female skull model was joined using a set of corresponding pegs and holes. The background gridlines represent 1 cm intervals. (In this image the join line has been parted slightly to allow better visualisation of the pegs.)

Evidently there needs to be a slight diameter differential between the peg and its corresponding hole, otherwise it would not be possible for one to accommodate the other. However, too great a difference in size, and pegs would not be gripped sufficiently to hold the parts together. As discussed in chapter 3, there are minor discrepancies between the dimensions of the virtual model and the final printed part. In order to establish the tolerances of the physical dimensions of these pegs and holes, tests were conducted to determine the range of acceptable values, and to attempt to identify an optimum value.

Fig 4.12 Initially, paired test cubes were designed to determine crude percentages of size difference that might be appropriate. Standard peg and hole of diameters of 5 mm were arbitrarily chosen, and percentage increments above and below this were trialled. The above image shows two cubes at the software design stage, prior to printing.

Fig 4.13 Fitting the printed cubes together to determine the crude dimensional discrepancy that provided a friction-tight fit.

The above cube trial included the following peg and hole options:

Pegs Holes

+ 2.5 % - 2.5 %

+ 1 % - 1 %

5 mm diameter 5 mm diameter

- 1 % + 1 %

- 2.5 % + 2.5 %

- 5 % + 5 %

Tab 4.1 Peg and hole options with initial crude cube trial.

The faces of the cubes were offered up to one another sequentially, testing in turn each possible combination of peg and hole interaction. When there was no discrepancy (i.e. a 5 mm peg and a 5 mm hole), no fit was possible at all. With a 1 % size discrepancy, the fit was too tight to allow complete insertion of peg into corresponding hole, and separation was equally difficult. A 2.5 % size discrepancy allowed complete insertion with a resulting friction-tight fit that allowed subsequent separation, but only with significant force application. A 5% size discrepancy was found to provide too loose a fit, offering barely any resistance to insertion and allowing the cubes to separate under the influence of gravity alone. Once the desired tightness of fit had been crudely ascertained to lie within the + 2.5 % to + 5 % range, a further experiment was conducted to refine the point at which the specified size discrepancy provided optimal results for the intended purpose of allowing multi-part models to be joined together and held by friction alone.

Fig 4.14 A similar procedure was then adopted to fine-tune the precise point at which friction grip became sufficient, but not excessive. Above is the virtual test bar prior to printing, with labelled holes at a range of incremental size increases from the standard 5 mm diameter peg.

Fig 4.15 The printed testing device, complete with marker post bearing a standard 5 mm diameter peg. This second assembly provided further detail about the optimum size differential for friction-fit purposes.

Using the above apparatus, the following trial results were obtained:

Size discrepancy Trial finding

+ 2 % Would not fit together

+ 2.5 % Very tight

+ 3 % Tight

+ 3.5 % Ideal

+ 4 % Ideal

+ 4.5 % Ideal

+ 5 % Loose

+ 5.5 % Too loose

+ 6 % Too loose

Tab 4.2 This slightly more refined sequence of trials revealed that a size discrepancy of around 4 % between the designed peg and hole dimensions provided a good balance between ease of insertion and subsequent holding friction.

The “peg and hole” approach proved adequate if models were to be permanently fused using cyanoacrylate (or similar) strong adhesive. However, as the option of repeated opening and closing of the join for inspection of internal details was considered a desirable feature, this method was found in practice to be unsatisfactory: repeated joining and separation actions caused sufficient surface interface wear that eventually the parts no longer held together tightly by friction alone. An alternative method was therefore developed whereby recesses were added (at the software stage) into the opposing flat surfaces of the model parts to be joined. After printing, small neodymium magnets can then be inserted into the recesses (which are designed to leave a slight empty space directly above the uppermost surface of the magnet). The empty space above each magnet is subsequently filled with a small volume of liquid resin, which is then photo- cured by light of an appropriate wavelength to seal the magnet permanently into position.

This method provided a favourable long-term solution, and is highly flexible on account of the availability of a multitude of different sizes, shapes and strengths of magnet. For critical applications, the strength of magnet required can be calculated in advance to ensure sufficient “pull” strength to support the mass of cured resin that will be present in the final printed part. Caution must be exercised in ensuring correct polarity of the magnets at the insertion stage, to ensure that model parts cling to rather than repel one another.

Fig 4.16 Here a precisely-sized recess is being engineered into an adjoining face, ready to accept a magnet after printing. For this 60% scale adult female human skull model, the magnets used had a diameter of 3 mm and a thickness of 1.5 mm. A hole diameter of 3.5 mm permitted flush-fitting of the magnets, and a depth of 2.5 mm allowed the upper surface of each magnet to be placed well below the surrounding surface in order to accommodate a layer of resin sealant.

Fig 4.17 Magnetic multi-part model skull. On the left, the intact skull showing the barely- perceptible join line. On the right, the model has been separated to reveal the internal features. Shadows can be seen at the sites of magnet inclusion (arrow). A closer view of one magnet embedded in cured resin at the face of the joint line is shown (inset).

4.3.3 Micro CT

The discussion so far has focused on producing models from clinical CT scanner hardware, as this is the technology that is most frequently used during forensic post- mortem examinations. Occasionally, cases are encountered where samples of bone are removed for more detailed examination, including micro CT scanning. The level of detail available from micro CT scan data is an order of magnitude greater than clinical CT scanning,69 but the requirement to remove bones from the body and transport them to a different location means that this is not a practical option that is justifiable in the vast majority of cases. It is also beneficial to “deflesh” the bones (i.e. remove the adherent soft tissues) prior to scanning, and so the additional processing and logistical burdens are most often considered to be worthwhile in cases such as criminal dismemberment, where analysis of so-called “tool marks” left by the dismembering implement can yield useful information for the police investigation. Preparation for this eventuality was part of the reasoning behind attempting to print models using micro CT data during the initial testing phase described in chapter 3. The exquisite detail contained within micro CT datasets is associated with a proportionate increase in file size. Although the printing process remains the same, regardless of the source data, the length of time required to prepare files for printing is greatly increased with micro CT scan cases if using standard computer

82 systems. Micro CT scanning facilities incorporate powerful computer hardware, vast storage systems and industry-standard software into their workflow processes, and are capable of routinely handling the complex data generated. However, the less capable computer systems found in forensic pathology and other office-based departments are likely to suffer noticeable time lags when opening and saving the large files, for example, and the individual steps taken during editing will also be far slower to complete. Valuable experience in managing these complex datasets on standard computer hardware was gained during this phase of the project, which ultimately proved worthwhile once police requests were later received for 3D printed models to be created following cases of criminal dismemberment (see chapter 6). The following images are intended to provide a visual example of the increased detail provided by micro CT scanning. However, even these images were created using markedly reduced STLs prepared from the scan data, and the true precision of the actual micro CT scan is even greater still.

Fig 4.18 This image provides a visual indication of the difference between clinical CT and micro CT scanning. The same piece of bone has been imaged both with clinical CT scanning (left) and micro CT scanning (right). The enhanced fine detail revealed with micro CT is readily apparent, in this case making assessment of saw mark characteristics a possibility. 3D printed versions of these and associated bone specimens are illustrated in chapter 6.

Fig 4.19 Equivalent magnification of the STL mesh from the same region of each of the virtual models featured in figure 4.18, revealing how the increased number (and corresponding decreased size) of the individual triangles allows far greater detail to be included in the micro CT scan file (bottom) when compared to the clinical CT scan version (top). This also illustrates why the digital files associated with micro CT scans require considerably greater memory and computer processing capacity than those of clinical scans. (It should be noted that the true level of detail contained within the original micro CT scan data is far greater still than the heavily-reduced STL depicted here.)

Chapter 5 – Potential uses for techniques being developed

5.1 Initial ideas

The precise benefits to individual forensic practitioners over the long term cannot be predicted with certainty, but it is clear that as this emerging technology becomes established,18 the likelihood is that previously unforeseen applications will continue to be identified when addressing specific challenges and opportunities that arise through ongoing casework. The following brief outline is intended to provide indications of the ideas that have either been trialled already, or that are immediately obvious avenues for further investigation. This should not be interpreted as a comprehensive list of what is possible, but as examples of different initial starting points. Before identifying specific case types, it is worth highlighting particular characteristics of 3D printers and considering why these factors render the technology so suitable for use in forensic pathology casework. The following table summarises the main perceived advantages:

Characteristic Advantage

Tactile output Models can be handled and inspected intuitively, enhancing understanding of complex 3D features and relationships.

Reproducibility Multiple copies of a file can be printed to allow simultaneous examination of the same object.

Non-destructive The original artefact can be kept safe whilst a faithful reproduction is created for potentially damaging handling.

Scale Small items can be scaled up to demonstrate fine detail, and large items scaled down to make objects more manageable.

Visibility Transparent windows or removable parts can be incorporated to enhance visualisation and understanding.

Versatility Anatomical shapes that would simply not be achievable using traditional manufacturing techniques can be produced easily.

Tab 5.1 Summary of 3D printing characteristics and relevant advantages in forensic casework.

5.2 Skeletal trauma

The identification and visualisation of bony trauma, which is highly prevalent within forensic pathology practice, is a key strength of CT scanning. Skeletal trauma depiction is one of the main reasons why forensic autopsy practitioners have been so keen to adopt this type of scanning in recent years, and therefore is the most readily-apparent subject for the implementation of 3D printing technology.

5.2.1 Fractures

Whether the injuries of greatest forensic interest in a particular case affect the skull, the cervical spine, pedestrian lower limbs or a paediatric ribcage, 3D prints of the fractures may provide additional demonstrative power or accessibility over standard 2D representations.70 However, as discussed in chapter 4, complex fracture patterns require a strategy for maintaining the spatial relationships of any dissociated fragments. Whether the intention is to re-approximate fragments into their pre-injury anatomical positions, or to display them in their post-injury locations, much can be done at the STL editing stage to ensure that the final print meets expected requirements. For example, bridges of permanent support structure can be added to join fragments that would otherwise become “free-floating” in the absence of supporting soft tissue elements.

Lower limb long bone fractures in pedestrians who are involved in road traffic collisions can provide pathological markers of the direction of impact.71-77 Demonstration of fracture patterns can therefore assist with collision sequence reconstruction, and so interactive 3D models of fractured limbs could potentially be of benefit. It may be that characteristic damage to a bone gives an indication of contact with a particular part of a vehicle, and providing a portable life-size model of that portion of bone would allow confirmatory “fit matching” not possible using the bone itself.

Spinal fractures, especially those of the upper cervical column, are of forensic importance and can be difficult to demonstrate in conventional autopsy photographs or even with CT imaging. Specific models could aid with lay understanding of this potentially confusing anatomical region. In cases of suspicious child death, the pattern of rib fractures can be of critical importance in determining whether inflicted injury has been a factor. Production of a ribcage model with highlighted fractures would introduce new challenges, but if deemed to be of sufficient benefit would not be beyond the bounds of

86 achievability. The contrast between posterior ribcage fractures caused by forceful squeezing, and anterior fractures caused by resuscitation might be more readily appreciated by non-medical jury members holding a ribcage model in their hands, rather than simply listening to a medical expert’s testimony.

5.2.2 Tool mark analysis

In addition to demonstrating fracture patterns, 3D prints containing specific features of injuries left by weapons not only allow the anatomical location and extent of injury to be displayed, but also enable dimensional characteristics to be compared with potential weapons. The option of producing 3D printed models for this task has practical advantages over the more traditional method of macerating the actual injured bones.78 Retrieving weapon characteristics from 3D printed models is likely be restricted to demonstrating simple shape characteristics (e.g. a rounded hammer impact site) and approximate dimensions if the original data has been obtained by standard medical imaging modalities. However, if micro CT scanning facilities are available then an opportunity exists to explore in more detail the so-called “tool marks” caused when weapons interact with the skeleton. The superiority of micro CT resolution over ordinary clinical scanners has already been described,57 but the vast datasets generated by micro CT scanning can be problematic to process in the manner outlined above using standard computer hardware. It is possible to 3D print from micro CT data,17,69,79 and the fine detail available makes the additional effort worthwhile. Very fine tool marks can be used in cases of sharp force bony injury to assist with identification of a knife, or a saw blade in a case of dismemberment.16,57,80,81 If very small details are visible within the digital files, but are not discernible on the 3D printed surface, it is possible to scale the model up to larger-than-life size in order to magnify these details to the extent that even the most subtle characteristics are not obscured by the limitations of the printing process. As an additional advantage, an “inverse cast” of a bone defect can be 3D printed to reveal potential information about the surface of the injuring implement. An example of this concept is illustrated in the form of damage to a Bronze Age shield artefact in chapter 8. Similarly, 3D printed models of a bony defect and the corresponding causative implement could be useful for the demonstration of a physical “fit match”, as such fit-matching has a long history of use within forensic science disciplines.82

5.3 Soft tissues

Bony trauma is not the only type of injury detectable by CT scanning, and soft tissue wounds are also frequently of forensic interest and importance.

5.3.1 Skin wounds

Extraction of soft tissue anatomy from medical imaging data is more challenging than with skeletal elements, as the interface boundaries between tissue planes are far less distinct. Whilst segmentation of individual muscles (for example) is extremely challenging, it is relatively straightforward to create 3D models of the skin’s surface on account of the clearly defined density interface between air and skin. Depending on the morphological features of the injury, and positioning of the body at the time of CT scanning, it may be possible to produce a useful model of a stab wound, or even a stab wound track through soft tissues. Stab wounds into skin may in some cases be fairly easily extracted due to the presence of air in the wound track. Figure 4.7 illustrates an example of a 3D model of a stab wound through both skin and bone. Production of a miniaturised body or partial body could provide jurors with an overview of the locations of multiple stab wounds in a complex case. It is said that a wax effigy of Julius Caesar was displayed to a gathered crowd to demonstrate the locations of his mortal wounds,83 and so it could be argued that using 3D printed models to achieve a similar effect is not a truly novel idea.

5.3.2 Ballistic trajectories

In cases of penetrating ballistic trauma, any precise wound trajectory determined radiologically or by autopsy examination can be added to a 3D model in the form of a linear rod extending outwards from the body at the correct angle. As with the stab wound model discussed above, a miniature human form bearing multiple trajectory markers could be made available to members of the jury to assist with comprehension in a complex ballistics case. The number of wounds, their physical relationship to vital anatomical structures and the angles of wound direction may be more rapidly and completely appreciable in a 3D model that can be handled and viewed from any perspective, than from a series of 2D representations. An example of this type of reconstruction is depicted virtually in chapter 8.

5.3.3 Projectile recovery

In forensic cases it is also customary to retrieve any projectiles remaining within the body for subsequent examination by ballistics experts. If projectiles or fragments are not physically recoverable due to their location or operational constraints (e.g. in the event that no invasive examination has been authorised), they could instead be virtually extracted and 3D printed to provide a non-invasive, non-destructive opportunity to study their characteristics, albeit with limited resolution. This concept has been proved in principle during the course of the project, as demonstrated in figure 5.1.

Fig 5.1 CT scan imaging revealed the presence of an air weapon projectile (air rifle pellet) within the substance of the brain (yellow arrow). A 3D printed model of this projectile (inset) is compared with the actual item recovered during invasive autopsy.

5.4 Identification

The need to confirm identity is a recurring theme within the practice of a forensic pathologist. Tried and tested means of achieving successful identifications have long been established, and it is unlikely that 3D printing is going to add value in a case where DNA or fingerprints are available. In cases where a dental identification is planned, there exists the very real prospect of 3D printing the teeth or jaws from CT scan data and using these to compare with ante mortem records should a physical examination in the mortuary not be a feasible option for operational reasons. Fortuitously, during the course of this project a mass fatality incident occurred that necessitated the use of dental identification, and 3D printing of dentition was used successfully to achieve a positive identification

89 without resorting to disfiguring incisions in an already severely fire-damaged body.84 This example will be discussed further in chapter 6.

Occasionally cases arise where standard methods of identification are not possible, and any available idiosyncratic characteristics may need to be taken into consideration. Where ante-mortem radiology data exists there is the option of 3D printing unique anatomical structures (e.g. degenerative spinal changes, prominent atheroma patterns, skull sinus architecture, etc.) to compare with post-mortem scan data and assist with establishing a convincing match.85-93

5.5 Anthropology

Forensic pathologists often work closely with anthropologists in cases of skeletonised, dismembered or fragmented remains. CT scanning has already become a useful tool in anthropology practice,51-51,94-98 and there may be additional advantages in having access to physical models rather than relying solely upon virtual, on-screen representations of scan data. By providing high quality 3D printed facsimiles, physical study can be undertaken without risk of damage to (or loss of) original specimens. Even if not deemed truly useful for diagnostic anthropology work, producing physical models has wider benefits as a record for future teaching and training. Anthropologists already use casts of bones for this purpose, and 3D printing provides a more economical and reproducible way of creating teaching or comparison sets of bone models. Virtual models can easily be shared electronically,99 enabling 3D printing at a geographically distant location and opening up the possibility of “tele-anthropology” (figure 5.2).

Fig 5.2 Digital 3D data derived from CT scan of a prehistoric fossil specimen on a different continent, and made available by the host institute for wider academic study. The virtual model (illustrated as a low-resolution mesh, left) can be used to produce detailed surface rendered images (centre) or even a 3D printed physical model (right). Intricate study of precious artefacts can effectively now be conducted by unlimited numbers of experts around the world, without the need for anyone to travel to examine the specimen in person, or to risk its damage through transport or direct handling.

5.6 Education

It is not just anthropologists who stand to benefit from having ready-made physical models of anatomical structures for teaching and training purposes. Whilst models of normal anatomy have been mass-produced and are widely available, examples of specific injuries and disease processes are not so ubiquitous. Previously, specimens of interest were collected and preserved in teaching museums for education of the next generation of medical and associated practitioners. Since the introduction of the Human Tissue Act 2004, such anatomical museums have all but disappeared with consequent loss of irreplaceable historical specimens. 3D printing is a way of creating physical models of interesting fractures and other pathological processes that can be copied, shared, handled and easily re-printed if damaged.100 In many respects, 3D printed models designed to emphasise a particular feature could be superior to a preserved anatomical specimen as structures of interest can be enhanced or idealised in ways not possible when simply dissecting and displaying human tissue.101 Possible natural disease processes of interest include coronary (or other) arterial models to demonstrate anatomical relationships or

91 pathological lesions. As discussed in chapter 4, such vascular models are relatively easy to segment from the scan data where angiography has been carried out, given the marked density difference between contrast and surrounding tissues. Aortic (and other) aneurysms would be demonstrable using this technique, and other incidental lesions such as tumour deposits may provide unexpected opportunities to create further educational models. Creative use of multi-part models with different coloured materials and detachable sections, although complex to plan and print successfully, could be of tremendous value in aiding understanding of the anatomical relationships and pathological impact. As an example, the vertebral arteries are of specific forensic interest, but are difficult to inspect during a post-mortem examination. The leap from anatomical textbook to physical examination in the mortuary could be bridged by the intermediate step of a 3D printed model of the relevant anatomy in an accessible format designed to aid subsequent dissection. As with the earlier discussion of tool mark analysis, it is possible to print small structures (such as the hyoid bone) scaled up to larger-than-life size to aid visualisation and understanding of anatomical features.102

5.6.1 Anatomical teaching

An early opportunity arose during the project to include 3D prints as part of the nursing education programme at nearby De Montfort University. This developed as a logical progression to the programme, which had already adopted the virtual 3D capability of CT scanning to assist learning complex anatomy structures such as the pelvis. By comparing use of traditional 2D graphical representations with 3D virtual models obtained from CT scans, and then adding the ability to handle and examine 3D printed models of the same anatomy, confirmatory evidence was obtained that this was of benefit to nursing students learning anatomy.103

5.6.2 Molecular teaching

Towards the later stages of the project, a similar process was used to improve the understanding of complex molecular shapes using both virtual and 3D printed means in the undergraduate chemistry course at the University of Leicester. Several students were sufficiently motivated to take part in an optional workshop that required them to produce their own 3D files, subsequently leading to the production of physical 3D printed models. Such models are said to aid comprehension of complex three-dimensional structures

92 encountered in the world of chemistry.104 Examples of this molecular 3D printing are illustrated in chapter 7.

5.7 Thinking outside the box

It is important not to restrict potential applications to the immediately obvious or directly relevant, as this technology can have advantages beyond straightforward representation of anatomy and injuries. Abstract or hard-to-visualise concepts (such as complex molecular shapes) can be demonstrated with additional impact in the form of a tactile 3D model. For example, toxicological cases may not at first appear to be likely beneficiaries of 3D printing technology, but an insulin molecule model with detachable C-peptide (for example) might provide useful assistance when explaining relevant matters to a lay audience. With ready access to a 3D printer, imagination and creativity are the only limitations.

Chapter 6 – Forensic case application

6.1 The best laid plans

During the planning stages it was envisaged that the project would entail the acquisition of 3D printing hardware and software, and the establishment of processes to produce anatomical models using CT scans from an existing database of cases where consent had been obtained to use the scan data for research purposes. The production of “real” models from active casework for use in genuine police investigations and court trials was conservatively viewed as aspirational, and accomplishment of this feat within the course of the project was by no means an expectation. However, as awareness of local availability of 3D printing technology became known, production of such models was indeed requested on several occasions. Brief details of example cases are included below for illustrative purposes, but specific information in relation to the cases themselves has been kept to a minimum to avoid divulging sensitive material.

6.2 Skull fractures

As predicted in chapter 4, demonstration of skull fractures has proved to be the most opportune application requested by the police for court use in criminal trial cases to date.

6.2.1 Assault with a weapon

In this particular case, the victim had been assaulted with a blunt object and left for dead, but ultimately survived. For this reason, there was no post-mortem examination and so no photographs were available to provide the court with visual evidence of the extent of traumatic injury. A clinical CT scan had been undertaken following initial admission to the hospital emergency department, and this was subsequently made available to the forensic pathology department with a request for an opinion as to the severity of the injury, in a case where the criminal trial would be concerned chiefly with differentiation between assault and attempted murder. In order to assist the jury in deciding this matter, a 3D printed model of the skull injury was requested.

The weapon used in this case was never recovered, but analysis of CCTV footage suggested that this may have been part of a concrete paving slab. Two models were produced from the clinical CT scan images: a scaled-down model of the skull to provide

94 an overview of the location and extent of the skull fracture, and a life-sized portion of the skull that would indicate the size and shape of the injuring implement as well as revealing the internal features of the skull injury.

Fig 6.1 Overview of entire skull, showing the location and extent of fracturing (highlighted in red on right hand image). This model was produced from a clinical CT scan carried out in life, where the scanned anatomical region is restricted to limit radiation dose, hence the absence of the lower parts of the skull.

Fig 6.2 Life-size portion of skull, demonstrating how the viewing angle can be altered to enable examination of both the external surface (left hand images) and the internal damage at the point of impact (right hand images). The squared-off profile of the striking object can be appreciated on the external surface of the skull, and the slight depression (inward displacement) of the fractured bone can be seen (and felt) on the inner surface.

N.B. The above prints were produced at a stage in the project when initial resin versions (with a shiny, translucent surface quality, especially in thin areas) were still in use. The aesthetic appearance of both the white and grey resins has improved substantially with modified versions released subsequently.

Having been able to examine these models during the trial, the jury decided that the extent and severity of the injury was sufficient to justify a conviction of attempted murder, rather than the less serious offence of grievous bodily harm (GBH). Following the conclusion of this trial, feedback was requested from the police officer who had requested the 3D print. It was verbally reported that the trial judge had commented positively upon the use of 3D printed anatomical models during his summing up remarks, but unfortunately no transcript of this remark is available for inclusion here. After recovery, the victim subsequently requested to attend the department to learn more about the process, and having done so was enthusiastic about the case being used as an example to inform others so that future victims could benefit from the application of such technology.

6.2.2 Shod foot assault – living victim

Shortly after court proceedings for the above case had concluded, an entirely unrelated case with uncanny similarities occurred in the same region. Again, the victim sustained severe head injuries, but again survived after a prolonged period of hospitalisation. Given the previous positive result, a request was again received to convert the available clinical CT scan data into a physical 3D model to illustrate the extent of the skull injury. However, on this occasion the prosecution team later elected not to introduce the model itself into the courtroom for fear of its potential prejudicial effect, or accusation by the defence team of sensationalism given the highly emotive appearance of the model in this case.

Instead, the model was made available to the forensic pathologist who had originally provided an opinion to the police based upon on an examination of the victim’s externally-apparent injuries whilst an intensive care unit patient. This pathologist examined the 3D printed skull model and provided an updated opinion as to the causation and severity of the inflicted injuries. The pathologist subsequently attended court to give this opinion in evidence, and a conviction for attempted murder was returned by the jury. As the pathologist had not seen the clinical CT scan images, and there had again been no post-mortem examination, the pathological evidence in relation to extent and severity of skull injury was based solely on examination of the 3D printed model.

Fig 6.3 This selection of views of the 3D printed skull gives some indication of the location and extent of the fracture, although tactile examination of the physical model provides far more information than is appreciable in 2D images alone. (This model was produced from a clinical CT scan carried out in life, where the scanned anatomical region is restricted to limit radiation dose, hence the absence of the lower parts of the skull.)

6.2.3 Shod foot assault – fatal

In this case, the victim died and a full post-mortem examination was conducted. However, photographs taken for evidential purposes during such examinations are rarely allowed to be shown to the jury in court for reasons (often cited by defence barristers) of causing undue distress, being overly emotive or otherwise prejudicing the outcome of the trial. For these reasons, alternative methods are found wherever possible to depict the injuries described during pathological examination. As a post-mortem CT scan had been conducted prior to autopsy in this case, a 3D printed model of the deceased’s skull could be printed to facilitate the jury’s understanding of the nature of the skull injury in a sanitised form. The defence argument was that the victim had struck the back of his head whilst falling, whereas the prosecution’s case was that the defendant had stamped upon the victim’s head whilst he lay on the ground.

After production of the model, the defendant pleaded guilty to the charge against him, and so there was ultimately no need for a court trial. The model was retained by the police as evidence, in case of any future appeal process or case review.

Fig 6.4 The area of injury (fracture) can be seen just above the right eye socket. This skull model is complete, as the scan was performed post-mortem, and there was therefore no need to truncate the scanning region to minimise radiation dose (compare with Figs 6.1 and 6.3).

6.3 Dental identification

As already mentioned in chapter 5, 3D printing was used successfully during the investigation of a mass fatality event to assist with identification of a victim using dental means.84 Following a building explosion, collapse and intense fire, several deceased victims were recovered during excavation of the rubble pile. Given the charred and fragmented nature of the remains, dental methods of identification were deemed most appropriate by HM Coroner. For one individual who was believed to have been involved, there were no available dental records to provide an ante-mortem comparison, but a good quality smiling photograph had been obtained from the deceased’s social media account. Consultation with the forensic odontologists confirmed that this photograph alone would be sufficient to provide a positive identification, as long as the morphology of the deceased’s dentition could be inspected adequately.

Severe charring meant that access to the teeth would not have been possible without resorting to disfiguring facial incisions, but invasive internal examinations had not been authorised by HM Coroner due to the non-suspicious nature of the incident. All victims had undergone a CT scan as part of the disaster victim identification (DVI) process, and so a life-sized model of the dentition visible in the smiling photograph was rapidly printed and provided to the forensic odontologists within the mortuary. This allowed a positive identification to be made without making any incisions in a severely charred body, and without adding any additional delay into the DVI mortuary process. For the avoidance

98 of doubt, the unique DVI reference number assigned to the deceased at the time of body recovery was included in the 3D printed model to ensure continuity and avoid the potential for confusion between different individuals.

Fig 6.5 Model teeth printed to allow dental comparison with smiling photograph, complete with unique DVI reference number (inset). The comparison smiling photograph is not reproduced here for reasons of confidentiality.

6.4 Anthropology

In an unrelated mass fatality event outwith the East Midlands region, a large number of deceased victims were identified using standard DVI processes. Due to a combination of trauma and severe fire damage, many victims required the intensive involvement of a team of forensic anthropologists. In one case, a fragment of bone was found during subsequent examination of clothing that had been removed from a particular individual. As that individual had already been identified, released to next of kin and cremated, there was no option of returning to the victim’s body to re-approximate the skeletal fragment by physical fit-matching. CT scanning had been conducted on each of the victims as part of the DVI process, and so a request was received from the anthropological team involved with the incident to search for the fragment of bone in the original CT scan data, as doubt had been raised as to the provenance of the bone fragment. Review of the scan images confirmed that the fragment had indeed been located within clothing inside the body bag. The CT scanning had taken place prior to opening of the body bag within the mortuary, and so this established that the fragment had not been transferred erroneously from a different body.

Following this successful confirmation, a further request was received to ascertain (if possible) whether or not the fragment originated from the deceased’s fractured proximal femur, as anthropological assessment had suggested femur as the mostly likely site of

99 origin. 3D prints were therefore made of the fragment in question and the comminuted (separated) fragments of the fractured proximal left femur. Having re-assembled the 3D printed bone fragments, it could clearly be demonstrated that the stray fragment could not have arisen from the suspected femoral fracture site within the body.

Fig 6.6 In these anterior and posterior views of the fractured section of femur (thigh bone), the grey 3D printed individual fragments have been tacked together with adhesive putty for convenience during photography. The separate (white) 3D printed fragment clearly would not fit anywhere into the fractured region, and therefore could not have originated from this site.

Anthropological assessment had considered the femoral fracture site to be the most likely origin of the stray fragment, and so a final request was received to identify (if possible) whether the actual site of origin of the bone fragment could be determined instead. Following review of the scan images, the fractured right distal humerus appeared to be the most likely source of the fragment. This region of the body was at the very edge of the scan field, and so had been incompletely scanned and was also affected by significant edge-artefact. Despite these limitations, it was still possible to 3D print the majority of the bone fragments from this region. Following assembly of the printed parts, the true

100 origin of the bone fragment could be confirmed as the right humeral fracture site, with a high degree of certainty.

Fig 6.7 Although this region of the body had been sub-optimally scanned, individual fragments of fractured right humerus (grey) could still be extracted from the CT scan, 3D printed and assembled. This time, the separate (white) fragment appears to fit very readily into the fractured region, providing a confident indication of its likely site of origin.

6.5 Penetrating weapon injury

In a region outwith the East Midlands, an adult male had been found in the street with a head injury, and had been taken to hospital where he underwent brain surgery. The damaged portion of skull was discarded along with other clinical waste from the operating theatre, but the patient later died and a post-mortem was carried out. The forensic pathologist was able to confirm that death was due to an unsurvivable head injury, but the mechanism could not be determined given the absence of the damaged portion of skull. A clinical CT scan had been performed prior to surgery, and the pathologist asked to be provided with the scan images. These clearly revealed that the deceased was likely to have been struck on the head with an implement, rather than having suffered a fall or having been the victim of a road traffic collision.

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The police enquired whether a 3D printed model could be produced both to assist with potential identification of the causative implement and for eventual use in any subsequent court trial. The local pathologist was unable to fulfil this request, but was aware of the existence of this project and so suggested that the police contact the department here in Leicester. Having been provided with the clinical CT scan, confirmation was rapidly given that a 3D print could indeed be produced. Two duplicate models were then requested given the proposed uses, and these were duly printed as depicted below.

Fig 6.8 3D printing allows identical copies of the same model to be produced if required, in this case to allow simultaneous demonstration by the pathologist and inspection by the jury during the court trial.

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Fig 6.9 By cropping the fractured region of the skull in this manner, both the external and internal features of the damage can be visualised. As the model is produced at life- size scale, the dimensions of the causative weapon can also be appreciated.

6.6 Dismemberment

Following the discovery of parts of a dismembered body, post-mortem examinations were undertaken as part of a murder investigation. As is often found in cases of dismemberment, “tool marks” (see chapter 5) had been left by the dismembering saw. Samples of the sawn bone were therefore retained and de-fleshed in the laboratory to allow detailed tool mark analysis, including micro CT scanning. The investigating police enquired about the possibility of using the micro CT data to produce 3D printed models of the saw marks for eventual demonstration to a jury in court. Given the greatly increased size of the datasets generated by micro CT scanning, this task involved much more software manipulation to prepare printable files. However, prints were indeed successfully created, and the surface detail was vastly superior to that which could have been achieved by standard post-mortem CT scanning.

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Fig 6.10 This 3D printed section of dismembered humerus (upper arm bone) clearly shows numerous linear striations in the surface adjacent to the main saw mark. These shallow cuts are termed “false start” kerfs. The fine grain of the printed bone’s surface can also be appreciated, giving an indication of the level of detail achievable even with a “desktop” 3D printer.

Fig 6.11 Similar views of a 3D printed section of dismembered femur (thigh bone). This time there is a very prominent false start kerf cutting deeply into the surface of the bone. Again, the fine grain of the printed bone’s surface can be discerned.

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Fig 6.12 This image compares the level of detail available from a clinical CT scan (left) and micro CT scan (right) of the same bone specimen. The additional technical processes and digital manipulation burden necessary mean that micro CT scanning would not be feasible for the majority of cases. However, these results demonstrate that the effort is worthwhile when this level of detail is deemed sufficiently important.

Fig 6.13 For this case the prosecution team also wanted an interactive model to be available for jury examination. This life-size model consists of a top and bottom half, which can be detached at the point of sawing to reveal the cuts through the spine used to separate the torso of the victim during dismemberment. Separate oblique cuts can be seen through the vertebral body and the posterior spinous process elements, which corroborated the defendant’s account of rolling the body over midway through the process to make the task easier to complete.

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Fig 6.14 As the spine model was produced from micro CT scan data, exquisite feature detail can be seen within the area of internal bone revealed by the saw cut, further emphasising the level of precision that is possible even with desktop 3D printing.

In a different case of dismemberment, the separately recovered body parts were again CT scanned, including micro CT scanning of the saw cuts at the points of dismemberment. In one particular bone (left proximal femur), an almost complete cut through the bone had been aborted and a further complete dismemberment cut made a short distance away. Prior to the court trial, the police requested a 3D print of this preserved, incomplete cut as a saw suspected of having been used to carry out the act of dismemberment had been recovered during the investigation. The saw was brought to Leicester so that it, too, could be CT scanned. This meant that a corresponding portion of saw blade could then also be 3D printed, to enable direct physical comparison of the saw blade with the bone cut mark. Following use in the court trial, verbal feedback from the investigating detectives was that both the 3D models and the accompanying 3D images were felt to have been extremely useful whilst explaining the pathological information to the jury. A request has been made for this feedback to be produced in written form, but as yet none has been provided.

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Fig 6.15 3D print from micro CT scan of portion of dismembered bone (left proximal femur). In addition to the main saw cut (oblique upper margin in these views) there is a deep, almost complete partial cut through the bone.

Fig 6.16 A separate 3D print was made of the saw blade (grey, inset) after it had been CT scanned. This enabled production of a physical “fit match” model of both the blade and the cut bone, which was provided to the police for use in the subsequent court trial.

In another case from outwith the East Midlands region, a human foot was discovered by a dog walker and reported to the local police. The foot was partially skeletonised and partially mummified, but a CT scan was conducted as part of the overall forensic investigation of the case, and a 3D print was requested so that a life-sized replica was available for examination and reference purposes where demonstration of the actual foot specimen would have been impractical or otherwise inappropriate. Rather than isolating the bone elements from the soft tissue, on this occasion the request was for an entire specimen to be printed including the mummified skin surface.

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Fig 6.17 This life-size 3D print of an entire dismembered foot specimen displays where mummified skin is still present, but also reveals underlying bones and tendons where the skin is absent. Details such as toe nails and the cut ends of protruding bone can also be seen. Transportation, display and examination of this clean, dry and odourless print is preferable to that of the original specimen for a variety of reasons.

6.7 Weapon reproduction

Finally, prior to a court trial for a fatal stabbing case, the police requested a 3D printed model of the knife that had been used during the incident. It was deemed necessary for the jury to understand the dynamics of the incident as fully as possible in order to determine whether the defendant’s account of events was indeed plausible. To this end, the intention was to ask the defendant to demonstrate to the jury in court exactly how the knife had been held and used. The actual knife could not be handled, as it remained sealed in a forensic evidence container, and to have used the genuine exhibit for such demonstration purposes was also considered highly inappropriate. A life-size replica to be used in court as a “prop” was therefore chosen as a suitable alternative to facilitate this process. The exhibit was brought by the police to the CT scanner, and scanned whilst still inside its forensic container. A major advantage of using an x-ray modality (rather than photogrammetry or laser scanning, for example) was that there was no need to

108 remove it from its wrappings or container (inside which it was also fastened to a backboard by means of multiple cable ties). After scanning, the knife was easily isolated from its container and the cable ties virtually, before printing was undertaken within a short timescale and at low cost. A pure white colour was chosen both for its neutrality and ease of visibility in court. Aside from its use as a method of demonstrating the grip used and movements made during the incident, the degree of permanent bending of the blade caused during the incident could also be seen in the model (something that was not readily apparent if the actual knife were to be viewed inside its container).

Fig 6.18 The top image shows the 3D printed knife model, with a 3D volume rendering view from the original CT scan (below) for comparison. Inset is a composite of superimposed “top” and “bottom” views of the knife model, to give an indication of the degree to which the blade has been permanently bent away from the midline by the force of the stabbing action.

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Chapter 7 – Bonus capability

7.1 Unforeseen advantages

Whilst the primary purpose of acquiring the ability to produce physical 3D objects was to demonstrate the anatomical features obtained by CT scanning in forensic cases, the hardware and software are not limited solely to the production of such models. Indeed, any shape that can fit inside the build volume of the printer can in theory be printed. Once 3D printing had become a reality within the department, alternate uses for this technology were therefore sought and investigated.

7.2 CAD prototyping

Computer-aided design, as the name suggests, is a process whereby objects are created within computer software programs, allowing computer-controlled manufacturing techniques to achieve high tolerances in terms of both dimensional accuracy and part-to- part consistency between production cycles. Such systems have largely replaced the stages of traditional paper drafting and production using hand tools in a wide variety of manufacturing industries. Depending upon the software processes chosen to prepare files for 3D printing, CAD functionality may already be available. This is certainly true of the Blender software mentioned already in chapters 2 to 4, which will be discussed again in greater detail in chapter 8. Complex objects can be created “from scratch” to exact specifications, and whilst the units of measure used are essentially arbitrary, Blender units transfer into 3D printing software in their default form with precision equivalent to thousandths of a millimetre.

Consideration was therefore given to the potential for design and manufacture of customised or entirely novel pieces of equipment that might be of practical use in departmental research or day to day practice.105-107 Traditional methods of prototype production are time-consuming and expensive, and justifiable only if such investment can be recouped later by up-scaled production and commercialisation. In post-mortem research, clinical equipment is often re-tasked in a mortuary setting. It may be that improved performance or additional functionality could be achieved, but that the required adaptor or modification does not exist “off the shelf”. In such a case, a specific part can be designed that is tailored to exact requirements, and printed as a single item. Any

110 adjustments or fine-tuning can then be undertaken based upon performance of the prototype, and further evolutions of the design created. This entire production cycle can now be achieved “in house” cheaply and quickly using the hardware and software acquired for this project, and so departmental research and forensic casework are no longer limited to the availability of pre-existing equipment and parts.

7.3 Examples

A previous research project by a different postgraduate researcher looked at the pressure and flow characteristics of fluid within coronary arteries, specifically measuring the differences caused by varying degrees of pathological obstruction.108 The early phases of that particular research project would have benefited greatly from the ability to produce tubular structures of known diameter and controlled percentage occlusion. However, this facility did not exist at that time, and so the project was undertaken without the ability to test apparatus under controlled conditions and prepare baseline data for subsequent comparison. If a similar project were to be conducted again, the availability of 3D printing technology within the department would enable such initial testing to be undertaken. With this in mind, current researchers within the department were made aware of this facility, and asked to consider the possibility of creating any custom items that may be of assistance with their research. The following examples illustrate the versatility of the available technology, and give an indication of the potential benefits for future projects.

7.3.1 Research specimen custom container inserts

One project being undertaken within the department required electron microscope examination of very small blood vessels from paediatric post-mortem examinations. No item of existing laboratory equipment was perfectly suited to the task of storing and transporting the small samples during the period of fixation in glutaraldehyde needed prior to processing for electron microscopy. A small, water-tight food storage container was considered by the researcher to be suitable for this task, but even this had a relatively large volume compared to the size of the samples taken. A method of separately holding each sample securely whilst within the larger container was therefore considered highly advantageous.

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Using basic CAD techniques, a shape was designed to fit inside, but significantly fill, the specific container (thereby considerably reducing the volume of glutaraldehyde necessary to be added). This had individual holder slots incorporated into it, so that the samples could be kept separate from one another whilst in the container. In order to control the manner in which the samples were suspended in the fixative solution, small plastic clamps were to be suspended from the frame of the designed insert, and contoured features were therefore incorporated into the slots at the top of each well in the insert, to allow a secure fit for the clamp suspension assembly. The addition of numerical indicators alongside each well would allow continuity tracking of individual specimens once placed inside the container. The various features described were refined through several iterations of the initial prototype. For example, the first prototype was formed with the intention of ensuring that the holder would fit snugly within the proposed outer container, whilst gripping of the plastic clamps was sufficient to prevent movement without hindering insertion and removal. The tolerances established during the process described above (see section 4.3.2) were used to design a crude shape to test these sole parameters, without consuming unnecessary resin. The first iteration printed with external dimensions that allowed the insert to be placed into and removed from the container without friction, but allowed very little vertical or lateral movement once sealed inside. These dimensions did not therefore require adjustment, but the slots for holding the plastic clamps allowed a degree of lateral movement, and so their simple flat bottom surfaces were redesigned to incorporate a curve of the same radius as the clamp devices, meaning that subsequent models held the clamps centrally and without lateral movement. After seeing the prototype holder in situ, it became apparent that the internal reservoir left for fixative could be reduced by substantially thickening the walls of the insert, such that only a small residual well remained within the container. This had the added advantage of providing somewhere to place well numbers to assist with specimen identification. These prototypes were produced with sufficient rapidity (i.e. print times of the order of two hours each) that multiple design cycles could be completed within in a single day, and with only modest consumables costs ranging from £2.50 - £3.60 per item. The creation of usable parts that were not available commercially, with a very short time lag between identification of need and final use in practice, has therefore become an everyday reality within the department’s research environment. (Custom parts such as this would never be required at anything approaching the scale necessary to make them commercially viable as a piece of generic laboratory hardware.)

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Fig 7.1 On the left is an early prototype created to confirm correct dimensional parameters with short print time and reduced material consumption. The central image is the final iteration of the design, with numerical indicators added. The resulting 3D printed part is shown on the right.

7.3.2 Enlarged inserts and associated items

After the initial results of the research experiment requiring the aforementioned specimen holder, that project was expanded. The subsequent studies were predicted to require many more samples, and the small containers and inserts prepared before would no longer have the necessary capacity.

Larger water-tight containers were sourced by the researcher, and a new insert was designed to fit these larger containers. Further refinements in the design based on feedback gained from use of the previous items were incorporated into the latest model. As a further aid to assist with specimen identification once large numbers of specimens had been collected, the same insert was printed in differing colours so that the numbering system could be expanded across multiple containers simultaneously.

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Fig 7.2 Further refinements of the previous model to accommodate increased specimen numbers, and to fill alternate container shapes. Note the individual wells for each specimen in the large container insert on the right. Not only does this mean that each specimen has its own separate zone to prevent cross-contamination or inadvertent movement between identifying numerical indicators, but it also drastically reduces the volume of fixative required when using the larger outer containers.

Fig 7.3 On the left is a 3D printed version of one of the computer-based designs depicted in figure 7.2, and the lower right part of the image shows how the insert fits inside a water-tight container for storage and transportation. Each specimen can be contained within its own individual fixative well, suspended by a pair of sprung clamps (insets).

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After being stored and transported in the above 3D printed container inserts, the specimens underwent electron microscopy. Attempts were made by the researcher to seek alternative methods of imaging the samples, and orientation during imaging was considered of paramount importance. One solution was to produce a microscope slide coverslip that featured a central well of sufficient depth to allow the specimen to be entirely encased within mounting fluid, whilst simultaneously trapped between transparent surfaces to maintain preferred orientation during imaging. Coverslips with the required dimensions were not available to purchase commercially, and so CAD techniques were used to create an object of the exact dimensions and a small number of these were 3D printed in transparent resin using stereolithography.

Fig 7.4 This rendering illustrates the central well within the small microscope coverslip adaptor design. Once glued to a microscope slide, this recess can be filled with mounting medium, and the specimen orientated in the correct alignment before the final coverslip is placed on top, sealing the entire preparation. Such items do not exist “off the shelf”, but can be designed to fit the readily-available standard microscope slide and coverslip dimensions exactly, whilst being of an appropriate thickness to accommodate the collected specimens precisely.

Prior to imaging, additional staining techniques were to be applied to the microscopy slides. A plastic container of suitable size had been acquired in which to complete this process, but the slides themselves had to be held in an elevated position above the floor of the container whilst staining was being carried out. The simplest solution was to 3D print pairs of parallel “rails” of appropriate dimensions, which could then be secured to the base of the container, enabling rows of slides to be arranged for simultaneous staining in an efficient manner.

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An additional step in the research project involved macrophotography of the individual specimens, and it was found to be most convenient to carry out this procedure with the specimen suspended in front of a tripod-mounted camera. A custom rig was designed and 3D printed to allow the specimen to be suspended by gravity directly over the centre of a rotating turntable to facilitate the most efficient completion of an otherwise awkward and potentially frustrating aspect of this particular piece of research.

Fig 7.5 A very simple design that could be printed quickly and at low cost enabled suspension of delicate specimens directly over the centre point of a rotating turntable, providing a simple method of enabling photographs from multiple angles of view to be taken of small specimens at very close range. Without ready access to on site 3D printing, use of small, purpose-built practical aids such as this would not be feasible.

This sequence of ongoing requests for additional small items to be designed and printed as required to assist an evolving research project illustrates that access to 3D printing capability can truly assist day to day practice within the department. Moreover, the very approach to research problem-solving has quickly adjusted from making do with what is available, to a more bespoke strategy in which an ideal setup is designed from the outset.

7.3.3 Clinical engineering collaboration

Through contacts with the local NHS Clinical Engineering department, requests began to be received to assist with 3D printing of several objects during the course of this project. This involved a combination of both the production of prototype designs that were being

116 developed by the department, as well as limited manufacturing runs of batches of small items to be used for several different end-user applications. Bespoke pieces of research apparatus were also produced to assist with clinical research experiments being undertaken. The advantages of being able to have items manufactured on site, rather than being produced elsewhere and delivered, simplified logistics and greatly improved turnaround times, especially when multiple design iterations were necessary to refine subtle fit and performance parameters. Costs were also minimised (tens of pounds internally versus hundreds or more quoted by external providers), and the risk of electronic CAD design files going astray was also felt to have been reduced by keeping the entire process within the hospital environment. As with forensic case material, avoiding the transmission of data to external, potentially insecure, sites has information security advantages. As the designs and their intended uses might be considered commercially-sensitive, the specific details are not reproduced here. However, the collaboration was considered sufficiently successful for the Clinical Engineering department to have acquired access to its own 3D printing device in order to continue the work started during this project.

7.3.4 Brain box

Having noted the implementation of pathological sectioning guides elsewhere,106 consideration was given to whether a similar device might be of benefit in forensic autopsy work. In cases where lesions within the brain are deemed to be of forensic importance, the brain is fixed in formalin solution and transported to a specialist neuropathologist for detailed examination. Where brain pathology is not thought to be of sufficient significance to justify such retention, the brain is sectioned in its fresh state at the time of the post-mortem examination. Sectioning a brain in this manner is suboptimal given its soft consistency in the unfixed state. The creation of a dedicated sectioning aid was therefore investigated. The general size and morphology of a human brain was digitally subtracted from a cuboid to leave a hollow, brain-shaped depression, and the resulting object then had parallel slices cut into it at regular 1 cm intervals.

Although in theory such an item could be used in a mortuary setting to facilitate more uniform and reproducible sectioning of an unfixed brain, discussion with colleagues around its perceived potential highlighted concerns over the ease with which thorough cleaning could be carried out to ensure adequate infection control measures. For this

117 reason, a physical print has not been produced to date for testing purposes. However, the design process was instructive in demonstrating the feasibility of a system of custom mortuary equipment production, and the file remains available for future testing if deemed of potential benefit.

Fig 7.6 Computer renderings of a prototype brain slicing aid. A generic brain-shaped recess was hollowed out of a solid cuboid, and then regularly spaced blade guides were added to a depth greater than the maximum extent of the brain recess, allowing cuts to be made cleanly through the entire thickness of the organ. Such an object would allow fresh (soft) brains to remain supported whilst even slices are made for subsequent examination.

7.3.5 Corpse screw

Historically, the department was involved in the development of a novel device aimed at rapidly obtaining DNA samples from large numbers of deceased individuals in the context of a mass fatality scenario. Known colloquially as the “corpse screw”, its official title was the Universal Biopsy tool for mass fatality DNA collection (international patent Nos. EP2171421, CA2693282, US121670172 and AU200827880). A series of different cutting blades had been produced for testing purposes, but a suitable handle test article to mount and trial these alternative blade types was not created at the same time. A small number of prototype handles had originally been produced for ergonomic and assembly

118 dimension evaluation, but these were not suitable for the swapping and testing of different blade types. Technical drawings of the proposed final production handle existed, but 3D CAD models were not available for direct 3D printing. Obtaining modified versions of the original prototype handle would have been time-consuming and expensive, as well as being excessive for the intended testing application.

As an exercise in overcoming this prototype manufacturing hurdle, a novel blank handle shape was designed with an aperture sized and shaped to accept the trial cutting blades. Although production of the corpse screw had been suspended at the time of this project, the advantages of “in house” 3D printing capability were demonstrated by the rapid production of a handle that would have proved suitable for assessing cutting performance of the prototype blades prior to committing to the expense of producing even small numbers of final specification devices.

Fig 7.7 Although rudimentary in terms of apparent sophistication, this basic tool was quick and simple to design and print, and entirely adequate for its intended purpose of mounting different blades for cut testing. The left-hand image shows a computer rendering of the designed virtual model, and the right-hand image shows the corresponding physical 3D print. The upper inset shows one of the cylindrical blades to be tested and the lower inset shows this blade after mounting in the tip of the handle.

7.3.6 Phantoms

A phantom is the term given to an object designed to stand in place of a human subject during testing and calibration of radiological imaging equipment for clinical or research

119 purposes. This may be a simple design containing geometric shapes of material of known density that can be imaged to check that the performance of clinical imaging equipment is within tolerable limits. More sophisticated models with recognisable anatomical features may be needed for specific research projects, and work of this nature is frequently carried out by a team located in the same building as the EMFPU. At the time of writing, projects are currently being devised that will require the production of high- fidelity models of both the heart and the skull. Using knowledge and skills gained during the course of this project, it is hoped that complex new phantoms can be created to fulfil the radiological and nuclear medicine research objectives, whilst retaining sufficient anatomical detail to make the results as relevant as possible to clinical practice. (At present this research remains in its early phase, hence why there are no images or further details to include in this thesis.)

7.3.7 Phone microscope adaptor

Diagnostic histopathology relies almost exclusively on the microscopic appearances of tissue samples, and it can be important to capture these appearances for inclusion in presentations or publications, or simply to allow rapid electronic conference with colleagues. Complex arrangements involving dedicated cameras attached via trinocular adaptors are not universally available to practising diagnosticians, and the results achievable do not always justify the acquisition and processing burdens associated with this method of image capture. The inclusion in modern smartphones of high-quality camera modules has led many pathologists to attempt rudimentary image capture simply by holding the camera lens up to the microscope eyepiece. Whilst perhaps not ideal, the results obtainable can be surprisingly good, and this method is certainly both quick and convenient, especially when combined with the ease with which the captured pictures can then be shared via the smartphone’s connectivity capabilities.

The main drawback with this smartphone photomicroscopy technique is the requirement to align the camera lens with the exit pupil of the eyepiece, and for movement to be minimised during image capture (a constraint worsened by the necessary action of touching the screen or pressing a button on the camera in order to capture the image). One pathology colleague had identified a freely-available online design for a microscope smartphone holder, and enquired whether it would be possible to print this device to assess its efficacy. Once printed (see figure 7.7), it was discovered that certain small

120 alterations could be made to the design to improve the alignment of the specific smartphone and microscope eyepiece being used, and also to allow the main cradle to be quickly removed and stowed separately whilst leaving the eyepiece adaptor in place permanently to speed up the alignment process when a photograph was required. These adjustments were quickly made to the design, and a second version was printed with the improvements successfully incorporated.

Fig 7.8 The 3D printed adaptor is shown attached to the microscope eyepiece (top left) and with smartphone mounted (top right). The adjustable arms of the adaptor, which allow a wide variety of shapes and sizes to be accommodated, can be seen in this original model. Minor alterations to this design were subsequently made, as discussed above. Along the bottom are examples of different histology slides imaged at various magnifications using the above setup.

Although only a small application of minimal impact to the working of the department or its research, this example again illustrates the power and flexibility gained by acquisition of 3D printing technology.

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7.3.8 Micro centrifuge vial

Having already worked closely with the chemistry department to produce a series of 3D printed molecular models for teaching and public engagement applications, a further enquiry was received about the feasibility of producing custom adaptors for laboratory work. Whilst specific vials already existed “off the shelf” for micro centrifuge work, these were prohibitively expensive for the small project proposed, and the high specification of the existing laboratory equipment consumables was not considered necessary for the initial experiments.

A rudimentary micro centrifuge vial adaptor design was forwarded by the chemistry department and duly printed. Initial tests proved that the physical fit of the prints was satisfactory, but the base shape was subsequently altered to facilitate easier removal of specimens from the centrifuge tubes. The improved designs were then also printed, and the exploratory laboratory work could proceed without the need to purchase a large quantity of expensive items simply to run a small experiment.

Fig 7.9 The left-hand image illustrates the external surface of an intact centrifuge vial design, whilst the right-hand image depicts cutaway sections of three different internal design variations. Access to 3D printing equipment enabled rapid, low cost trials to be undertaken in order to establish the optimum morphology for sample stability and balance during centrifuge operation as well as ease of extraction from the vial after the centrifuge run.

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7.3.9 Molecular models

Close collaboration with the chemistry department has resulted in the production of a variety of molecular models primarily for visualisation and teaching purposes. The complex 3D shapes of molecules, the chemical importance of their geometry and the difficulty some observers have in interpreting 2D representations of molecular structures all mean that 3D printing is an ideal medium for enhancing understanding through the use of physical, tactile models. To begin with, simple molecules were created to perfect the technique of transforming theoretical molecular structures into 3D printable files. Larger and increasingly complex shapes were then attempted, with occasional failures but predominantly positive results. Public engagement activity has taken place using the 3D printed models, and a novel 3D printing workshop has been commenced within the undergraduate chemistry course as a result of this collaboration.

Fig 7.10 This model was created as a way of helping to explain how a new cancer treatment works, and was used for a specific presentation by the team who helped to develop the drug. As the drug molecule itself binds very tightly inside the much larger protein molecule, the model was designed so that it could be taken apart to allow closer inspection of the drug binding site. On the left, the two halves of the molecule have been separated (and the small, circular magnets that hold the parts together can be seen). The top right image shows the intact protein molecule, with the bound drug molecule barely visible inside. Visibility of the drug molecule is markedly enhanced by ultraviolet illumination (bottom right).

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Fig 7.11 This large model was produced to help students comprehend the multiple helical ribbon structures as well as the binding formation of the decamer subunits, which may not be immediately apparent when viewing 2D representations of this complex structure. The ability to use soluble support structures during printing was invaluable, as the process of manually removing supports from all areas of this model is highly likely to have been unfeasible.

When asked for feedback, the haematology professor who demonstrated the drug development model remarked the model generated much interest at the meeting, and very clearly demonstrated the physical concept, which appeared to have been grasped by the audience. He did lament that he suspected a general lack of comprehension by the audience when it came to the finer haematological points of his wider presentation, but this was not felt to be the fault of the 3D printed molecular model! The chemistry department’s molecular model experiment was sufficiently successful that it has now been published,109 although the ongoing restrictions of the Covid-19 coronavirus pandemic mean that at the time of writing this particular project is currently paused.

7.3.10 Mathematical models

An initial small-scale collaboration with the mathematics department resulted in the production of 3D printed models of mathematical shapes. These consisted of a series of so-called minimal surfaces, that existed only virtually. By converting the mathematical functions into STL files, and then editing out certain errors, the files could be turned into physical, 3D printed models. This was a complex process that involved first giving a

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“thickness” to curved surfaces that were effectively two-dimensional (and therefore unprintable) in their theoretical form, and then subsequently overcoming the various anomalies that arose at the intersections between these surfaces. The STL editing techniques that had been learned during the other aspects of this project meant that this was a relatively straightforward, albeit time-consuming, manual process.

The production of these physical models was not only an academic one, in that the original request was to have tactile representations of objects previously only visible in a virtual, theoretical form for the purpose of a mathematical public engagement exercise. This proved sufficiently popular for an ongoing mathematics and art collaboration to be established, resulting in the formation of a University of Leicester “Tiger Team” to promote continued public engagement and education. This has led to the development of a specific project within the undergraduate mathematics teaching programme, with the goal of encouraging students to master all the techniques necessary in order to convert mathematical functions into 3D printed shapes. At the time of writing, the first of these students has been recruited and is currently acquiring these new skills.

Fig 7.12 This particular model (the first of the Chen-Gackstatter family of minimal surfaces) proved so popular that several members of the mathematics department have subsequently requested personal copies.

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Chapter 8 – Wider interpretation of “technology”

8.1 Software still counts as technology

This project set out to evaluate the potential application of additive manufacturing technology to forensic pathology casework, and this evaluation was not confined simply to the hardware component of such technology. What was not appreciated at the planning stage, but became increasingly apparent as the project progressed, was how useful the software element of this 3D technology could be in assisting with the investigation of genuine forensic cases.

As mentioned previously, Blender was adopted early on in the project as an open-source platform that would enable editing of 3D datasets prior to printing. Professional 3D software packages with full functionality and high-powered processing capacity are available, but at a premium. For example, the annual subscription rates for industry standard 3D design and editing packages such as Fusion 360 and 3DS Max are $495 and $1,620 respectively at the time of writing, and a one-time licence for ZBrush (a package commonly used by 3D artists for creating and sculpting models, as well as creating realistic images) costs $895. Whilst this cost may be entirely appropriate for full-time 3D modellers and animators, occasional tweaking of medical models as a minor, non- core operation within a small forensic pathology department does not justify such expenditure. Instead, the ability of free software to carry out the necessary model editing steps was explored, using suggestions made by other 3D printer users in various online fora. Initially, Sketchup was trialled, but this seemed more suited to the creation of architectural-type models from scratch, rather than the editing of complex anatomical models. Netfabb basic was then tested, but the free version of this software was restricted to relatively simple tasks, and access to more enhanced functionality required additional expenditure. Meshmixer appeared promising at first, but the automated functions did not achieve the desired results and numerous software crashes meant that little progress could be made in practice.

Eventually, perseverance with Blender (which had been avoided at first due to its relatively daunting interface) resulted in not only acquisition of the skills necessary to carry out precise preparation of model files for 3D printing, but in the development of additional 3D capability that has so far already proved to be of unexpected use in real- world casework. Whilst the “slicer” programs discussed in chapter 2 are essential when

126 preparing files for 3D printing, they have very specific functionality and are of limited use in extended roles. Blender is, in contrast, extremely versatile and provides a comprehensive suite of automated and manual editing options. Whilst a direct comparison between the free Blender and an expensive industry-focused package may reveal limitations or deficiencies in the former, the broad capability and flexibility of Blender at least provides the user with multiple different strategies of achieving outcomes without the need to purchase a specific piece of software. This is important because it means that techniques developed using this software are then available to any practitioner in any setting, regardless of funding, potentially allowing widespread adoption of techniques that may previously have been perceived as unobtainable. Examples of what has been achieved to date are illustrated below, but a detailed description of the steps taken would be beyond the scope of this chapter. As such, a “how to” guide has already been published in order disseminate this specific information to a wider audience.110

8.2 3D file editing

Given the initial printing anomalies caused by artefacts within the “raw” models exported by the CT scan interpretation software, the immediate challenge was to harness the power of Blender to enable simple steps such as removal of debris or closing of holes.

8.2.1 Basic “cleaning”

In the first instance, the goal was simply to remove unwanted items (such as background debris) from the 3D file to simplify the printing process. This can be achieved quickly and simply by selectively highlighting the objects required in the final printed model, and then inverting the selection to identify all the other (i.e. unwanted) items within the file. These items can then be deleted, leaving behind only the elements of interest. Figure 8.1 illustrates this process.

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Fig 8.1 a Files imported from medical imaging often contain numerous small items of unwanted debris, b The required bones can be selected (highlighted yellow) quickly and easily, c By inverting the selection, all of the background debris becomes highlighted instead, d The unwanted items have been deleted to leave only the items of interest (in this case fractured leg bones).

Whilst it would be possible to remove many of the unwanted items in the medical imaging software prior to exporting the 3D file, in practice this is a fairly crude method and can result in alteration of the surface of the 3D model if cropping is attempted close to the edge of an object to be retained. Editing afterwards in Blender affords a far greater degree of control and precision. This precision also extends to cropping. When using the medical imaging software to crop objects, the cropping plane can be controlled with relatively limited precision and the finished edges appear roughened or incomplete. By waiting until the Blender editing stage to carry out any required cropping procedures, far greater control of the cropping plane is achievable, and the edges of the cuts can be made perfectly flat and completely sealed, as illustrated in figure 8.2.

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Fig 8.2 The top row shows a cropped section of skull bone that includes a round, penetrating injury from a ballistic projectile. This was excised virtually using the relatively crude cutting tools available within the OsiriX medical imaging program. The bottom row shows the same section of bone, but this time having been cropped using the more precise tools available within the Blender software package.

Other procedures, such as smoothing rough or otherwise irregular surfaces or features can also be carried out quickly and easily. Evidently such a process would not be appropriate where this detail is of forensic importance, but where for aesthetic or other reasons some form of smoothing is desirable then this is another example of a software feature that can be employed to good effect, and with minimal effort.

8.2.2 Complex mesh editing

So far, the editing discussion has covered only simple steps that can be quickly applied to whole datasets or object parts. For more detailed 3D problem-solving, editing at the scale of individual mesh faces and vertices can be carried out (albeit with increasing effort and time expenditure). With each file where such editing was deemed necessary, experience was gained and additional skills were acquired, leading to greatly improved efficiency and abilities for use in subsequent cases.

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Fig 8.3 Where bone is very thin, its density can be insufficient to register as solid in the 3D model extracted from the CT scan data (visible as holes within the eye socket of the left-hand image). Such holes can be filled in using Blender (right-hand image), although this is not an automated process and can be very time-consuming in complex cases.

Fig 8.4 Metallic dental fillings are a common source of x-ray artefact during CT scanning, and the left-hand images illustrates the degree of anomaly that can be present in the imaging dataset. The right-hand image shows how drastic improvement can be achieved with careful editing in Blender.

The scope of what is potentially possible is vast, and so only limited examples have been presented here for illustrative purposes.

8.2.3 Joining different datasets

When dealing with cases of criminal dismemberment (i.e. when a dead body has been separated into several parts in order to facilitate concealment, transport or disposal), the

130 individual body parts are invariably CT scanned separately. When using medical imaging software alone, these scans can therefore be viewed only one at a time: there exists no facility in the standard software to re-approximate limbs with the torso, for example. Images could be produced of each body part, and then these images could be electronically “stitched” together, but this is not an ideal solution. By importing the individual body parts into Blender as separate objects, they can be realigned as required, and images made to exact requirements for inclusion in subsequent reports.

Fig 8.5 Blender images prepared for court use in a case of criminal dismemberment. These depict the complete skeleton after reconstruction (left), as well as providing an indication of how the individual body parts had been separated from one another (right).

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Fig 8.6 An attempt to recreate the previous image using multiple pictures exported from standard medical imaging software (OsiriX). The individual component scans cannot be “cleaned” of background debris and adherent soft tissue as carefully or in such a uniform manner, and the various bones cannot be manipulated back into their normal anatomical relationships. The joins cannot therefore be as closely realigned, and there are also discrepancies of scale between the separate elements.

8.2.4 Creating custom parts

As has been discussed previously, CAD functionality was necessary during the initial testing and calibration phases (chapter 3), for the creation of complex models (chapters 4, 5 and 6) and during the development of prototypes and equipment (chapter 7). Whilst establishing these techniques, one feature of Blender consistently provided a solution to a variety of encountered challenges. The so-called “Boolean modifiers” function in a mathematical manner, allowing “addition” or “subtraction” of whole 3D models to or

132 from one another. This enables complex shapes to be joined together seamlessly, or for an object to be used effectively to “cut” a shape away from another object to allow complex, snug-fitting joins to be made between surfaces. These processes are illustrated pictorially using the following examples.

8.2.4.1 Medieval skeleton

When the remains of Richard III were excavated, the uppermost cervical vertebral body (C1) was found to be fragmented and incomplete. This bone could not therefore be included in the 3D printed models originally created in 2013. However, using the software techniques acquired during this project, the fragmented bone could be repaired such that, although still incomplete, it could now be incorporated into an updated model (Fig 8.7).

Fig 8.7 Fusing objects: the fragments that had been CT scanned as separate pieces (left) have now been digitally joined together (right) to produce a vertebral body sufficiently intact to be incorporated into a 3D printed model.

As well as simple fusion (addition) of separate objects to one another, the same Boolean modifier technique can be used to “cut” (subtract) part of one object by using another object to determine the precise region(s) to be deleted (Figs 8.8 and 8.9).

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Fig 8.8 Once the bones (left) have been aligned with one another satisfactorily, a new object (white sphere) is created. This is then moved into position to fill the gap as required. By making the bones semi-transparent (right), the overlapping portions can be appreciated.

Fig 8.9 The Boolean modifier subtracts (deletes) the parts of the white sphere that were overlapping with the bone. These objects can then be 3D printed separately, but will have flush-fitting surfaces, as shown more clearly in the detailed surface view of the new disc in isolation (right).

This process was repeated multiple times to create custom intervertebral discs and facet joints for each of the articulations of the entire spine. Previous attempts to reconstruct the spine of Richard III have relied upon modelling putty to fill the spaces between adjacent bones.

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Fig 8.10 Blender rendering of the upper neck section of the reconstructed spine of Richard III, including the fused, previously-fragmented C1 body (see Fig 8.7) and numerous examples of custom-made intervertebral discs and facet joints.

Fig 8.11 The completed, life-size 3D printed model of the spine of Richard III.

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As can be seen from figure 8.11, the base of the model has been made circular, so that there is no predetermined “front” or “back”, and it can be infinitely rotated to observe the effect of scoliosis (pathological curvature) from any angle. This model is self-supporting, as the angle of intersection between the sacrum (pelvis) and circular base has been offset to counteract the curvature of the spine and balance out the overall vertical vector force exerted by gravity. These steps were achievable only by acquiring knowledge of 3D software that extends well beyond the minimum required for 3D printing alone.

8.2.4.2 Bronze Age artefact

A similar technique was used to create virtual “casts” of defects in a Bronze Age wooden shield recovered from an archaeological dig site, as previously mentioned in chapter 5. The artefact was said to be truly unique, and as such of exquisite archaeological and historical importance. Highly conservative excavation and preservation methods were therefore used on account of its extreme fragility, precluding direct manipulation of its surface. The “front” surface (i.e. that exposed to potential battle damage) had been covered with a layer of protective plaster during recovery, and could not be viewed for a prolonged period during the preservation process. A CT scan of the object was conducted, and this revealed focal defects considered to represent possible weapon damage.

By extracting and 3D printing the localised areas of surface damage, robust (and replaceable) life-size facsimiles were created. These could be transported, probed, measured and manipulated without any risk of damaging the unique original. As determination of potential causative weapons was being attempted by the archaeological team, Boolean modifier subtraction of the 3D surface defects from solid cuboids was carried out to produce raised profiles of each of the defects, in a manner reminiscent of how casts of patterned surfaces at crime scenes might enable comparison with blunt force injuries.111 According to the archaeology team, these 3D printed surface defects and their corresponding inverse casts were of immense value in the interpretation of the shield’s surface damage and consideration of the potential Bronze Age weapons that may have been responsible.

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Fig 8.12 A portion of shield surface extracted from the CT scan data (left), and its corresponding inverse “cast” (right). The 3D printed versions of each of these defects underwent detailed examination by the archaeology team, who were unable to inspect the actual shield’s surface even visually for a prolonged period due to it being covered by protective plaster for much of the duration of the preservation process.

8.3 Image rendering

When viewing 3D representations in standard medical imaging programs, the default colouration and textured appearance is considered adequate for diagnostic and demonstration purposes. However, the user has fairly limited control over the final appearance. Using Blender to “render” a 3D scene (as opposed to simply manipulating the 3D data) opens up virtually limitless potential for fine-tuning the precise viewing angle, perspective and relationships between objects, as well as their colour and texture. The necessary process steps of adding and adjusting a camera, setting up lighting and background, etc. are beyond the scope of this thesis, but some examples of the benefit of using Blender for 3D rendering are demonstrated below.

8.3.1 Basic images

To begin with, the creation of a simple depiction of the 3D anatomy was explored as a way of improving upon the type of visualisation normally provided by standard medical imaging programs. The aim was to create images with a more natural-looking aesthetic quality, as opposed to the obviously computer-generated appearance of common medical images. Initial results were received very positively, at first by colleagues and then subsequently by police officers and barristers who had been provided with these

137 improved images for court purposes. An example of the difference in appearance is shown in figure 8.13:

Fig 8.13 The same foot has been rendered using both standard medical imaging software (OsiriX, left) and Blender (right).

This type of software manipulation is not only useful for its aesthetic appeal. Using rendered images in conjunction with simple mesh editing steps as outlined above can help to answer specific questions arising during investigations. Figure 8.14 illustrates just one example of how a software-only solution was provided to answer a query in a case that was initially considered for 3D printing. In this case, a number of individuals had died as a result of a house fire, and post-mortem examinations had been conducted to exclude foul play as well as to confirm causes of death and to establish the identities of the victims. Following severe fires, it is not uncommon to find that parts of bodies have become detached or completely burned away due to the intensity of the heat. In this case, two of the burned bodies had lost their left feet, but still had right feet attached. A single left foot was subsequently recovered from the fire debris at the scene. Anthropological assessment and DNA sampling would necessitate disruption of the already fire-damaged tissue to gain access to the internal aspects, and would require prolonged (and expensive) additional forensic analysis. As the bodies (including the separate foot) had undergone CT scanning as part of the body recovery and examination process, a quick, costless and non-invasive alternative was available. By extracting a 3D model of the calcaneus (heel) bone from the separated foot and digitally “flipping” it from left to right to create a

138 mirrored version, this could be compared with the extracted virtual calcaneus bones from the two bodies in question. As the shape of the calcaneus bone is complex, and can vary considerably from individual to individual, establishing a “match” with one of the bodies would provide a strong indication as to the likely origin of the separate foot.

The original request from the pathologist who was dealing with this particular case was to 3D print the bones to allow a physical examination. However, the quality of the rendered images was deemed satisfactory to achieve a confident match and so the entire process was completed within a matter of minutes and without unnecessary expenditure. Even though the bones had been partially damage by fire, sufficient information remained within the scan dataset to enable a detailed morphological assessment. The actual bones were all surrounded by adjacent bones and encased in charred soft tissue, meaning that a physical examination in the mortuary would have been both time-consuming and destructive. The advantages of using such a strategy were readily apparent in this case, and similar techniques are now likely to be used in the department during future forensic investigations.

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Fig 8.14 The morphological appearance of the bone from the separate foot much more closely resembles its counterpart from body 1 than from body 2. (No colour was added to this rendering sequence as the goal was simple morphological assessment.)

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8.3.2 Enhanced representation

In the vast majority of cases, a plausible “bone” colour was all that was considered necessary to display the bone fragments required to demonstrate the anatomy and pathology in question. Where a skull was included in the image, the presence of teeth presented a minor problem in that teeth rendered in exactly the same colour as the surrounding bone appeared disagreeable. Learning how to manipulate the software to render the teeth differently then opened up the possibility of deliberately false-colouring selected areas or structures for emphasis. Now the facility exists to produce clear, unambiguous images with essentially any combination of finished appearances depending on the required objective. Illustrative examples are shown in figure 8.15.

Fig 8.15 Left image: distracting lines on the bone’s surface have been smoothed, and the individual bone fragments have been separated slightly to emphasise the fracture pattern, following re-approximation of previously displaced fragments. A particular fragment of interest has been highlighted red. Centre image: one fragment of bone has been rendered partially transparent to enable visualisation of the entire red fragment. Right image: further transparency has been added to reveal additional internal bone details.

As discussed in chapter 5, it may be necessary to reconstruct wound trajectory angles in ballistic or other penetrating trauma cases. Advanced manipulation of the CT scan data using software can facilitate this type of reconstruction without the need to produce a physical model. The following example illustrates a fatal brain injury resulting from a self-inflicted air rifle discharge.

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Fig 8.16 Precise orientation of a virtual rod (red) through the point of penetration of the base of the skull to the final resting position of the projectile (yellow) can provide an easy to interpret visual representation of the straight-line trajectory, either using a semi- transparent (left) or cutaway (centre) visualisation technique. By adding the jaw (right) and rotating this to the position that would accommodate the straight-line trajectory, it can be seen that an unfeasibly large mouth opening would have been necessary, indicating that a degree of deflection or rebound must have occurred at some point along the projectile’s true course.

8.3.3 Texture wrapping

Following on from these more advanced rendering options, the ability to incorporate textures onto the rendered surfaces was considered. As an example, the surface of the skin can be extracted as a 3D model from the CT scan data (described in chapter 4). This surface is generated from x-ray data, and so there is no data about the colour of the surface. If a photograph of an injury (such as a bruise) has been taken, the 2D photograph can be “wrapped” around the surface prior to rendering, recapitulating the real-world appearance more correctly than simply “cutting and pasting” a 2D image of a bruise onto a 2D image of the skin’s surface. This process is illustrated schematically in figure 8.17, using a simple mannequin shape and text labels in lieu of genuine injury photographs.

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Fig 8.17 The left-hand image gives an indication of how the surface of a body appears when extracted from a CT scan (i.e. a monochrome shape only, with surface colours such as bruising not detectable by x-ray imaging). The outside of the body is photographed by the police during a forensic post-mortem examination, and specific photographs are taken of any injuries (simulated here by the central 2D labels). The right-hand image shows how the surface of a body can be rendered as a 3D, coloured shape, with any injuries in their correct positions and “wrapped” around the contours of the body.

Currently, computer-generated images are often prepared for court in criminal trials where injuries have been inflicted, as it is deemed either potentially distressing or unduly emotive to allow jury members to examine actual post-mortem photographs. However, these diagrams are created using standard anatomical mannequins that often do not resemble the deceased’s body habitus, and 2D photographs of any injuries are added at their approximate locations (with the inherent limitations of trying to align a 2D photograph on a 2D representation of a 3D body model that does not match the true shape of the injured anatomy). Using the technique described above, the actual anatomical data from the scanned deceased body is used to form the shape, and the injuries can therefore be applied more accurately to give an appearance that more closely resembles the true appearance, but whilst still maintaining the element of computer-generated “sanitisation” for the benefit of the jury.

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8.4 Animation

Once the principles of mesh editing and image rendering had been learned, experiments were undertaken with some of the animation tools available in Blender to evaluate potential additional applications.

8.4.1 Movement

The most immediately-apparent use for these tools was the enhanced visualisation of complex fractures afforded by dynamic animation over still images. The CT scan data is acquired with bone fragments in their “end” positions. Using the editing functions described above, individual fragments can be returned to their “start” positions by reconstructing normal anatomical relationships. The software can then be used to animate a smooth sequence displaying how the fragments migrated from start to end, and this is potentially of assistance when demonstrating the mechanism of fracturing. Whilst an experienced pathologist may be able to appreciate the significance of the eventual arrangement of bone fragments in a complex fracture, a lay audience may benefit greatly from watching a slow-motion animated sequence when attempting to comprehend how a particular fracture has occurred.

Fig 8.18 This fatal pedestrian road traffic collision resulted in severe fracturing of both lower limbs (left). Blender manipulation of the bone fragments enabled reconstruction of the legs (right), facilitating enhanced visual inspection of fracture patterns and appreciation of dynamic interactions over that possible with standard medical imaging software providing static images of the end result only.

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Fig 8.19 This image show a spinal fracture before (left) and after (right) reconstruction in Blender. These spatial arrangements were used as “end” and “start” positions respectively to create a slow-motion 3D animation of the fracturing sequence, providing a dynamic illustration of the high vertical loading forces that were the cause of this particular fracture.

8.4.2 Mesh deformation

Production of moving images is not the only way in which animation tools can be used to assist with evidence presentation. Animators use tools to deform 3D models so that movement appears realistic. For example, a walking character will need legs that bend at the hip, knee and ankle in a natural manner. This process is controlled by “rigging” the 3D mesh with a series of joints that dictate where deformations occur and in what direction. These tools can be used in just the same way to deform meshes created from CT scans. Figure 8.20 depicts the use of this technique to deform a straight limb into a flexed pose.

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Fig 8.20 After digitally dismembering a virtual body (left), the artificially straight limbs can have their joints bent into positions more commonly associated with criminal body part storage and disposal (right).

8.5 The whole is greater than the sum of its parts

The subject depicted in the previous image (figure 8.20) represents a good example of how all of the aforementioned techniques were combined in a single case to answer a very specific question. A police force outwith the East Midlands region needed to establish whether a missing person could have been dismembered and then stored for a period of time within a specific domestic freezer. As the individual was missing, presumed deceased, no CT scan existed of the individual in question. A complete 3D body had to be constructed by joining the head, torso and legs from three separate scans of an individual matching the size, sex and body habitus of the missing person. Once this surrogate intact 3D body shape had been created, it was dismembered virtually at sites typically used in cases of criminal dismemberment. Animation techniques were then used to bend the arms and legs in a realistic manner. CAD techniques were used to demarcate a 3D volume that matched exactly the internal dimensions (determined by

146 laser scanning) of the freezer in question. The bent and dismembered body parts could then be arranged inside the freezer volume, demonstrating that it would have been possible for a body of the designated size to have been dismembered and stored within the available space inside the known freezer compartment.

Whilst this process could have been completed entirely virtually, the act of determining how best to fit the various body parts inside the freezer was made more intuitive by 3D printing miniature physical models that could be directly manipulated. Once a suitable “best fit” had been determined using the physical models, the arrangement was recreated in the digital version and a series of rendered still images and an animation were produced for use by the police in the subsequent court trial.

Fig 8.21 Assorted 3D printed body parts (left), and 3D printed freezer compartment volume at the same scale (right) enabling physical trials to be undertaken to establish “best fit” packing orientation prior to attempting virtual reconstruction for court image production purposes.

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Fig 8.22 A series of frames reproduced from a revolving animation created for use in court. The semi-transparent freezer walls allow the tightly-packed body parts to be seen fitting within the internal compartment.

It should be remembered that no body was recovered in this case, and all that was known was the internal shape and dimensions of a freezer compartment, as well as the height, weight and general build of a missing person. All that was asked for by the police was a demonstration that it would have been theoretically possible for a body of this size to be dismembered and stored within the known freezer compartment volume, and this task was completed successfully using a variety of techniques that had already been accumulated through different experiments and previous case applications. This case demonstrates clearly the power of the 3D technology capability acquired during the course of this project, and provides an indication of its likely potential for future case application as even more procedures are conceived, trialled and subsequently mastered. Increasingly complex 3D graphical representations, often involving the fusion of data from multiple modalities, appears to be gaining in both popularity amongst practitioners and acceptability with court systems.112-116

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Chapter 9 – Moving forward

9.1 Where now, where next?

At the time of writing, the basic processes of manipulating data and creating physical models have now been realised. The practical work started with this project will continue long after submission of this thesis. The overall direction and precise avenues to be followed are at this stage uncertain, as they will of necessity be dictated to a large extent by the emergence of new cases presenting novel problems to solve. Examples of current themes already highlighted for further study are outlined below.

9.2 Continued research

Due to the realities of balancing full-time clinical and teaching commitments with a part- time research project of a finite duration, not all of the investigations considered during the project could feasibly be completed by the time of thesis submission. Although the initial research project has come to an end, the research process has not stopped.

9.2.1 Reconstruction aids

One topic that has yet to be examined in detail is the potential for 3D printing to provide custom reconstruction aids as part of the mortuary autopsy service. In theory, specific anatomical substitutes can now be manufactured as required to assist with complex reconstruction procedures.117 Knowing in advance which anatomical shapes might be beneficial to the reconstruction process will allow them to be manufactured prior to the autopsy, so that they are ready when required and their production introduces no delay into the overall process. In a case where severe traumatic destruction of the head has rendered sympathetic reconstruction extremely challenging, a 3D printed shape (possibly based on a mirror-image of surviving anatomy) could allow a potentially viewable reconstruction to be achieved more easily. In paediatric cases, numerous bones (such as limb bones or the entire ribcage) are often removed during post-mortem examination and sent away for destructive analysis that precludes their use in reconstruction. Currently, various strategies are employed using materials ordinarily found within the mortuary to fashion crude shapes of the required dimensions. However, custom bone facsimiles derived from the PMCT scan conducted prior to autopsy would allow reconstruction to

149 proceed at the conclusion of the examination using anatomically-correct substitutes where required.

9.2.2 Skin models

The majority of the work completed to date has been concerned with the production of skeletal element (i.e. bone) models. Whilst skin surface models have been shown to be possible (see chapter 4), the creation of such models has so far not been specifically requested during an active forensic case. With bone models, continued improvements were seen, both in the reduction of digital file preparation time and in the quality of the finished prints, after repeated case applications. Similar improvements through refinement of processes may therefore be obtainable with skin surface models, and so there is likely to be benefit in carrying out pre-emptive further testing in order to optimise skin model production prior to a request being received in a future case, with the accompanying time and other pressures generated under such circumstances. Numerous cases of fatal stabbing have already undergone local PMCT scanning, and so ample opportunities exist to test the possibilities and limitations in advance of any request for production of a court-ready model.

9.2.3 Wound trajectories

Another logical application that has yet to be tested in full for an active forensic case is the ability to depict wound trajectories using 3D data from CT scans (discussed in chapter 5 and illustrated in chapter 8). This has recently been proposed as an important potential role for 3D printing in forensic practice.18 Trajectory angles are typically considered in ballistic (i.e. gunshot) cases, but any penetrating injury could also be displayed in the form of a trajectory map. Once pathological data about wound trajectories has been confirmed during post-mortem examination, this can be correlated with radiological features in scan imaging and incorporated into 3D models, whether virtual, physical or both. It is anticipated that high-quality 3D images, or possibly even animations, would suffice in many cases to demonstrate the angle and passage of a particular injury through the body. However, the theoretical possibility of creating physical models of derived trajectories must be put into practice in advance of any police request, so that the practical limitations can be known and expectations managed accordingly.

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9.2.4 Software applications

Continuing to explore what is achievable purely within a software environment will form a major constituent of the ongoing capability development. Whilst basic images and animations have already become a reality, more complex reconstructions involving significantly fragmented or otherwise damaged remains are likely to represent a challenge that will take time to resolve, but with genuine benefit to the local and wider forensic community. For example, at the time of writing this thesis, collaboration with a facial reconstruction expert at a different university was being undertaken in order to attempt to establish the identity of an unknown deceased male with severe head trauma, who had been discovered by the side of a motorway.

Fig 9.1 The left side shows the degree of skull destruction at the time of CT scanning. Blender software was used first to separate these fragments from one another, and then to reassemble them so that a facial reconstruction could subsequently be attempted.

9.3 Dissemination of information

Throughout this project, as knowledge and experience have been gained and images of successful applications acquired, the results have been demonstrated across a variety of platforms. Through long-established links with the College of Policing (COP), regular lectures are given by EMFPU pathologists to senior detectives and coroners on national Senior Investigating Officer Development Programme (SIODP), Senior Identification Manager (SIM) and Police Mortuary Operations Co-Ordinator (PMOC) courses.

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Forensic pathology input is also provided to the University of Leicester Medical School’s undergraduate pathology teaching programme, and to postgraduate investigative courses at the UK Defence Academy (Cranfield University) and De Montfort University. RAF Medical Officers receiving aviation medicine training and updates at the Centre of Aviation Medicine also receive regular forensic pathology tuition from EMPFU staff, and the University of Leicester’s Department of Genetics and Genome Biology have been provided with updates (in both oral and poster presentation form) about this project’s work during scheduled post-graduate research seminars. Attendees of the EMPFU-run postgraduate PMCT courses also receive specific 3D printing and 3D technology input.

In addition to the fora listed above, the project’s work has also been included in formal publications. At the time of writing, these consist of original articles in Nurse Education Today103 and the Journal of Forensic Radiology and Imaging,84 a technical report in the journal Forensic Science, Medicine and Pathology110 and an entire chapter in the textbook Essentials of Autopsy Practice: Reviews, Update and Advances.118

9.4 Collaborations

At the time of writing there continues to be collaboration with the mathematics department in the production of increasingly complex models of mathematical surfaces. As this field of mathematics expands, there is expected to be increased interest amongst undergraduate students in 3D manipulation and possibly also 3D printing in relation to mathematical concepts. It is hoped that when current Covid-19 coronavirus restrictions subside, there will be a return of 3D molecular model workshops for the chemistry department undergraduates, with greater emphasis on expanding this component of the course from the niche to the mainstream.

There is at the present time a multi-centre collaboration, including personnel from University College London, Cranfield University and several police forces, that aims to collate forensic 3D printing experience from as many different sources as possible and begin to establish a comprehensive evidence base for its use. This is a long-term goal that it is hoped will continue to grow and develop over time, and the work of this project is to be included within the larger national effort to increase this evidence base and promote use of this emerging technological application.

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The department is also heavily involved in a current project with a USA-based team researching the potential for identification of deceased individuals using 3D anatomical methods based on CT scan data. By continuing to be involved in local education, national clinical practice development and international research, the work currently being undertaken within the East Midlands Forensic Pathology Unit remains both relevant and important at a time when 3D technology continues to expand across multiple academic and practical disciplines. These collaborations will be actively nurtured over the immediate and longer-term future.

9.5 Consolidation and expansion of the forensic service

With continued demonstration of this technology to coroners and detectives via the channels listed above, as well as word of mouth dissemination through police personnel following successful active case implementation, increased interest in 3D applications is being experienced by the EMFPU at the present time. Initially only physical models were requested, but as the aesthetic qualities of rendered 3D images have been experienced these have also become frequently requested for court purposes. It remains to be seen whether 3D printed physical models or increasingly-sophisticated visualisations will become the preferred method of informing jury members about complex medical and pathological data, but research into the different approaches is ongoing.119

Naturally, as simple applications prove successful, the requests are becoming increasingly complex. This has introduced an additional complication in that, although the general software manipulation stages have become much more streamlined with increasing experience, the considerably greater complexity now required at these stages means that a greater proportion of total model production time is now taken up at the software stage, rather than by the physical printing and model finishing stages. It was entirely acceptable during the research and refinement phases of this project to spend lengthy periods of time mastering these vital steps. If future requests for model production become sufficiently numerous or complex, this additional time needs to be evaluated and costed appropriately in order to maintain viability. It will not be sufficient simply to cover the costs of consumables and equipment maintenance or replacement: compensation will need to be made for any diversionary effect that this small facet of practice might have on the far greater income-generating workload of the EMFPU. With time, greater familiarity with timescales involved and standardisation of practice should

153 allow realistic estimates to be made in advance of any work being undertaken. During the transition from research to service provision, close attention will have to be paid to these issues to avoid a fledgling service being perceived as prohibitively expensive for its likely benefit, or not being viable in the long term due to failure to recoup costs or achieve results within acceptable timeframes.

9.6 Limitations and weaknesses

Despite the project’s success in realising the potential for introducing forensic pathology department-sourced 3D printed models into the courtroom, several areas have been identified that could have been improved upon, and therefore provide elements to focus upon during future work undertaken.

The aim had always been to evaluate devices specifically situated at the smaller, “desktop” end of the market. Lower grade printers undoubtedly suffer limitations in comparison to their industrial counterparts. The chief limitations of the devices used in this project were found to be as follows:

Printer Limitation Impact Formlabs Form 2 Small build volume Full-size, intact skull models not possible Attached supports Surface degradation and difficult / impossible access for removal Ultimaker S5 Inferior surface finish Full-size models not chosen, with quality police opting instead for superior finish of smaller Form 2 models

Tab 9.1 Chief limitations of devices used in project

It would have been preferable, with sufficient resources, to have acquired a larger range of printers, rather than only the two devices selected for evaluation. 3D printing technology has continued to develop throughout the course of this project, and at the time of writing improved versions of current models and new models altogether have joined the marketplace. By way of an example, the Form 2 has already been superseded by the Form 3, which has redesigned several key aspects of the desktop SLA process, meaning

154 that prints can be produced more quickly and more reliably, with supports that are more easily removed and a better overall surface finish. A larger version of this device, the Form 3L, has simultaneously been launched, enabling this technology to achieve the full- size human skull build volume requirement. In future, as the 3D capability of the department is grown, it may be possible to upgrade or add to the current limited device inventory, with potential gains in terms of print quality, turnaround time or model production cost as well as increased overall printing capacity. It goes without saying that such financial outlay would need to be justified, and almost certainly recuperated, by ongoing income generation associated with the printed models.

The project’s initial financial burden was absorbed by the department in order for the printing hardware and consumables to be purchased at the outset, and the prices paid by the police for the first few “real” models did not reflect the professional time expended in their creation. This would not be a sustainable business model for future expansion, and so significant price elevation is now to be expected. This alone may discourage uptake, and careful consideration will need to be given to this area of development. With the more complex cases, many hours were expended at the software editing and preparation stages. Forensic pathology practitioner time is a finite resource, and therefore represents a potential barrier to future model production. One solution to this may be to improve the workflow efficiency through increasing experience, but in reality there are limits to what might be achievable through use of freely-available software alone. Investment in dedicated software, with enhanced automated functionality, is one possible remedy to this situation, and expansion of the number of individuals within the department who can undertake this type of work may help to relieve this bottleneck.

Whilst there were, fortuitously, very few print failures during the course of the project, occasional occurrences of total print loss did occur. This leads not only to wastage of consumables, but also of valuable time. In the context of a research project this may not have been critical, but in the context of a criminal trial with tight turnaround deadlines and cost considerations, such technical failures could prove disastrous for a fledgling 3D printing service. Continued attention to detail and investigation of the causes of any print failures are therefore essential to minimise recurrence of such episodes.

As already described, the measurement techniques used to date have been rudimentary, and it is feasible that with increased use of 3D printed models within the courtroom various challenges might arise concerning perceived or demonstrable accuracy. It may

155 be necessary, and would certainly be desirable, to revisit this area of work with a more robust approach making use of the advanced metrology techniques available elsewhere.

Despite the range of examples illustrated above, the truth is that the total number of cases successfully used in court to date remains very low, and the quality of objective feedback received even lower. It is therefore difficult to be certain about whether the introduction of printed models into the courtroom was merely tolerated by the judicial system, accepted neutrally or embraced enthusiastically. Increasing numbers of cases with associated formal feedback gathering will hopefully address this current data shortfall, whilst simultaneously helping to establish whether this remains a niche tool for exceptional cases only, or has more generally-applicable benefits for the Criminal Justice System.

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Chapter 10 – Conclusions

The introductory section of this thesis outlined the reasons for the project’s conception, with the primary aim being to demonstrate the feasibility in practice of introducing 3D printed models from forensic CT scan data into the courtroom environment, using recently available desktop grade devices. Ultimately this aim was achieved, as illustrated in chapter 6 by the inclusion of genuine cases where the acquired departmental 3D printing capability has now been used in routine forensic pathology casework.

On the way to achieving this aim, both the background research findings and the knowledge progressively accumulated during the course of the project allowed specific conclusions to be drawn about the relative strengths and weaknesses of the different technological approaches to 3D printing. As discussed in the sections detailing the general concept of 3D printing and types of technology available, these determinations used during the decision-making process outlined when selecting hardware for project acquisition.

Following the initial chapters were descriptions of a series of early experiments conducted to test the function of the acquired hardware, and to ascertain its performance parameters. The acceptance of a 2 mm margin of error within forensic anthropology practice was taken as a suitable comparator, and the initial test prints were found (albeit by rudimentary means) to fall well within this margin of acceptability. In addition, whilst confirming adequate performance within the limited scope of the project, this process also reinforced familiarity with the whole 3D printing workflow process and determined the bounds of what was technically achievable with the acquired hardware based upon success and occasional technical failures (e.g. due to suboptimal placement of support structures or anomalies within 3D files).

The ability to produce the complex models required for court cases using modest equipment and software relied heavily upon the lessons learned and experience gained during the earlier experimentation and testing phases. However, in addition to achieving the aim of establishing an “in house” 3D printing capability, and successfully applying this to actual forensic casework, unanticipated advantages were also discovered during the work of the project. The most notable of these was the improvement in 3D visualisation techniques possible using the software capability developed over the course of the project. Prior to the arrival of 3D printing in the department, the pathologists had

157 the facility only to view CT images using standard medical imaging software. There was no facility to join data from different scans into a single scene, manipulate individual fragments to re-approximate fractures or create photo-realistic rendered images of 3D pathological data. Given the enthusiasm with which the results to date have been received by police and legal teams, a further conclusion arising from this project is that increasingly sophisticated ways of using 3D data to enhance courtroom representation of evidence is likely to become an expectation. It remains to be seen whether 3D printing itself will become widely adopted, or whether it will turn out to be unnecessary altogether with a preference instead for purely virtual means of demonstrating pathological findings to jury members.

What has certainly become apparent during the course of this project is that a large number of individuals both in the UK and internationally are working simultaneously to establish an evidence base for the use of 3D technology in forensic applications. This project’s modest contribution to the evidence base is confirmation of the ability to create court-ready models quickly, “in house” and on a small scale without the need for external providers and large financial outlay. It is hoped that this will encourage others to follow suit, and expand the existing forensic 3D capability further.

The 3D printing techniques learned, and the bonus capability acquired by exploration of the associated 3D software, have facilitated a rapid change in the way in which forensic pathological evidence is being presented in court locally. Not only have publications to this effect already been accomplished during the project, but further dissemination of this information is ongoing. The use of 3D technology in forensic investigation appears set to become both more routine and more widespread, and so the completion of this thesis is not considered merely the completion of a standalone research project: the research project itself is considered to represent the beginning of something greater and more long- lasting.

The very first image in this thesis demonstrated the start of the 3D process by showing the skull of Richard III being CT scanned, and so it seems fitting to conclude with related illustrations of the end result and progress made to date. The following full-size skull was produced from micro CT scan data of three individual bone fragments: the majority of the skull, the separated frontal portion and the disarticulated jaw. The jaw section contained an artefactual post-mortem crack, which had caused it to splay and no longer fit correctly into the skull base, and the frontal skull portion had also become slightly

158 distorted over time. The crack in the jaw was repaired digitally within the virtual model, correcting the splay, and a so-called lattice modifier was used to correct the frontal bone distortion without altering the appearance of the surface geometry, enabling the skull fragments to fit neatly together once again. If photographs of the actual skull taken at the time of excavation are viewed, the original slight discrepancies of fit are discernible. The Blender rendered image in figure 10.1 shows the result following digital restoration.

Fig 10.1 Micro CT scans of three individual pieces of bone (after artefact repair and distortion correction) re-approximated and rendered in Blender.

Finally, a print material of suitable colour was sourced that also contained particulate inclusions to add texture and break up the visible lines of the individual print layers, imparting an aesthetic appearance to the surface that is more in keeping with medieval bone than the cold, clinical finish provided by a pure white material. The combined print time for the three fragments totalled more than 8 days of continuous printing, and soluble supports were essential to achieve the desired finish, but figure 10.2 demonstrates what can currently be achieved even with “desktop” grade 3D printers. As this technology continues to be refined, increased availability of ever-improving devices is set to bring about further improvement still.

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Fig 10.2 Life-size 3D prints demonstrating the level of detail attainable even with a “desktop” grade printer, including internal depressed fracture fragments (inset).

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