A finite element study of the human cranium : the impact of morphological variation on biting performance

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

Musculoskeletal Models in a Clinical Perspective

This book includes a selection of papers showing the potential of the dynamic modelling approach to treat problems related to the musculoskeletal system. The state-of-the-art is presented in a review article and in a perspective paper, and several examples of application in different clinical problems are provided

The effect of isometric cervical strength, head impact location, and impact mechanism on simulated head impact measures in female ice hockey players

Head injuries in sport have become a growing concern due to the negative acute and chronic health effects manifested from concussion injuries. Ice hockey is a sport associated with a high rate of concussions, although most research has focused on concussions in men’s hockey. Comparatively, women’s hockey has not only seen a drastic increase in participation rates, but female hockey players also exhibit a higher concussion rate than male players, despite the “no body contact” rule that is founding characteristic of women’s hockey. In fact, female hockey players may be more at risk for concussions than their male counterparts. The concerning prevalence of concussions in women’s hockey has been identified, yet the factors contributing to the high risk of concussions are still unclear. Among others, factors such as cervical muscle strength, head impact location, and impact mechanism have all been discussed in the literature as potential variables influencing the risk of concussion in athletes. The influence of these factors on head impact biomechanics, however, have not been thoroughly investigated. Furthermore, women experience high rates of concussion that have been potentially linked to decreased cervical muscle strength; however, there is little research that has characterized cervical muscle strength among female hockey players and limited research that has developed a set of normative data for female hockey players. Consequently, the purpose of this study was twofold. The first purpose was to develop normative data on the isometric cervical muscle strength and anthropometrics of female hockey players. The second purpose was to examine the effect of neckform torque, head impact location, and impact mechanism on simulated head impact measures of peak linear acceleration, shear force, and injury risk in female hockey players. To address the first purpose, the isometric cervical strength of a sample of female hockey players (n= 25) was measured in flexion, extension, and side flexion. An average of the muscle strength in these three directions was then calculated to develop an average overall isometric cervical strength measure for each athlete. Overall cervical strength measures of 58.64 N, 76.01 N, and 108.27 N (SD=17.52 N) represented the 10th, 50th, and 90th percentiles, respectively, of the normally distributed dataset created from the sample. These measures were then scaled and transformed into torque measures to be appropriately modelled on a mechanical neckform to address Part II of the simulation study. The 10th, 50th, and 90th percentile isometric cervical strength measures corresponded to torque measures of 1.36 Nm (weak), 2.94 Nm (average), and 4.62 Nm (strong), respectively, as established through calibration and transformation of the data. To address the second purpose, three neckform torques (weak, average, and strong), three helmet impact locations (front, rear, and side), and two impact mechanisms (direct and whiplash+impact) were tested at 16 different drop speeds using a dual-rail vertical drop system. The outcome measures included peak linear acceleration, shear force, and Gadd Severity Index, as these are variables commonly used to assess concussions in athletes

Biomechanical Constraints on Molar Emergence in Primates

abstract: Across primates, molar-emergence age is strongly correlated to life-history variables, such as age-at-first-reproduction and longevity. This relationship allows for the reconstruction of life-history parameters in fossil primates. The mechanism responsible for modulating molar-emergence age is unknown, however. This dissertation uses a biomechanical model that accurately predicts the position of molars in adults to determine whether molar emergence is constrained by chewing biomechanics throughout ontogeny. A key aspect of chewing system configuration in adults is the position of molars: the distal-most molar is constrained to avoid tensile forces at the temporomandibular joint (TMJ). Using three-dimensional data from growth samples of 1258 skulls, representing 21 primate species, this research tested the hypothesis that the location and timing of molar emergence is constrained to avoid high and potentially dangerous tensile forces at the TMJ throughout growth. Results indicate that molars emerge in a predictable position to safeguard the TMJ during chewing. Factors related to the size of the buffer zone, a safety feature that creates greater stability at the TMJ during biting, account for a large portion of both ontogenetic and interspecific variation in the position of emergence. Furthermore, the rate at which space is made available in the jaws and the duration of jaw growth both determine the timing of molar emergence. Overall, this dissertation provides a mechanical and developmental model for explaining temporal and spatial variation in molar emergence and a framework for understanding how variation in the timing of molar emergence has evolved among primates. The findings suggest that life history is related to ages at molar emergence through its influence on the rate and duration of jaw growth. This dissertation provides support for the functionally integrated nature of craniofacial growth and has implications for the study of primate life history evolution and masticatory morphology in the fossil record.Dissertation/ThesisDoctoral Dissertation Anthropology 201

Determining impact intensities in contact sports

Most sports Personal Protective Equipment (PPE) consist of varying levels of foam – more foam equals more protection. This has led to bulky, cumbersome PPE which restricts user movement. However, before existing PPE can be modified, their performance must be assessed and a baseline for necessary protection must be explicitly determined. This is a major limitation since current techniques for assessing PPE performance and impact intensity measurements from sport have used surrogate anvils and impactors which were not validated for the sports-related impact they tried to replicate. Through a series of independent studies, a better understanding of human impact response in sporting impacts was sought. This included investigating methods for improving the measurement of impact intensities in sports and the assessment of PPE performance. Human impact response revealed that tensed muscle led to a significant increase in impact force but was associated with less perceived discomfort. At low impact intensities common to sport, the increased local stiffness helped to dissipate impact energy and reduce soft tissue compression. As previous anvils omitted this soft tissue response, modifications were made to a martial arts dummy, BOBXL, to increase its biofidelity. This anvil was validated using in vivo kicks and an impact force – impact velocity relationship. Using this validated anvil, existing methods of assessing PPE performance were evaluated. Current methods were found to create artificially comparable levels of force but did so by using an incorrect effective mass and impact velocity. In all tests, PPE performance was found to depend on weight providing evidence of the ‘more protection, more foam’ concept. As it is impractical to use in vivo kicks to assess PPE performance, kick kinematics were investigated to assess its variability in terms of the impact force – impact velocity relationship and its accuracy. This aided in the development of a mechanical kicking robot which could more properly assess PPE performance. This research was applied to the design of form-fitting, impact-mitigating sports PPE with the capability for integrated technology. Proposed amendments to the current methods of assessing PPE will help to develop better testing and better performing PPE in the future

Explore the Dynamic Characteristics of Dental Structures: Modelling, Remodelling, Implantology and Optimisation

The properties of a structure can be both narrowly and broadly described. The mechanical properties, as a narrow sense of property, are those that are quantitative and can be directly measured through experiments. They can be used as a metric to compare the benefits of one material versus another. Examples include Young’s modulus, tensile strength, natural frequency, viscosity, etc. Those with a broader definition, can be hardly measured directly. This thesis aims to study the dynamic properties of dental complex through experiments, clinical trials and computational simulations, thereby bridging some gaps between the numerical study and clinical application. The natural frequency and mode shapes, of human maxilla model with different levels of integrities and properties of the periodontal ligament (PDL), are obtained through the complex modal analysis. It is shown that the comprehensiveness of a computational model significantly affects the characterisation of dynamic behaviours, with decreasing natural frequencies and changed mode shapes as a result of the models with higher extents of integrity and preciseness. It is also found that the PDL plays a very important role in quantifying natural frequencies. Meanwhile, damping properties and the heterogeneity of materials also have an influence on the dynamic properties of dental structures. The understanding of dynamic properties enables to further investigate how it can influence the response when applying an external stimulus. In a parallel preliminary clinical trial, 13 patients requiring bilateral maxillary premolar extractions were recruited and applied with mechanical vibrations of approximately 20 g and 50 Hz, using a split mouth design. It is found that both the space closure and canine distalisation of the vibration group are significantly faster and higher than those of the control group (p<0.05). The pressure within the PDL is computationally calculated to be higher with the vibration group for maxillary teeth for both linguo-buccal and mesial-distal directions. A further increased PDL response can be observed if increasing the frequency until reaching a local natural frequency. The vibration of 50 Hz or higher is thus approved to be a potential stimulus accelerating orthodontic treatment. The pivotal role of soft tissue the PDL is further studied by quantitatively establishing pressure thresholds regulating orthodontic tooth movement (OTM). The centre of resistance and moment to force ratio are also examined via simulation. Distally-directed tipping and translational forces, ranging from 7.5 g to 300 g, are exerted onto maxillary teeth. The hydrostatic stress is quantified from nonlinear finite element analysis (FEA) and compared with normal capillary and systolic blood pressure for driving the tissue remodelling. Localised and volume-averaged hydrostatic stress are introduced to describe OTM. By comparing with clinical results in past literature, the volume average of hydrostatic stress in PDL was proved to describe the process of OTM more indicatively. Global measurement of hydrostatic pressure in the PDL better characterised OTM, implying that OTM occurs only when the majority of PDL volume is critically stressed. The FEA results provide new insights into relevant orthodontic biomechanics and help establish optimal orthodontic force for a specific patient. Implant-supported fixed partial denture (FPD) with cantilever extension can transfer excessive load to the bone surrounding implants and stress/strain concentration which potentially leads to bone resorption. The immediate biomechanical response and long-term bone remodelling outcomes are examined. It is indicated that during the chewing cycles, the regions near implant necks and apexes experience high von Mises stress (VMS) and equivalent strain (EQS) than the middle regions in all configurations, with or without the cantilever. The patient-specific dynamic loading data and CT based mandibular model allow us to model the biomechanical responses more realistically. The results provide the data for clinical assessment of implant configuration to improve longevity and reliability of the implant-supported FPD restoration. On the other hand, the results show that the three-implant supported and distally cantilevered FPDs see noticeable and continuous bone apposition, mainly adjacent to the cervical and apical regions. The bridged and mesially cantilevered FPDs show bone resorption or no visible bone formation in some areas. Caution should be taken when selecting the FPD with cantilever due to the risk of overloading bone resorption. The position of FPD pontics plays a critical role in mechanobiological functionality and bone remodelling. As an important loading condition of dental biomechanics, the accurate assignment of masticatory loads has long been demanded. Methods involving different principles have been applied to acquire or assess the muscular co-activation during normal or unhealthy stomatognathic functioning. Their accuracy and capability of direct quantification, especially when using alone, are however questioned. We establish a clinically validated Sequential Kriging Optimisation (SKO) model, coupled with the FEM and in vivo occlusal records, to further the understanding of muscular functionality following a fibula free flap (FFF) surgery. The results, within the limitations of the current study, indicates the statistical advantage of agreeing occlusal measurements and hence the reliability of using the SKO model over the traditionally adopted optimality criteria. It is therefore speculated that mastication is not optimally controlled to a definite degree. It is also found that the maximum muscular capacity slightly decreases whereas the actual muscle forces fluctuate over the 28-month period

Recent hominim cranial form and function

This thesis aims to assess if biting mechanics drives craniofacial morphology in recent hominins. To that end, a virtual functional morphology toolkit, that includes Finite Element Analysis (FEA) and Geometric Morphometrics (GM), is used to simulate biting, measure bite force and quantify deformations arising due to simulated biting in Homo sapiens and its proposed ancestral species, Homo heidelbergensis. Moreover, the mechanical significance of the frontal sinus and of the brow-ridge is also assessed in Kabwe 1 (a Homo heidelbergensis specimen). The frontal sinus is examined by comparing the mechanical performance in three FE models with varying sinus morphology. A similar approach is applied to the brow-ridge study. This approach relies on the assumption that FEA approximates reality. Thus, a validation study compares the deformations experienced by a real cranium under experimental loading with those experienced by an FE model under equivalent virtual loading to verify this assumption. A sensitivity analysis examines how simplifications in segmentation impact on FEA results. Lastly, the virtual reconstruction of Kabwe 1 is described.Results show that prediction of absolute strain magnitudes is not precise, but the distribution of regions of larger and smaller (i.e. pattern of) deformations experienced by the real cranium is reasonably approximated by FEA, despite discrepancies in the alveolus. Simplification of segmentation stiffens the model but has no impact on the pattern of deformations, with the exception of the alveolus. Comparison of the biting performance of Kabwe 1 and H. sapiens suggests that morphological differences between the two species are likely not driven by selection of the masticatory system. Frontal sinus morphogenesis and morphology are possibly impacted by biting mechanics in the sense that very low strains are experienced by this region. Because bone adapts to strains, the frontal sinus is possibly impacted by this mechanism. Lastly, biting mechanics has limited impact on brow-ridge morphology and does not explain fully the enormous brow-ridge of Kabwe 1. Hence, other explanations are necessary to explain this prominent feature