Procrustes, Euclidean and cephalometric analyses of the morphology of the mandible in human Class III malocclusions.

The role of mandibular phenotype in the development of Class III malocclusion remains unclear. The purpose of this study was to determine whether the form of the mandible differed between prepubertal individuals with Class I and Class III malocclusions. Lateral cephalographs of 73 children of European-American descent aged between 5-11 years with Class III malocclusion were compared to those of 60 counterparts with a normal, Class I molar occlusion. The cephalographs were traced and checked, and eight homologous mandibular landmarks were digitized. Average mandibular geometries, scaled to an equivalent size, were generated using Procrustes superimposition. Euclidean distance matrix analysis (EDMA) was undertaken to corroborate the Procrustes analysis, and bivariate analysis utilizing eight linear and five angular measurements was also performed. Residuals and F-values from Procrustes analysis indicated that mandibular configurations differed statistically for Class I and Class III types. EDMA confirmed that the Class I and Class III geometries were significantly different, revealing that the greatest differences in morphology arose in the anterior-most mandibular regions. As well, most variables showed statistically significant differences when the Class I and Class III mandibular types were compared. When the sample was subdivided into seven age- and sex-matched groups, nearly all age-based comparisons were significantly different. It is concluded that the morphology of the mandible differs in individuals with Class III malocclusions when compared to the normal Class I configuration, and that these alterations may indicate dichotomous postnatal mandibular ontogeny.