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 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

The transition to agriculture and industrialization changed the human face. Can vegetarianism be a new factor of change?. Review of the literature

Hace aproximadamente once mil años el ser humano pasó de alimentarse de los animales que cazaba o pescaba y los frutos y plantas que recolectaba, a aquellos que podía crecer a voluntad mediante la agricultura. Este cambio alteró dramáticamente la forma de la cabeza ósea y particularmente de la cara, que se hizo más grácil, y además se redujo el aporte de nutrientes claves. Con la industrialización, hubo un gran deterioro de la salud oral. Al abandono de una dieta cazadora recolectora incluso se le ha atribuido el origen de maloclusiones dentales, debido a la reducción del estrés masticatorio y con ello un menor tamaño relativo del maxilar y la mandíbula respecto a los dientes. Hoy en día, existiendo una mayor conciencia de la población respecto de sus cuidados y de su entorno, la adopción de dietas que excluyen cierto tipo de alimentos como los animales ha ganado adeptos. La dieta vegetariana presenta varias características que pueden afectar el metabolismo general y el óseo en particular, de manera similar a como lo hizo la agricultura en el pasado. La presente revisión busca analizar los cambios de la dieta humana, del punto de vista nutricional y mecánico y cómo estos afectaron la forma de la cara. Esto con el objetivo de comprender los posibles efectos de la introducción de alimentaciones de tipo restrictivas, como la vegetariana, en el organismo y particularmente en la anatomía facial.Approximately eleven thousand years ago humans beings went from feeding on animals they hunted or fished and fruits and plants they gathered, to crops they could grow through agriculture. This change dramatically altered the shape of the skull, particularly the face, which became more gracile, and also reduced the contribution of key nutrients. Along with industrialization, there was great deterioration of oral health. Leaving behind the hunter-gatherer diet has even been attributed to the origins of dental malocclusions, as masticatory stress was reduce, and reducing the size of the maxilla and mandible with respect to the teeth. Nowadays, there is greater awareness in the general population regarding personal care and their surroundings. Diets that exclude certain types of foods such as animal products are becoming more prevalent. The vegetarian diet has several characteristics that can affect metabolism, particularly the bones, as did the change from hunter-gatherer to agriculture in the past. The present review seeks to analyze the changes of the human diet, from the nutritional and mechanical point of view and how these have affected the shape of the face. This in order to understand the possible effects of the introduction of restrictive type feeds, such as the vegetarian diet in the body, particularly in facial anatomy

Differences in masticatory loads impact facial bone surface remodeling in an archaeological sample of South American individuals

The reduction of masticatory strains is considered one of the main factors that led to a pronounced morphological variation of the facial skeleton among modern humans. Although the archaeological record has provided evidence of bone remodeling activity being linked to craniofacial variation, its link with subsistence strategies has been proposed but not yet tested. Here, we evaluate the relationship between the strains arising from masticatory loads in the facial bones and the observed surface bone remodeling activity in adults and subadults from archaeological sites from South America that exerted different masticatory loads during life. We simulated the impact of mechanical loading during I1 and M1 bite using finite element analysis in six skulls from two archaeological samples, one of hunter-gatherers from Patagonia and the other of horticulturists from Northwest Argentina. The extension and distribution of bone formation and resorption were registered by a periosteal bone surface analysis on facial bones. We found a similar spatial distribution of high and low strains between samples and across ages, but different magnitudes. In general, compression strains corresponded with resorption activity, while tension strains corresponded with formation activity. Our results show a relationship between mechanical bone response to masticatory loading and bone remodeling activity, which can ultimately shape cranial morphology. We propose that although there are differences in skull morphology among populations that are established early in ontogeny, mechanical loading produced during mastication can enhance such differences. These results then support the idea that craniofacial morphology can contribute to reconstructing the history of past populations.Fil: Brachetta Aporta, Natalia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; Argentina. Universidad de Chile; Chile. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Departamento de Antropología; ArgentinaFil: Toro Ibacache, María Viviana. Universidad de Chile; Chile. Institut Max Planck for Evolutionary Anthropology; Alemani

The Predictability from Skull Morphology of Temporalis and Masseter Muscle Cross-Sectional Areas in Humans

Artículo de publicación ISITo carry out functional simulations of the masticatory system that aim to predict strain magnitudes it is important to apply appropriate jaw-elevator muscle forces. Force magnitude estimation from directly measured muscle physiological cross-sectional area or anatomical cross-sectional area (CSA) is not possible for fossils and skeletal material from museum collections. In these cases, muscle CSAs are often estimated from bony features. This approach has been shown to be inaccurate in a prior study based on direct measurements from cadavers. Postmortem alterations as well as age changes in muscle form might explain this discrepancy. As such, the present study uses CT images from 20 living individuals to directly measure temporalis and masseter muscle CSAs and estimated cross-sectional areas (ECSAs) from bony features. The relationships between CSAs and ECSAs were assessed by comparing mean values and by examining correlations. ECSAs are up to 100% greater than CSA and the means of these variables for each muscle differ significantly. Further, ECSA is significantly correlated with CSA for temporalis but not masseter. Cranial centroid size is only significantly associated with CSA for temporalis. These findings indicate that ECSAs should be employed with caution in simulations of human masticatory system functioning; they do not reflect CSAs and it is plausible that this also applies to studies of closely related living and fossil taxa. When ECSAs are used, sensitivity analyses are required to determine the impact of potential errors

Jaw kinematics in South African Plio-Pleistocene hominins inferred from maxillary molar root morphology: implications for species identification

Max Planck Society FONDECYT (Chile) 11150175 FR-TAF3890 CGL2010-2086

Mandibular Bone Loss after Masticatory Muscles Intervention with Botulinum Toxin: An Approach from Basic Research to Clinical Findings

The injection of botulinum toxin type A (BoNT/A) in the masticatory muscles, to cause its temporary paralysis, is a widely used intervention for clinical disorders such as oromandibular dystonia, sleep bruxism, and aesthetics (i.e., masseteric hypertrophy). Considering that muscle contraction is required for mechano-transduction to maintain bone homeostasis, it is relevant to address the bone adverse effects associated with muscle condition after this intervention. Our aim is to condense the current and relevant literature about mandibular bone loss in fully mature mammals after BoNT/A intervention in the masticatory muscles. Here, we compile evidence from animal models (mice, rats, and rabbits) to clinical studies, demonstrating that BoNT/A-induced masticatory muscle atrophy promotes mandibular bone loss. Mandibular bone-related adverse effects involve cellular and metabolic changes, microstructure degradation, and morphological alterations. While bone loss has been detected at the mandibular condyle or alveolar bone, cellular and molecular mechanisms involved in this process must still be elucidated. Further basic research could provide evidence for designing strategies to control the undesired effects on bone during the therapeutic use of BoNT/A. However, in the meantime, we consider it essential that patients treated with BoNT/A in the masticatory muscles be warned about a putative collateral mandibular bone damage

Geometric morphometrics and the study of biologic shapes: From descriptive to quantitative morphology Morfometría geométrica y el estudio de las formas biológicas: De la morfología descriptiva a la morfología cuantitativa

Morphometrics is the study of co-variation of biological form and its causes. Its development over the last decades has reached several biological sciences with a traditional descriptive approach, such as morphological sciences. The new geometric morphometric tools allow not only objective quantitative analysis, but also to assess qualitative traits due to the chance of recovering the form under study. This is possible because of the application of biometry techniques, instruments and software that allow the acquisition and analysis of shape coordinates that represent the geometry of the specimen, and that are not limited to obtaining linear data that lack of precision and amount of information of geometric data. Geometric morphometric analysis consists of three fundamental steps: obtaining primary data, obtaining shape variables and statistical analysis. The extensive use of this technique in areas related to morphological sciences over the last years makes geometric morphometrics a