Structural Failure and Fracture of Immature Bone
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STRUCTURAL FAILURE AND FRACTURE OF IMMATURE BONE Vee San Cheong Department of Bioengineering Imperial College London A thesis submitted for the degree of Doctor of Philosophy of Imperial College London and the Diploma of Imperial College October 2014 DECLARATION This dissertation is the result of my own work and all work that is not my own, including any assistance received, have been acknowledged. It has not been previously submitted, in part or whole, to any university of institution for any degree, diploma, or other qualification. Copyright © 2014 Vee San Cheong The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute, or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform, or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work. Every child deserves a chance at a life filled with love, laughter, friends and family. -Marlo Thomas Oh God, I am thinking Thy thoughts after Thee - Johannes Kepler ABSTRACT Radiological features alone do not allow the discrimination between accidental paediatric long bone fractures or those caused by child abuse. Therefore, for those cases where the child is unable to communicate coherently, there is a clinical need to elucidate the mechanisms behind each fracture to provide a forensic biomechanical tool for clinical implementation. 5 months old ovine femurs and tibiae were used as surrogates for paediatric specimens and were subjected to CT scans to obtain their geometrical and material properties. A novel methodology to align long bones so that they would be loaded in a state of pure bending and torsion was developed and compared against the use of a standard anatomical coordinate system. The second moment of area and its coefficient of variation (COV) for each alignment method were calculated to ascertain the reference axes that minimised the effect of eccentric loading. Wilcoxon-signed rank test showed a significant reduction in COV of the second moment of area using this new method, indicating that the bone has a more regular cross-section when this methodology is implemented. The algorithm generated the locations of subject-specific landmarks that can be used as a reference to align the bones in experimental testing. A low-cost platform that synchronized the data acquisition from the tensile testing machine and the strain gauges was built and used with a high speed camera to capture the fracture pattern in four-point bending at three strain rates and in torsion at two different strain rates, following commonly reported case histories. Finite element (FE) models of ovine tibiae in their optimised alignment were generated to replicate the fracture patterns that were obtained. Fracture initiation and propagation was simulated through the use of element deletion with a maximum principal strain criterion. The experiments produced transverse, oblique, and spiral fractures consistently, which were correlated with the finite element analysis, demonstrating the ability of this pipeline to now be adapted for use in forensic analysis. v ACKNOWLEDGEMENTS I am immensely thankful to my supervisor Professor Anthony Bull for all his help and guidance over the past few years. His passion and enthusiasm for research has inspired and influenced me greatly while his critical analysis and innovativeness have honed my thinking skills and analytical capabilities. Dr Pocha Meher has shown me clinical insights that I never imagined I would have the opportunity to be a part of, and has inspired me through her constant belief that the quality of life can always be improved. I would like to extend my deepest thanks to Professor Andrew Amis and Camilla Halewood for their kindness and support given in the experimental work. They have provided valuable resources and advice that made all the testing work possible. I am especially grateful for the friendship of Dr Kaiyu Zhang, who provided valuable counsel, thought-provoking discussions, and encouragement throughout the course of my PhD. A thesis of this nature would not have been made possible without the pastoral support of various people. I owe my since thanks to Rikke Juul, Leila Steeds, Sarah-Jo McClellan and Mary Bowe who walked with me at different stages of my PhD journey. I am also immensely thankful to Marsh Moyle and Merram Paul for showing me how to order chaotic thoughts and made the production of this thesis a reality. The drawing of a sheep in this dissertation is especially dedicated to Marsh. Special thanks also to John Barrs who helped to proofread the initial drafts of this thesis and gave valuable suggestions on developing my writing style. I am also thankful to my family for the feedback provided on the diagrams and pictures in this thesis. I am extremely grateful for the financial support from the Department of Bioengineering and the Singapore Institute of Management, without which this research would have never happened. Finally, I like to thank the creator of wonders, without whose help none of these would have happened. vi CONTENTS 1 INTRODUCTION ............................................................................................ 20 1.1 AIMS ............................................................................................................................ 21 1.2 THESIS ORGANISATION .............................................................................................. 22 2 CLINICAL DRIVERS FOR IMMATURE BONE FRACTURE RESEARCH ................. 23 2.1 INTRODUCTION ........................................................................................................... 23 2.2 INCIDENCE RATE AND EPIDEMIOLOGY OF LONG BONE FRACTURES ........................ 24 2.2.1 Fracture location ........................................................................................................ 25 2.2.2 Femoral shaft fracture ................................................................................................ 25 2.2.3 Humeral shaft fracture ............................................................................................... 26 2.2.4 Tibial and fibular fractures ........................................................................................ 26 2.3 FAILURE ANALYSIS ..................................................................................................... 27 2.4 FRACTURE MORPHOLOGY .......................................................................................... 30 2.4.1 Spiral fracture ........................................................................................................... 31 2.4.2 Oblique fracture ......................................................................................................... 32 2.4.3 Transverse fracture .................................................................................................... 33 2.4.4 Buckle fracture ........................................................................................................... 33 2.4.5 Greenstick fracture ..................................................................................................... 34 2.4.6 Comminuted fracture ................................................................................................. 34 2.4.7 Longitudinal fracture................................................................................................. 34 2.5 DETECTION OF CHILD ABUSE ..................................................................................... 34 2.6 PROPOSED FRAMEWORK ............................................................................................. 36 2.7 SUMMARY .................................................................................................................... 38 3 ANIMAL MODELS FOR BONE FRACTURE RESEARCH IN CHILDREN: A REVIEW........................................................................................................... 39 3.1 INTRODUCTION ........................................................................................................... 39 3.2 FEATURES OF THE IMMATURE PAEDIATRIC BONES ................................................... 39 vii 3.2.1 Strength .................................................................................................................... 39 3.2.2 Strain and plastic deformation ................................................................................... 41 3.2.3 Fracture pattern ........................................................................................................ 42 3.3 FACTORS OF CONSIDERATION IN SELECTING AN ANIMAL MODEL ........................... 43 3.3.1 Bone architecture and material properties .................................................................... 43 3.3.2 Comparative osteology ............................................................................................... 45 3.3.3 Age selection of animal model ..................................................................................... 47 3.3.4 Summary of literature review ..................................................................................... 48 3.4 COMPARATIVE ANATOMICAL FEATURES OF OVINE AND HUMAN FEMUR AND TIBIA 50 3.4.1 Ovine femur .............................................................................................................. 50 3.4.2 Ovine