Finite Element Analysis of Infant Skull Trauma using CT Images ARNA Ó SKARSDÓ TTIR Master of Science Thesis Stockholm, Sweden 2012 Finite Element Analysis of Infant Skull Trauma using CT Images ARNA Ó SKARSDÓ TTIR Master’s Thesis in Scientific Computing (30 ECTS credits) Master Programme in Computer simulation for Science and Engineering 120 credits Royal Institute of Technology year 2012 Supervisor at KTH was Svein Kleiven Examiner was Michael Hanke TRITA-MAT-E 2012:07 ISRN-KTH/MAT/E--12/07--SE Royal Institute of Technology School of Engineering Sciences KTH SCI SE-100 44 Stockholm, Sweden URL: www.kth.se/sci Finite element analysis of infant skull trauma using CT images Abstract Some cases of infant skull fracture fall under the category of forensic study where it is not obvious whether the head trauma happened due to an accident or abuse. To be able to determine the cause of the head trauma with sufficient accuracy, biomechanical analysis using finite element modeling of the infant cranium has been established. By simulating the trauma, one may be able to obtain the fracture propagation of the skull and from it determine if the scenario narrative is plausible. Geometry of skull, sutures, scalp and brain of a 2 month old infant head was obtained using CT images and meshed using voxel hexahedral meshing. Simulation of an impact to the head from a fall of 0.82 m height, to a rigid floor, was carried out in the non-linear finite element program LS-Dyna. Two scenarios were simulated: an impact to the occipital- parietal bones and an impact to the right parietal bone. The fracture propagation was obtained using the Chang-Chang Composite Failure Model as a constitutive model for the skull bones. The amount of material parameters gathered in the present study to predict fracture of the infant skull has not been obtained before, to the best knowledge of author. Validation of the models’ ability to show relatively correct fracture propagation was carried out by comparing the obtained fracture pattern from the parietal-occipital impact against published fracture patterns of infant PMHS skulls from a free fall onto a hard surface. The fracture pattern was found to be in good compliance with the published data. The fracture pattern in the parietal bone from the impact was compared against a fracture pattern from a previously constructed model at STH. The patterns of the models show some similarities but improvements to the model and further validations need to be carried out. Finit elementanalys av skallbensfraktur hos småbarn med datortomografibilder Sammanfattning Några skallskador hos spädbarn ger grund till kriminaltekniska studier där det inte är självklart om skallskadan skett på grund av en olycka eller misshandel. För att kunna fastställa orsaken till skallskadan med tillräcklig noggrannhet har biomekaniska analyser med finita element modeller av barns huvud genomförts. Genom att simulera traumat kan man kunna få sprickpropagering i skallbenet och från den avgöra om scenariot är rimligt. Geometrin för skallen, suturer, hårbotten och hjärnan hos ett 2 månader gammalt spädbarns huvud erhölls genom CT-bilder och Voxel hexahedermeshning. Simulering av påverkan på huvudet från ett fall på 0,82 m höjd mot ett hårt golv simulerades i det icke-linjära finita element programmet LS-Dyna. Två scenarier simulerades: ett islag mot nack-hjässbenet och ett mot det högra hjässbenet. Sprickpropagering simulerades med en Chang-Chang Composite konstitutiv frakturmodell för skallbenet. Den omfattande mängd materialparametrar som sammanfattades i denna studie för att prediktera skallbensfrakturer hos spädbarnets har, enligt författarens kännedom, inte erhållits tidigare. Validering av modellernas förmåga att visa relativt korrekt sprickpropagering genomfördes genom att jämföra det erhållna frakturmönstret från simuleringarna med publicerade frakturmönster från spädbarn för fritt fall mot en hård yta mot nack-hjässbenet. Frakturmönstret befanns vara i god överensstämmelse med publicerade data. Brottmönstret i hjässbenet jämfördes med frakturmönstret från en tidigare konstruerad modell på KTH. Brottmönstren från modellerna visar vissa likheter men förbättringar av modellen och ytterligare valideringar måste genomföras. Table of contents Introduction ................................................................................................................................... 1 1.1 Thesis overview ............................................................................................................. 1 1.2 Background ................................................................................................................... 2 1.3 Aim of the project ......................................................................................................... 4 1.4 Anatomy of an infant cranium ....................................................................................... 4 1.4.1 The skull bones ...................................................................................................... 5 1.4.2 Sutures and fontanelles .......................................................................................... 7 1.4.3 Dura mater ............................................................................................................. 7 Biomechanical & material properties of infant skull and suture ................................................... 9 2.1 Bone and suture elastic behavior ................................................................................... 9 2.1.1 Isotropic material properties ................................................................................ 10 2.1.2 Transversely isotropic material properties .......................................................... 12 2.2 Parameter selection...................................................................................................... 14 2.2.1 Parameters obtained from literature .................................................................... 15 2.2.2 Parameters estimations ........................................................................................ 18 Creating Finite Element mesh from CT data ............................................................................... 20 3.1 Introduction to computed tomography imaging .......................................................... 20 3.2 Image segmentation ..................................................................................................... 21 3.2.1 3D Slicer .............................................................................................................. 22 3.2.2 DeVIDE ............................................................................................................... 25 3.2.3 Matlab .................................................................................................................. 27 3.3 Hexahedral meshing .................................................................................................... 28 3.3.1 Dicer .................................................................................................................... 28 3.3.2 Mapping .............................................................................................................. 29 3.3.3 Mesh quality ........................................................................................................ 32 Finite element Modeling ............................................................................................................. 35 4.1 Finite element simulation in LS-Dyna ........................................................................ 35 4.1.1 Governing equation ............................................................................................. 35 4.1.2 Discretization: Hexahedral element and its shape function ................................. 36 4.1.3 Time integration .................................................................................................. 38 4.1.4 Matrix calculation................................................................................................ 39 4.1.5 Volume integration .............................................................................................. 41 4.1.6 Hourglass control ................................................................................................ 42 4.2 Simulation in LS-Dyna ................................................................................................ 42 4.2.1 Model preparation ............................................................................................... 43 4.2.2 Constitutive models ............................................................................................. 45 4.2.3 Contact constraints .............................................................................................. 49 Simulation results ........................................................................................................................ 51 5.1 Parietal-Occipital impact ............................................................................................. 51 5.2 Parietal impact ............................................................................................................. 56 Discussion ................................................................................................................................... 61 6.1 Material properties ...................................................................................................... 61 6.2 Image segmentation