Functional significance of strain distribution in the human mandible under masticatory load: Numerical predictions

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Abstract

A common feature of studies of mandibular morphology is the assumption that there is some functional relation between the form of the lower jaw and masticatory stress. It was noted that the local variation in cortical bone thickness in the mandibular corpus appears to be stereotypical among anthropoids. This occurs at sections under the molars, where the lingual cortical plate is thinner than buccal one. In this study we investigate and contrast the strain pattern along buccal and lingual surfaces of the mandibular corpus during mastication using a numerical model of a human mandible. We show that strain distribution differs in alveolar and mid-corpus segments of the mandible and that the latter develops an alternate pattern between the buccal and lingual aspects of the working and balancing sides of the jaw. We then relate the magnitude of these strains to Frost's mechanostat. Our results suggest that the cortical asymmetry of the human mandible is in fact not related to strain patterns generated during mastication.

Introduction

To preserve its competence bone requires functional mechanical stimulation. Absence of functional loading will cause changes to the microstructure of the bone and may possibly lead to atrophy.1, 2, 3 A common feature of studies of mandibular morphology is the assumption that there is some functional relation between the form of the lower jaw and masticatory stress.4 This relationship between diet and mandibular bone mass was linked to allometric factors in comparative studies.5, 6, 7 Furthermore, it was noted that the local variation in cortical bone thickness in the mandibular corpus appears to be stereotypical among anthropoids. This occurs at sections under the molars, where the lingual cortical plate is thinner than buccal one.8

The functional significance of this postcanine cortical asymmetry is still obscure.

It is widely accepted that local variation in bone area or mass is linked with differences in strain magnitude during function and hence strain analysis can be employed to investigate whether cortical asymmetry is linked with biomechanical demands in mastication. Following a theoretical analysis, Demes et al.9 suggested that during mastication, occlusal force combined with the twisting moment can result in an increase of stresses on the buccal cortical region and their decrease on the lingual cortical area. In vitro investigations8 provided some backing for this approach but strains determined in the lateral midcorpus and alveolar segments were inconsistent and did not support the model.

Because of obvious practical reasons, a direct strain measurement in the functioning human mandible cannot be achieved, and hence these have to be inferred. It was suggested that finite element modelling may be used instead to predict real biomechanical responses in mandibular models.10

The objective of the present study is to investigate and contrast the strain pattern along buccal and lingual surfaces of the mandibular corpus during mastication. To do this we construct a 3D numerical model of a human mandible and evaluate strains generated during lateral mastication. We show that strain distribution differs in alveolar and mid-corpus segments of the mandible and that the latter develop an alternate pattern between the buccal and lingual aspects of the working and balancing sides of the jaw. We then relate the magnitude of these strains to Frost's mechanostat of bone remodelling. Our results suggest that the cortical asymmetry of human mandible is negatively related with the structural load developed during mastication.

Section snippets

Finite element model

A three-dimensional fully dentate model of an adult human mandible was reconstructed based on the data obtained from CT taken at 1 mm thickness with 0.5 mm interpolations.

As in such previous studies, parameters of our model were determined by measurements on a single skull.11, 12 Based on greyscale profiles of the CT slices, we initially generated polygonal volumes for the cortical bone, medullary bone and teeth using in-house software. These meshes were then patched with rational surfaces

Strains

The strains peaked along the alveolar margin, on the buccal aspect of the working side, adjacent to the teeth which receive the occlusal load (Fig. 3A and B). In the alveolar segment, the values of the strain on the buccal aspect are constantly higher than the corresponding lingual ones. On the working side the bucco-lingual strain gradient is more accentuated in the postcanine segments of the corpus with a premolar bite, resulting in a more marked difference than a molar bite (Fig. 3B). On the

Discussion

In this study we investigated the strain and stress profiles of the loaded mandible along the alveolar bone, mid-corpus and lower border using a numerical model validated experimentally (Fig. 2). A proper assessment of structural loading pattern during mastication may be used to explain the anatomical features of the mandible, like the asymmetry of the postcanine cortical bone thickness in humans.

It is known that morphological variation in bone is due predominantly to its self-optimisng

Acknowledgements

The authors would like to thank Dr. David Daegling from Department of Anthropology, University of Florida for his very useful comments and suggestions regarding the present work. This paper was partially funded by a University of Otago Research Grant as well as a Deputy Vice-Chancellor's award to the Craniofacial Biomechanics Group (Otago University). Also, the authors would like to thank to the School of Mechanical Engineering, James Cook University, Queensland, Australia for providing access

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