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Modelling the Human Skull: the Available Tools

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Modelling the Human Skull: the Available Tools A finite element study of the human cranium; The impact of morphological variation on biting performance María Viviana Toro Ibacache PhD in Medical Sciences The University of Hull and the University of York Hull York Medical School December 2013 Abstract This thesis investigated the relationship between craniofacial morphology and masticatory mechanics using finite element analysis (FEA). Chapter 1 is a literature review of the relevant background: bone mechanics, jaw-elevator muscle anatomy, imaging techniques, FEA and geometric morphometrics. The second, third and fourth chapters comprise experimental work aiming to provide a framework for FE model construction and loading. The second chapter aimed to validate the method for FE model building and assess the sensitivity of models to simplifications. Models with simplified bone anatomy and resolution predicted strains close to those measured experimentally. The third chapter assessed the predictability of muscle cross- sectional area (CSA) from bony features. It was found that muscle CSA, an estimator of muscle force, has low predictability. The fourth chapter assessed FE model sensitivity to variations in applied muscle forces. Results showed that a cranial FE model behaved reasonably robustly under variations in the muscle loading regimen. Chapter 5 uses the conclusions from the previous studies to build FE models of six human crania, including two individuals with artificial deformations of the neurocranium. Despite differences in form and the presence of deformation, all performed similarly during biting, varying mainly in the magnitudes of performance parameters. The main differences related to the form of the maxilla, irrespective of neurocranial deformation. The most orthognatic individuals with the narrowest maxilla showed the most distinctive deformation during incisor and molar bites, and achieved the greatest bite force efficiency. However, bite forces were similar among individuals irrespective of the presence of artificial deformation. This appears to relate to the preservation of normal dental occlusion, which in turn maintains similar loading and so morphogenesis of the mid face. Altogether, the results of this thesis show that FEA is reliable in comparing masticatory system functioning and point to how variations in morphology impact skeletal performance. 2 Table of contents Abstract ............................................................................................................................................... 2 List of figures ..................................................................................................................................... 7 List of tables ..................................................................................................................................... 10 Acknowledgments ........................................................................................................................... 11 Author’s declaration ........................................................................................................................ 13 Chapter 1 : Introduction and literature review ........................................................................... 14 1.1. Introduction ......................................................................................................................... 14 1.2. Bone morphology and biomechanics ............................................................................... 16 1.2.1. Mechanical response to loading: bone as a material ............................................... 17 1.2.2. Bone response to loading: mechanoreception and mechanotransduction.......... 20 1.2.2.1. Mechanoreception................................................................................................. 21 1.2.2.2. Mechanotransduction ........................................................................................... 23 1.2.2.3. Effector cell response ........................................................................................... 25 1.2.3. Concluding remarks .................................................................................................... 25 1.3. Jaw-elevator muscles: Anatomy and function ................................................................. 26 1.3.1. Skeletal muscle structure............................................................................................. 26 1.3.2. Jaw-elevator muscle structure .................................................................................... 28 1.3.3. Temporalis muscle anatomy ...................................................................................... 30 1.3.4. Masseter muscle anatomy ........................................................................................... 32 1.3.5. Medial pterygoid muscle anatomy ............................................................................. 34 1.3.4. Concluding remarks .................................................................................................... 35 1.4. Modelling the human skull: the available tools ............................................................... 36 1.4.1. Biomechanical models of the human skull .............................................................. 36 1.4.2. Building the structure for biomechanical analysis: three-dimensional imaging techniques ................................................................................................................................ 38 1.4.2.2. Magnetic resonance imaging (MRI).................................................................... 39 1.4.3. Finite element analysis and the study of skull biomechanics ................................ 41 1.4.3.1. The finite element method in mechanical analyses .......................................... 43 1.4.3.3. Interpretation of FEA results .............................................................................. 49 3 1.4.4. Geometric morphometrics and the study of skull shape ....................................... 50 Chapter 2 : Validation of a voxel-based finite element model of a human cranium using digital speckle interferometry ........................................................................................................ 57 2.1. Introduction ......................................................................................................................... 57 2.2. Materials and Methods ....................................................................................................... 60 2.2.1. Sample ........................................................................................................................... 60 2.2.2. In vitro strain measurement ......................................................................................... 60 2.2.3. Finite element analysis ................................................................................................ 64 2.2.3.1. Finite element model under experimental loading ........................................... 64 2.2.4. Comparison of cranial and model performance ..................................................... 67 2.2.4.1. Measured vs. predicted strains ............................................................................ 67 2.2.4.2. Experimental and predicted bite force ............................................................... 71 2.2.4.2. Large scale model deformation. .......................................................................... 71 2.2.5. Measurement error. ..................................................................................................... 73 2.3. Results ................................................................................................................................... 74 2.4. Discussion ............................................................................................................................ 83 2.5. Conclusion ............................................................................................................................ 86 Chapter 3 : Can muscle cross-sectional area be predicted from skull morphology? ............. 88 3.1. Introduction ......................................................................................................................... 88 3.2. Materials and Methods ....................................................................................................... 91 3.2.1. Sample ........................................................................................................................... 91 3.2.2. Data acquisition ........................................................................................................... 92 3.2.2.1. Skull shape and centroid size ............................................................................... 92 3.2.2.2. Measurement of jaw-elevator muscle CSAs ...................................................... 97 3.2.2.3. Estimation of muscle areas from bony proxies ................................................ 99 3.2.3. Analyses ....................................................................................................................... 101 3.2.3.1. Predictability of muscle CSA from muscle areas on bone and skull size ... 101 3.2.3.2. Association between skull shape and CSA of jaw-elevator muscles ........... 101 3.3. Results ................................................................................................................................. 104 3.3.1. Descriptive statistics
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