The Effect of Tooth Loss on Craniofacial Morphology

The morphology of the craniofacial skeleton is known to be responsive to changing biomechanical loads produced by feeding. Edentulism, or tooth loss, has previously been shown to have an impact on the structure of the craniofacial bones, particularly the alveolar bone and palate, as well as on the material properties of the cortical bone. However, it is less clear what effect tooth loss has on the masticatory muscles and their bony attachment sites. In this study, we use geometric morphometrics on human skulls to compare the shape of the mandible and facial skeleton in edentulous and dentate individuals. Following from previous work, we expect that edentulous individuals will exhibit decreased alveolar bone height in both the mandible and maxilla. We further hypothesize that edentulous individuals will display a recession of bone at the insertion sites of the masticatory muscles, namely the zygomatic arch and the mandibular ramus. CT scans of individuals greater than 50 years of age (n=40) were obtained from the New Mexico Decedent Imaging Database (NMDID). Individuals with no teeth were sorted into the edentulous group, while individuals with more than 22 of 36 teeth were sorted into the functionally dentate. 3D Slicer software was used to generate 3D models, and 3D fixed and sliding landmarks were placed along the mandible, maxilla, zygomatic arch, palate, and orbit. Principal component analysis (PCA) and Procrustes ANOVAs were performed separately on the Procrustes-aligned coordinate data for the crania and mandibles. We identified significant differences in facial and mandibular shape between groups. In both cases, differences were largely driven by resorption along the alveolus. Variation in the morphology of edentulous individuals suggests that the time since tooth loss and behavioral factors, such as denture wearing, may impact the degree of alveolar resorption. Interestingly, edentulous individuals tended to have relatively smaller coronoid processes and posteriorly extended mandibular angles. This suggests that tooth loss may result in greater relative atrophy of the temporalis muscle, a jaw elevator/retractor, compared to the masseter muscle, another jaw elevator which is involved in resisting gravity and positional jaw closure. Further studies are needed to investigate the impact of tooth loss on muscle morphology to confirm this. These additive effects between bone resorption and muscle atrophy likely explain the altered facial morphology demonstrated by edentulous populations.