Thin-plate spline analysis of the mandible in subjects with Class III malocclusion

The objective of this research was to determine the deformations that contribute to a Class III mandibular morphology, employing thin-plate spline (TPS) analysis. Lateral cephalographs of 100 Taiwanese young adults (50 males and 50 females) with Class III malocclusion, were compared to those of 100 counterparts with normal occlusion. The cephalographs were traced, and 12 homologous landmarks were identified and digitized. Average mandibular geometries were generated by means of Procrustes analysis. The mandibular morphology differed statistically between normal and Class III subjects for F-test in Procrustes analysis (p<0.001). TPS analysis allowed for graphic display of shape changes between normal and Class III configuration. The total spline revealed that an extension in the regions of mandibular condyle and ramus, and of the antero-inferior portion of mandibular symphysis. The partial warp with the largest magnitude confirmed the extension in the chin area. It is concluded that the size and shape of the mandible differs in Class III subjects. The deformations may represent a developmental elongation of the mandible anteroposteriorly, leads to the appearance of prognathic mandibular profile. (This study was supported by the National Science Council, Grant # NSC 89-2314-B-181, Taiwan).