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UBC Theses and Dissertations

Biomechanics of the mammalian jaw Zhang, Futang


Mammalian jaw biomechanics are not fully understood. They can be studied by different approaches, including, but not limited to, jaw cross-sectional measurements, stress and strain analysis, and computer modeling. Five studies comprise this thesis: In the first study, three cross-sections were examined with high-resolution computed tomography (CT) in the human jaw. Although the cross-sectional areas varied among the three locations, the cross-sectional masses were homogeneous, suggesting uniform shear rigidity. Despite similarities in shape among the three cross-sections, cortical bone thickness and density varied, indicating regional loading conditions may be determinants in the cross-sectional design. The second study tested two hypotheses. The first postulated that symphyseal stress and strain are similar in pigs and humans. The second proposed that the symphyseal orientation in the pig jaw keeps the stress and strain level within a functional range. Individual muscle lever arms, cross-sectional moments of inertia, symphyseal centroids, and mean muscle tensions were considered in the pig and human jaws. The estimated stress and strain levels were markedly similar for pigs and humans with their symphyses in normal "functional" orientations. However, the estimated strain for the pig mandible was higher than the reported maximum functional strain when the symphysis was in a simulated "upright" orientation. In the following two studies, pig and human jaw mass properties were estimated from CT scans. The mass and geometric centers were close in both pig and human mandibles, and consistently located at the last molar region, suggesting imaging methods revealing 3D shape alone can be used to estimate mass properties. Jaw mass and moments of inertia could also be predicted by simple dimensional measurements of the jaw. Dynamic modeling of individual jaws is, therefore, possible. The sensitivity of mass properties in dynamic modeling was confirmed in a previously published dynamic human jaw model. In the final study, the respective mass properties were estimated by CT for each half of a pig jaw split into two halves, and rejoined with a rigid link. Dorsoventral shear, medial and lateral transverse bendings were predicted in the pig jaw symphysis during a unilateral chewing stroke. The prediction supported the hypothesis that the pig symphyseal orientation is essential to keep symphyseal stresses and strains within functional levels.

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