Comparison of relative mandibular growth vectors with high-resolution 3-dimensional imaging

Introduction: The mandibular rami and their endochondrally growing condyles develop in many directions relative to the variable anatomic patterns of the nasomaxilla and middle cranial fossae during growth and response to orthopedic treatment. Methods: High-resolution magnetic resonance images were used to compare 3-dimensional (3D) growth vectors of skeletal displacement and bone remodeling in 25 untreated subjects with Class II malocclusions, 28 subjects with Class II malocclusions who were treated with Fränkel appliance therapy, and 25 subjects with normal occlusions. Marked differences were noted over an 18-month observation period. The 3D coordinates of anatomic landmarks were registered by Procrustes fit to control for rotation, translation, and scale differences. Results: Compared with untreated Class II and normal-occlusion subjects, the treated group showed highly significant differences in the 3D displacement/remodeling vectors of gonion and pterygomaxillary fissure relative to condylion and middle cranial fossae bilateral skeletal landmarks, by using both permutation tests (P < .001) and a general linear multivariate model (P < .0001). Conclusions: In a prospective and systematically controlled study, we quantitatively described significant 3D rami skeletal compensations in the structural assembly of facial morphogenesis at the beginning of the adolescent growth spurt using novel modeling techniques. These techniques have facilitated quantification of relative 3D growth vectors to illustrate skeletal changes with Fränkel appliance therapy. Future studies are required to assess the long-term clinical significance of our findings.