The Biomechanics of Partial Symphyseal Fusion in Hapalemur Examined Using Finite Element Analysis

Abstract

A wealth of research on primate feeding supports the hypothesis that fused mandibular symphyses, ubiquitous among anthropoids, strengthen the joint against “wishboning” stresses that occur as a result of transversely-oriented muscle forces. However, in mammals that include extensive transverse occlusal movements, fusion has been suggested to increase bite forces in the horizontal plane by more efficiently transferring balancing-side muscle forces to the biting side. Some strepsirrhines, including Hapalemur and Propithecus, exhibit varying degrees of partial fusion. Hapalemur feeds extensively on extremely tough bamboo, so increasing transverse bite forces could be selectively important. However, steep molar cusps limit the potential for transverse occlusal movements. We used finite element analysis to examine symphyseal mechanics in Hapalemur. We constructed one completely fused Hapalemur mandible model and five with “symphyseal plates” assigned increasingly stiff elastic moduli (5MPa – 600MPa). Models were constrained at the second molar and loaded with muscle forces. By comparing nodal displacements, we found that modest increases in symphyseal stiffness (0MPa – 50MPa) drastically reduce wishboning, suggesting partially fused symphyses may be nearly as strong as those that are completely ossified. In contrast, we found that stiff/fused symphyses do not increase transverse bite forces.