Geometric Morphometric Analysis of the Influence of Three-Dimensional Basicranial Shape on Facial Asymmetry
The basicranium is a key organizer of the skull. Importantly, the basicranium ossifies early during craniofacial development and is centrally located in the skull, thus it is thought to act as a structural “foundation” that organizes later growth of the facial skeleton. The goal of this study is to test the hypothesis that basicranial asymmetry directly and additively influences asymmetry of the facial skeleton. To do this, we used cone beam computed tomography (CBCT) scans obtained from the Orthodontic Clinic at the University of California San Francisco and analyzed them with Geometric Morphometrics (GM), a set of methods for quantifying and comparing three-dimensional shape. This approach enables us to evaluate hypotheses about the nature of craniofacial patterning and variation in different planes, including deviations from midline symmetry. In this study, we first obtained homologous 3D coordinates from the basicranium and facial skeleton of adolescent and adult patients (N=105). Next, we used Principal Components Analysis (PCA) and Partial Least Squares (PLS) to test between alternative models of covariation between basicranial and facial asymmetries. Our results support the hypothesis that the basicranium plays a significant role in patterning facial asymmetry, and this relationship is consistent with an additive model. Given the earlier relative timing of ossification of the basicranium, this suggests that basicranial shape is important for establishing the direction of later facial growth, which is relevant for clinical considerations.