Morphological variations and differences of the mandible and midfacial skeleton across different sagittal skeletal patterns and sexes: a three-dimensional geometric morphometric analysis.

BACKGROUND: Maxillofacial morphological analysis is essential for orthodontic diagnosis, treatment planning, and outcome evaluation. However, previous two-dimensional or landmark-based studies can hardly comprehensively analyze three-dimensional morphology. Therefore, this study aimed to analyze morphological variations and differences of the mandible and midfacial skeleton across different sagittal skeletal patterns and sexes using three-dimensional geometric morphometrics, while also providing novel perspectives and methodologies for morphological analysis in orthodontics.

METHODS: The midfacial skeleton and mandible were analyzed separately. Three-dimensional dense templates were first constructed and template registration was validated. Through template registration, all segmented skeleton images (n = 186) obtained a homologous configuration with tens of thousands of quasi-landmarks. After generalized Procrustes analysis, principal component analysis (PCA) was conducted to examine shape variations. The average skeletal shapes were calculated and compared according to sagittal skeletal pattern and sex. Partial least squares regression (PLSR) was used to evaluate the independent effects of ANB angle and sex on skeletal shape.

RESULTS: PCA revealed the principal patterns of shape variations. The first principal component (PC1) of the mandible primarily showed variation in sagittal length and inter-ramal width (explaining 31.80% of the total variance), whereas PC1 of the midfacial skeleton mainly showed variation in vertical height and inter-zygomatic width (24.07%). Morphological differences were demonstrated through comparisons of average skeletal shapes grouped by sagittal skeletal pattern or sex. PLSR indicated significant effects of ANB angle and sex on the three-dimensional morphology of both the mandible and midfacial skeleton (all P < 0.001). Specifically, with increasing ANB angle (skeletal Class II tendency), the length of the condyle (condylar head and neck) decreased, the antegonial notch became more prominent, and the maxillary alveolar bone plane rotated downward and backward (clockwise). Compared with females, males exhibited a more prominent mandibular angle, increased inter-zygomatic width, and more protrusive nasal bridge and anterior nasal spine.

CONCLUSIONS: Three-dimensional dense geometric morphometrics enables a more comprehensive analysis of facial skeletal morphology. Three-dimensional morphological variations and differences of the mandible and midfacial skeleton across different sagittal skeletal patterns and sexes were characterized.