Geometric morphometric analysis of sexual dimorphism in the mandible from panoramic X-ray images

The human mandible is routinely utilised as part of the assessment of biological identity in forensic anthropological and odontological practice. The research introduces a novel geometric morphometric technique to investigate and quantify shape variation in the morphology of the mandibular corpus and ascending ramus and consequently highlights the potential for forensic purposes. Human mandibles from digital clinical orthopantomogram X-ray images, based on a sample of 50 male and 50 female adults from a modern Italian population, were examined. Three fixed landmarks were applied to the symphysis and condyle and 50 semi-landmarks re-sampled along the inferior corpus and the posterior ramus. Symmetrical reflection was applied yielding 200 configurations of 53 landmarks. Shape analyses were undertaken via: Procrustes superimposition; principal components analysis to investigate patterns of variation; classification using linear discriminant analysis with leave-one-out cross-validation; partial least squares (PLS) to test for structural modularity; and finally, retitle page sampling and re-analysis following PLS to optimize shape classification criteria. Stepwise re-sampling of landmarks reached an optimum cross-validated classification of 94.0% based on 25 landmarks; the results are strongly significant and suggest that the shape relationship between the mandibular corpus and ramus offers significant potential for forensic identification purposes using this method.