Non-rigid Point Set Registration with Application to Human Masticatory Muscle Deformation

Ph.DDOCTOR OF PHILOSOPH

How has the morphology of the human mandible varied in response to the dietary changes that have occurred in Britain between the Neolithic and Post-Medieval periods?

Unlike cranial morphology, human mandibular morphology has been found to be influenced primarily by environmental as opposed to genetic factors. Previous research has demonstrated that significant morphological changes have occurred in the mandible during the Agricultural and Industrial Revolutions when widespread dietary changes occurred, and diet became softer. During this time the size of the mandible decreased, and mandible morphology became more gracile. This research however has typically focussed on comparisons between two time periods associated with a large dietary transition. For this reason, it is not known if the reported changes in mandible morphology represent a consistent reduction in mandibular robusticity or rather fluctuation between periods associated with dietary variability. Furthermore, it is unclear how susceptible the mandible is to smaller dietary changes. The aim of this thesis is to investigate how responsive mandible morphology is to the dietary changes that have occurred in Britain from the Neolithic through to the Post-Medieval periods. It is hypothesised that mandible morphology is responsive enough to mechanical stimuli that more minor dietary changes will also result in morphological variation. Results indicated that in general mandible morphology became increasingly gracile over time, while more pronounced morphological changes are associated with major dietary transitions such as the intensification of agriculture and the Industrial Revolution. Moreover, significant increases in gracilisation occurred between the Anglo-Saxon and Medieval period, potentially indicating that the dietary changes that occurred during the Industrial Revolution were more gradual than previous research has indicated. In addition to the more prominent morphological changes, smaller morphological fluctuations occurred such as during the Roman occupation. It is concluded that the mandible is more responsive to smaller dietary changes than demonstrated by previous research and the morphological variation associated with these major dietary transitions may not have been as simple or rapid as previously assumed

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

Multibody dynamics modelling of the masticatory system of the house mouse (Mus musculus)

Understanding the function of masticatory system of the mouse, the model of choice for craniofacial studies is invaluable, yet has been investigated poorly. Multibody dynamic analysis (MDA), which is a 3D computer modelling technique used in this study, is ideally suited to replicate and investigate this complex system. To mechanically solve this intricate system, system indeterminacy should be tackled using optimisation algorithms. The mouse has two types of teeth, hence two very different types of biting: incisal and the molar biting. To understand the masticatory function, modelling these two types of biting is invaluable. This study aims to investigate the differences in muscle function between incisors and molar biting. It was hypothesized that the generated bite force in the first molar would be higher than the incisor, due to the mechanical advantage of the latter. Moreover, the model sensitivity to the optimisation algorithms and the constraint types as well as muscle attributes such as intrinsic stress value and cross sectional area were studied. Functional development of the masticatory system of the mouse was an additional interest in this study.The first MDA model of the adult mouse masticatory system was developed and two optimisation algorithms, Dynamic geometric optimisation (DGO) and minimisation of overall muscle energy (MOME), were used to overcome the system indeterminacy. Furthermore, individual-specific adult model were developed and maximal and sub-maximal incisor and first molar biting were simulated. In addition, a simplistic model of the juvenile incisal biting was developed in which maximal incisal bite force and muscle activation pattern was studied.Some divergences were predicted from DGO and MOME, which were resulted from different basis of the activation factors in the two algorithms. Nevertheless, DGO was chosen as the optimisation algorithm mainly because it allowed for the simulation of a full biting cycle and for inclusion of some key developments in the future. The maximum predicted bite force in incisal biting was lower than the in vivo measurement, which was possibly due to averaging PCSA across specimens. A correction factor of 25% was added to muscle intrinsic stress value to compensate for this underestimation. Moreover as expected, the maximum predicted bite force at the first molar position was larger than that of the incisor. It was also found that the ratio of muscle forces between incisal and molar biting did not remain constant, however, was more consistent for simulation of low bite forces. In addition, incisal bite force in juvenile model was in agreement with in vivo bite force measurement from the same individual.MDA presented here provides a model which may be used to study many functional tasks and to investigate functional development and intertwined relationship between function and development in the mouse and similar rodents

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

The Functional Morphology of the Primate Zygomatic Arch in Relation to Diet

abstract: Craniofacial morphology in primates can vary on the basis of their diet because foods are often disparate in the amount and duration of force required to break them down. Therefore diet has the potential to exercise considerable selective pressure on the morphology of the masticatory system. The zygomatic arch is a known site of relatively high masticatory strain and yet the relationship between arch form and load type is relatively unknown in primates. While the relative position and robusticity of the arch is considered a key indicator of craniofacial adaptations to a mechanically challenging diet, and central to efforts to infer diet in past species, the relationships between morphology and diet type in this feature are not well established. This study tested hypotheses using two diet categorizations: total consumption percent and food material properties (FMPs). The first hypothesis that cortical bone area (CA) and section moduli (bone strength) are positively correlated with masticatory loading tests whether CA and moduli measures were greatest anteriorly and decreased posteriorly along the arch. The results found these measures adhered to this predicted pattern in the majority of taxa. The second hypothesis examines sutural complexity in the zygomaticotemporal suture as a function of dietary loading differences by calculating fractal dimensions as indices of complexity. No predictable pattern was found linking sutural complexity and diet in this primate sample, though hard object consumers possessed the most complex sutures. Lastly, cross-sectional geometric properties were measured to investigate whether bending and torsional resistance and cross-sectional shape are related to differences in masticatory loading. The highest measures of mechanical resistance tracked with areas of greatest strain in the majority of taxa. Cross-sectional shape differences do appear to reflect dietary differences. FMPs were not correlated with cross-sectional variables, however pairwise comparisons suggest taxa that ingest foods of greater stiffness experience relatively larger measures of bending and torsional resistance. The current study reveals that internal and external morphological factors vary across the arch and in conjunction with diet in primates. These findings underscore the importance of incorporating these mechanical differences in models of zygomatic arch mechanical behavior and primate craniofacial biomechanics.Dissertation/ThesisAppendix AAppendix BAppendix DDoctoral Dissertation Anthropology 201

Craniofacial integration, plasticity and biomechanics in the mouse masticatory system

The craniomandibular skeleton is a complex, dynamic structure, housing many vital tissues and required to perform critical functions. This region is however subject to substantial morphological change during development, and required to adapt to its environment and individual variance. The capacity of this region to maintain correlated form and appropriate functional performance despite these challenges is not fully understood. The sample consists of three strains of mice; a wild-type strain and two mutant strains from the same genetic background strain. Both mutations selectively affect chondrocranial growth, and thus influence of both are limited to the crania. The brachymorph mutant phenotype is characterised by a shortened cranium, while the pten is elongated. This sample therefore allows exploration of a potential plastic response in terms of the mandible, the masticatory lever system, and in turn mechanical advantage, when cranial length and the out-lever are varied. Three dimensional landmarks were applied to micro-CT scans and partial-least-squares analysis carried out to determine covariance between crania and mandibles. Mechanical advantage was calculated as a ratio of muscle in-lever and jaw out-lever for three key masticatory muscles. A common pattern of both variance and covariance was found among all three strains, with mandibular morphology in each strain covarying with cranial phenotypes. Jaw out-lever lengths were found to be significantly different in all three strains, and yet little significant difference between strains was found in mechanical advantage for any muscles. This maintenance of mechanical advantage is attributed to plastic adaptation in regions influencing muscle in-lever length, the latter which were found to be significantly different in the three strains. These results show the potential of the craniomandibular complex to plastically adapt to maintain both correlated form and functionality when variation occurs in one region, and thus these results have significant implications for the evolvability of the complex

Assessment of mandibular surface area changes in bruxers versus controls on panoramic radiographic images: A Case Control study

BACKGROUND: Bruxism was the commonest of the many parafunctional activities of the masticatory system. Opinions on the causes of bruxism were numerous and widely varying. It can occur on sleep as well as wakefulness. Bruxism was for long considered a major cause of tooth wear. Other effects of bruxism may include tooth movement and tooth mobility, as well as changes in oral soft tissues and jaw bone. Since the exact etiology and manifestations are unclear it was difficult to diagnose Bruxism. In this study we evaluated the area change that can occur on the lower jaw bone in those with Bruxism and comparing the results with nonbruxers. AIMS AND OBJECTIVE: To determine the surface area changes of the mandible, condylar and coronoid processes in Bruxers from Panoramic radiographs and to compare and contrast the changes with age and gender matched controls. MATERIALS AND METHOD: The study was conducted in the department of Oral Medicine and Radiology. The total sample size was 40.The sample was divided in to two groups, Bruxers and nonbruxers with 20 subjects in each group. Healthy volunteers aged between 20- 30 years diagnosed with Bruxism and Healthy volunteers aged between 20- 30 years diagnosed without Bruxism were included in group II (Non Bruxers). Bruxchecker was made use of in confirming the Bruxism in Group I. The Orthopantomogram was used as the imaging modality for the study. The measurements were made with the help of a software, Image J. All the measurements were tabulated and statistical analysis was made using ANOVA (Post hoc) followed by Dunnet t-test and unpaired t-test. RESULTS AND DISCUSSION: The present study was conducted to assess the mandibular surface area changes in bruxers and nonbruxers. It was carried out on a study group comprising 20 healthy individuals as controls in comparison with 20 bruxers (10 males and 10 females).A comparison of the mandibular surface area as a whole and also condylar and coronoid processes individually were carried out. Significant results were obtained in case of condylar and coronoid processes between the two groups. The surface area of condylar process of Group I was found to be lower than that of Group II. The surface area of the right coronoid process of group I was found to be less when compared to that of group II but the values of the left coronoid process of group I was found to be more when compared with group II. The surface area of the mandible showed no significant difference between the groups. There was significant difference between the genders in case of mandible, condyle and coronoid. The surface area of mandible and condylar process was found to be lower in female when compared to male. The surface area of coronoid process was found to be more in case of females when compared to that of males in Group I. The results of our study show that while the overall surface area of bruxers remain unaffected when compared to controls, the condylar and coronoid process show significant change. The hitherto belief that the primary brunt of bruxism is borne by the masseter would require a revisit since alteration in tonicity of the masseter would reflect in surface area change of the mandible as a whole. An increase in the surface area of the coronoid process in bruxers was observed in our study which could be attributed to altered activity of the temporalis, a muscle largely responsible for the posture of the mandible. This could imply that bruxers show alteration in temporalis activity which would explain several clinical manifestations such as headache, neck pain, shoulder pain and altered posture and so on which we have observed in the clinical practice of neuromuscular dentistry. Further studies examining the activity of the temporalis and masseter would further corroborate our findings and for the basis for future research in this arena. Conclusion: This study is an original study that was carried out to assess the surface area changes in mandible and condylar and coronoid processes of Bruxers and nonbruxers. The results showed significant changes in the surface area of condylar and coronoid process in Bruxers when compared to the controls. This study was a step made to assess the bony changes in Bruxers which is seldom carried out by other researchers. We hope this study would be a stepping stone for the future studies in this field

The Form-Function Complex of the Primate Masticatory Apparatus

Craniofacial morphology varies considerably between primate species with many aspects of structural variation occurring within the masticatory apparatus. These variations in masticatory fonn are frequently attributed to differences in diet however elucidating the relationship between masticatory form and function is complex. This study combines the techniques of shape analysis in a comparative study of the primate masticatory apparatus with three-dimensional biomechanical modelling of primate jaw mechanics. The relationship between masticatory form and function is investigated within a subfamily ofprimates, the Cercopithecinae. Species within this group display a range of masticatory forms, inhabit a wide range of environments, have varying diets and share close phylogenetic relationships making them an ideal group to investigate the relationship between structure and function. Using the techniques of geometric morphometrics a shape analysis was conducted in which variations within the masticatory forms of the Cercopithecinae were quantified. Functional predictions of observed shape differences were made and considered in light of known environmental and ecological factors with particular emphasis on dietary specialisations and fall back foods. The results of the shape analysis indicate that differences in body size, strata utilisation, and diet appear to play a major role in structuring adult morphological diversity within and among the Cercopithecinae. Many of the shape differences associated with an increased body size appear to reflect the selective pressures of increased predation risk, including increases in the length ofthe jaw and canine teeth. These morphological traits are associated with the production of a large gape and are hypothesised to be functional adaptations to the use of canines as weapons. Other major shape differences apparent between the Cercopithecinae were those associated with the proportion of leaves and fruit in the diet. The highly frugivorous Mandril/us species possessed features associated with increasing gape and retractile motions of the mandible ideal for incision of large fruits. By contrast the dietary specialist Theropithecus gelada possessed features associated with increasing the efficiency of food breakdown and muscle force production, necessary to process their more abrasive foodstuff (i.e. grasses)