Biomechanical studies of the zygoma: A review of in vivo (italic) and FEM studies of the lateral orbital wall and zygomatic arch

The zygomatic region and lateral orbital wall are important structural components of the primate craniofacial skeleton, providing attachment for muscles, housing and protecting the eye, and resisting and transmitting forces incurred during feeding and fighting. The design of this region must therefore meet the needs of a number of different functional systems. The relative importance of these different systems in driving zygomatic morphology across different primate lineages remains to be elucidated. The role of this region in resisting, transmitting and dissipating feeding forces is amenable to study using in vivo bone strain and finite element modeling techniques. Based on these techniques Ross et al. (2011) suggested that there is a common pattern of deformation of the catarrhine facial skeleton, namely bending in frontal/coronal planes. Following Endo (1966) and Rak (1983), this bending was hypothesized to be due to inferiorly directed muscle forces acting on the zygomatic arches, producing torsion and sagittal bending of the arches. The highest strains and stresses associated with this deformation regime are experienced by the anterior root of the zygoma. The bone in this area is hypothesized to experience high shear stresses resulting from inferiorly directed muscle forces laterally and superiorly directed bite forces medially. This model was based on in vivo bone strain data from Macaca, Papio, and Chlorocebus, frame-modeling of Gorilla and Homo, and finite element modeling (FEM) data from Macaca. Review of the available bone strain and/or FEM data for Cebus,Pan, and several fossil hominins reveals common patterns of strain across taxa with a range of morphologies and diets. Bone strain magnitudes in the arch and anterior root are high relative to strain magnitudes elsewhere in the cranium, suggesting an important role for structural strength in the design of this region. The notion of a fundamental framework of the catarrhine face receives some support from these data.