Divided Zygomatic Bone in Primates With Implications of Skull Morphology and Biomechanics

Typically the zygoma is a single bone in the facial skeleton whose shape uniquely copes with loads associated with mastication. Rarely but naturally, the zygoma is divided into two or more parts by supernumerary sutures. These extra intrazygomatic sutures are located at an area of critical morphological and biomechanical importance, yet their impacts have not been studied. In this study, the morphological and possible biomechanical consequences of the divided zygoma (DZ) were investigated in primates including rhesus macaques (Macaca mulatta), orangutans (Pongo abelii and P. pygmaeus), and modern humans (Homo sapiens). Results demonstrated that a unilateral supernumerary suture within the zygoma affected facial symmetry. The superior division of the divided zygoma was normally slender along with the adjacent frontal bone parts; while the inferior division of the divided zygoma was normally more robust, along with stronger temporal and maxillary bones. These were possible biomechanical consequences, in which the stresses incurred during normal masticatory activities were shunted from the upper face to the lower face, especially along the zygomatic arch. These findings revealed that the DZ condition would alter overall morphology of the midface of the affected side, and unfavorably affect the pattern of stress distribution in the loaded side of the face during mastication. The developmental mechanisms for the supernumerary sutures dividing the zygoma were unclear. Further insights into this rare condition may deepen our understanding of craniofacial form, adaptation, developmental plasticity, and evolution, and help to improve therapeutic philosophies in corrective and regenerative medicine. Anat Rec, 299:1801-1829, 2016. © 2016 Wiley Periodicals, Inc.