Shape variability and sexual dimorphism of the chin: a 3D geometric morphometric analysis

BACKGROUND: The chin is a distinctive anatomical trait unique to Homo sapiens, with unclear origin and function. This study aimed to investigate chin morphology, describe shape variability, and identify potential sex differences. MATERIALS AND METHOD: The sample consisted of 95 CBCT images, from university archives. Inclusion and exclusion criteria were: age above 21 years, absence of craniofacial deformities, no missing or extracted teeth in the anterior mandibular area and no alveolar bone resorption. Raw DICOM data were converted into mesh surfaces. The following methodology comprised three steps: one manual and two automated resulting in coverage of the chin area by 640 landmarks. All landmark configurations were then superimposed with Procrustes Superimposition and the resulting shape information was reduced to Principal Components (PCs) of shape. These were used to describe shape variation in the sample. Sex differences were evaluated with permutation tests and allometry was assessed by regressing on the logarithm of centroid size. RESULTS: Symphyseal shape variation was primarily linked to its height-to-width ratio and to symphyseal thickness. Significant sex differences were found both in shape and size; male symphyses were 8.7% larger. Also, there were notable sex-related shape differences in mental tubercle prominence and mandibular curvature, however, these were minimized after removing the allometry effect. CONCLUSIONS: We suggest a fast and user-friendly 3D geometric morphometric method to assess chin morphology. We identified the main symphyseal shape patterns and sex differences, which were mostly related to size, indicating a strong allometric effect.