Variation of mandibular sexual dimorphism across human facial patterns

This study analysed how sex-specific features differed in male and female adult mandibles throughout the spectrum of vertical facial patterns (i.e., meso-, dolicho- and brachyfacial) and sagittal variations (the so-called skeletal Classes I, II and III; normal maxillo-mandibular relationship, maxillary prognathism vs. mandibular retrognathism, and maxillary retrognathism vs. mandibular prognathism, respectively). Specifically, we test the hypothesis that sexual dimorphism in the mandible is independent of such facial vertical and sagittal patterns. A sample of 187 European adults (92 males, 95 females; age range, 20-30 years; mean age 25.6 years, sd=4.2 years) from Granada (southern Spain) were randomly selected and grouped according to the standard cephalometric criteria of the sagittal and vertical patterns. Geometric morphometrics were used to analyse the size (centroid size) and shape (principal components analysis, mean shape comparisons) of the mandible. The patterns of sexual dimorphism were evaluated with a generalised linear model with interaction term. We found that sagittal and vertical facial patterns are associated with different mandibular morphologies (size and shape). Also, sexual dimorphism was present in all comparisons. The hypothesis was rejected only for vertical facial patterns. That is, the nature of sexual dimorphism was similar among the skeletal classes but different (e.g., distribution of dimorphic variables, interaction term) in meso-, dolicho-, and brachyfacial mandibles. In conclusion, sex-specific mandibular traits behave in a different way across vertical facial patterns. These results imply that an assessment of the vertical facial pattern of the individual is required before a sexual diagnosis of the mandible is proposed.