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

Finite element analysis of the cranium : Validity, sensitivity and future directions

Finite element analysis (FEA) is increasingly applied in skeletal biomechanical research in general, and in fossil studies in particular. Underlying such studies is the principle that FEA provides results that approximate reality. This paper provides further understanding of the reliability of FEA by presenting a validation study in which the deformations experienced by a real cadaveric human cranium are compared to those of an FE model of that cranium under equivalent simulated loading. Furthermore, model sensitivity to simplifications in segmentation and material properties is also assessed. Our results show that absolute deformations are not accurately predicted, but the distribution of the regions of relatively high and low strains, and so the modes of global deformation, are reasonably approximated

The effect of varying jaw-elevator muscle forces on a finite element model of a human cranium

Finite element analyses simulating masticatory system loading are increasingly undertaken in primates, hominin fossils and modern humans. Simplifications of models and loadcases are often required given the limits of data and technology. One such area of uncertainty concerns the forces applied to cranial models and their sensitivity to variations in these forces. We assessed the effect of varying force magnitudes among jaw-elevator muscles applied to a finite element model of a human cranium. The model was loaded to simulate incisor and molar bites using different combinations of muscle forces. Symmetric, asymmetric, homogeneous and heterogeneous muscle activations were simulated by scaling maximal forces. The effects were compared with respect to strain distribution (i.e. modes of deformation) and magnitudes; bite forces and temporomandibular joint (TMJ) reaction forces. Predicted modes of deformation, strain magnitudes and bite forces were directly proportional to total applied muscle force and relatively insensitive to the degree of heterogeneity of muscle activation. However, TMJ reaction forces and mandibular fossa strains decrease and increase on the balancing and working sides according to the degree of asymmetry of loading. These results indicate that when modes, rather than magnitudes, of facial deformation are of interest, errors in applied muscle forces have limited effects. However the degree of asymmetric loading does impact on TMJ reaction forces and mandibular fossa strains. These findings are of particular interest in relation to studies of skeletal and fossil material, where muscle data are not available and estimation of muscle forces from skeletal proxies is prone to error. This article is protected by copyright. All rights reserved

Can diet be inferred from the biomechanical response to simulated biting in modern and pre-historic human mandibles?

Differences among mandibular remains of past and present populations might be expected to reflect differences in loading history and so, diet. This is because evolutionary and experimental studies and orthodontic observations in modern humans indicate that adult mandibular form is influenced by genetic and loading history. In this study, we apply geometric morphometrics and biomechanical modelling to the mandibles of Upper Palaeolithic, Mesolithic hunter-gatherers and recent and living humans in order to assess if and how differences in adult form reflect subsistence strategies and so, masticatory system loading history. We show, using analyses of size and shape variation, that mandibular form in humans varies in a way that is consistent with the differences among subsistence groups. In particular mandibles from individuals who habitually fed on prepared and softened foods are small and show relative shortening of the mandibular body, among other differences. Using finite element analysis to simulate central incisor and first molar loading, we demonstrate that the performance of the human mandible in terms of resisting deformation covaries with mandibular form (size and shape). However, biomechanical performance in incisor or molar bites reflects only a proportion of the total variance in mandibular morphology; different aspects of morphology contribute to resisting different bites. Nevertheless, differences in biomechanical performance do reflect subsistence mode to some extent, especially for anterior bites. These differences are most strongly associated with mandibular size, the relative length of the body and the form of the gonion; which in turn reflect the degree of mandibular development, and so, loading history. While small, modern mandibles are more efficient at converting muscle to biting forces because of their shortened out lever arm (the body) they are not as capable of withstanding these loads and, for the same input force, deform more relative to upper Palaeolithic and Mesolithic individuals. Thus, we conclude that the differences between modern and prehistoric humans principally arise due to reduced mandibular loading during ontogeny rather than as adaptations to softer diets; they reflect underdevelopment. As such, it is unlikely that morphological and functional comparisons of mandibles across cultural transitions can differentiate anything other than broad aspects of loading history at a population level

Effects of different segmentation methods on geometric morphometric data collection from primate skulls

1. An increasing number of studies are analysing the shapes of objects using geometric morphometrics with tomographic data, which are often segmented and transformed to three‐dimensional (3D) surface models before measurement. This study aimed to evaluate the effects of different image segmentation methods on geometric morphometric data collection using computed tomography data collected from non‐human primate skulls. 2. Three segmentation methods based on a visually selected threshold, a half‐maximum height protocol and a gradient and watershed algorithm were compared. For each method, the efficiency of surface reconstruction, the accuracy of landmark placement and the level of variation in shape and size compared with various levels of biological variation were evaluated. 3. The visual‐based method inflated the surface in high‐density anatomical regions, whereas the half‐maximum height protocol resulted in a large number of artificial holes and erosion. However, the gradient‐based method mitigated these issues and generated the most efficient surface model. The segmentation method used had a much smaller effect on shape and size variation than interspecific and inter‐individual differences. However, this effect was statistically significant and not negligible when compared with intra‐individual (fluctuating asymmetric) variation. 4. Although the gradient‐based method is not widely used in geometric morphometric analyses, it may be one of promising options for reconstructing 3D surfaces. When evaluating small variations, such as fluctuating asymmetry, care should be taken around combining 3D data that were obtained using different segmentation methods

Validity and sensitivity of a human cranial finite element model: Implications for comparative studies of biting performance

Finite element analysis (FEA) is a modelling technique increasingly used in anatomical studies investigating skeletal form and function. In the case of the cranium this approach has been applied to both living and fossil taxa to (for example) investigate how form relates to function or infer diet or behaviour. However, FE models of complex musculoskeletal structures always rely on simplified representations because it is impossible completely to image and represent every detail of skeletal morphology, variations in material properties and the complexities of loading at all spatial and temporal scales. The effects of necessary simplifications merit investigation. To this end, this study focuses on one aspect, model geometry, which is particularly pertinent to fossil material where taphonomic processes often destroy the finer details of anatomy or in models built from clinical CTs where the resolution is limited and anatomical details are lost. We manipulated the details of a finite element (FE) model of an adult human male cranium and examined the impact on model performance. First, using digital speckle interferometry, we directly measured strains from the infraorbital region and frontal process of the maxilla of the physical cranium under simplified loading conditions, simulating incisor biting. These measured strains were then compared with predicted values from FE models with simplified geometries that included modifications to model resolution, and how cancellous bone and the thin bones of the circum-nasal and maxillary regions were represented. Distributions of regions of relatively high and low principal strains and principal strain vector magnitudes and directions, predicted by the most detailed FE model, are generally similar to those achieved in vitro. Representing cancellous bone as solid cortical bone lowers strain magnitudes substantially but the mode of deformation of the FE model is relatively constant. In contrast, omitting thin plates of bone in the circum-nasal region affects both mode and magnitude of deformation. Our findings provide a useful frame of reference with regard to the effects of simplifications on the performance of FE models of the cranium and call for caution in the interpretation and comparison of FEA results

The biting performance of Homo sapiens and Homo heidelbergensis

Modern humans have smaller faces relative to Middle and Late Pleistocene members of the genus Homo. While facial reduction and differences in shape have been shown to increase biting efficiency in Homo sapiens relative to these hominins, facial size reduction has also been said to decrease our ability to resist masticatory loads. This study compares crania of Homo heidelbergensis and H. sapiens with respect to mechanical advantages of masticatory muscles, force production efficiency, strains experienced by the cranium and modes of deformation during simulated biting. Analyses utilize X-ray computed tomography (CT) scan-based 3D models of a recent modern human and two H. heidelbergensis. While having muscles of similar cross-sectional area to H. heidelbergensis, our results confirm that the modern human masticatory system is more efficient at converting muscle forces into bite forces. Thus, it can produce higher bite forces than Broken Hill for equal muscle input forces. This difference is the result of alterations in relative in and out-lever arm lengths associated with well-known differences in midfacial prognathism. Apparently at odds with this increased efficiency is the finding that the modern human cranium deforms more, resulting in greater strain magnitudes than Broken Hill when biting at the equivalent tooth. Hence, the facial reduction that characterizes modern humans may not have evolved as a result of selection for force production efficiency. These findings provide further evidence for a degree of uncoupling between form and function in the masticatory system of modern humans. This may reflect the impact of food preparation technologies. These data also support previous suggestions that differences in bite force production efficiency can be considered a spandrel, primarily driven by the midfacial reduction in H. sapiens that occurred for other reasons. Midfacial reduction plausibly resulted in a number of other significant changes in morphology, such as the development of a chin, which has itself been the subject of debate as to whether or not it represents a mechanical adaptation or a spandrel

A sensitivity study of human mandibular biting simulations using finite element analysis

The form of human mandible reflects both genetic history and loading. In the context of archaeology, it has been used to retrodict loading history as a means of inferring subsistence strategy and paramasticatory use of the dentition. Rather than relying on form to retrodict function, an alternative is to simulate function and compare performance. Finite element analysis (FEA) offers the prospect of predicting and comparing the performance of mandibles under specific loading scenarios, for instance, simulated biting. However, its application depends on the sensitivity of the approach to variation and error in the initial and boundary conditions such as size and shape of the mandible, material properties of the bone tissue, muscle load vectors and the spatial constraints of the model. In the present paper we investigate the sensitivity of an FE model of a modern human mandible to simplifications in material properties and variations in boundary conditions. A medical CT scan of a living patient is used to create a range of FE digital models with different combinations of material properties, spatial constraints and muscle vectors. We then use ten individual CT scans of human mandibles to create simplified FE models all constrained and loaded in a standard way. We compare the development of von Mises strains over the surface of the mandibles, the output forces at the bite points and the modes and magnitudes global of deformations. Our results suggest that potential errors in segmentation, muscle force vectors, and constraints can have an appreciable effect on predictions of performance from FE analysis. Therefore, prediction of absolute strain magnitudes is uncertain. However, the errors are not large compared to the differences we find among the sample of mandibles, and FE analysis performs robustly in predicting relative, if not absolute, strains over the surface of a model. We suggest that a sensible approach in future comparative studies is to identically constrain and load ‘solid models’ comprising one homogenous material (e.g. with the properties of cortical bone). This limits studies to comparison of the effects of varying mandibular external form but such models reasonably predict relative strains, modes of global deformation and bite forces and so allow comparisons of these limited aspects of performance

Probabilistic biomechanical finite element simulations : whole-model classical hypothesis testing based on upcrossing geometry

Statistical analyses of biomechanical finite element (FE) simulations are frequently conducted on scalar metrics extracted from anatomically homologous regions, like maximum von Mises stresses from demarcated bone areas. The advantages of this approach are numerical tabulability and statistical simplicity, but disadvantages include region demarcation subjectivity, spatial resolution reduction, and results interpretation complexity when attempting to mentally map tabulated results to original anatomy. This study proposes a method which abandons the two aforementioned advantages to overcome these three limitations. The method is inspired by parametric random field theory (RFT), but instead uses a non-parametric analogue to RFT which permits flexible model-wide statistical analyses through non-parametrically constructed probability densities regarding volumetric upcrossing geometry. We illustrate method fundamen- tals using basic 1D and 2D models, then use a public model of hip cartilage compression to highlight how the concepts can extend to practical biomechanical modeling. The ultimate whole-volume results are easy to interpret, and for constant model geometry the method is simple to implement. Moreover, our analyses demonstrate that the method can yield biomechanical insights which are difficult to infer from single simulations or tabulated multi-simulation results. Generalizability to non-constant geometry including subject-specific anatomy is discussed