Phenotypic diversity in white adults with moderate to severe Class II malocclusion

Abstract

Introduction: Class II malocclusion affects about 15% of the population in the United States and is characterized by a convex profile and occlusal disharmonies. The specific etiologic mechanisms resulting in the range of Class II dentoskeletal combinations observed are not yet understood. Most studies describing Class II phenotypic diversity have used moderate sample sizes or focused on younger patients who later in life might outgrow their Class II discrepancies; such a focus might also preclude the visualization of adult Class II features. The majority have used simple correlation methods resulting in phenotypes that might not be generalizable to different samples and thus might not be suitable for studies of malocclusion etiology. The purpose of this study was to address these knowledge gaps by capturing the maximum phenotypic variations in a large sample of white Class II subjects selected with strict eligibility criteria and rigorously standardized multivariate reduction analyses.

Methods: Sixty-three lateral cephalometric variables were measured from the pretreatment records of 309 white Class II adults (82 male, 227 female; ages, 16-60 years). Principal component analysis and cluster analysis were used to generate comprehensive phenotypes to identify the most homogeneous groups of subjects, reducing heterogeneity and improving the power of future malocclusion etiology studies.

Results: Principal component analysis resulted in 7 principal components that accounted for 81% of the variation. The first 3 components represented variation on mandibular rotation, maxillary incisor angulation, and mandibular length. The cluster analysis identified 5 distinct Class II phenotypes.

Conclusions: A comprehensive spectrum of Class II phenotypic definitions was obtained that can be generalized to other samples to advance our efforts for identifying the etiologic factors underlying Class II malocclusion.

Figure I

Principal Component Analyses. Seven principal components accounted for 81% of the variation in the Class II sample.

Figure II

Examples of individuals in opposite extremes of the distribution of principal component scores for each of the seven principal components accounting for 81 % of the variation, together with the highest loading ceph variables on each principal component. PC1 refers to variation in the inclination of the mandibular plane angle and explains 25% of the variation. PC2 explains 15% of the variation and refers to the maxillary incisor angulation. PC3 refers to the mandibular AP and vertical lengths as well as the posterior facial height and explains 12% of the variation. PC4 references the position of the maxilla, especially in regards to the maxillary incisor angulation and accounts for 9% of the variation. PC5 represents mandibular incisor angulation relative to the mandibular plane and the degree of facial taper and explains 8% of the variation. PC6 refers to the angulation of the cranial base and the AP position of the maxilla and explains 6% of the variation. PC7, explains 5%, and refers to variation on the WITS analysis (A–O to B–O) and the amount of overjet.

Figure III

3-D Plot showing 5 spatially distinct clusters of CII malocclusion subjects.

Figure IV

Cluster centroids. Cluster 1 represented the mildest Class II skeletal profile of all groups with individuals presenting a slightly retrusive maxilla, mildly retropositioned mandible and normal vertical dimensions. Cluster 2 represented a moderate Class II malocclusion with a retrusive mandible, dental compensations with retrusive upper incisors and protusive lower incisors and a normal vertical dimension. Cluster 3 represents a more severe Class II skeletal profile in which patients present with both maxillary protrusion and mandibular retrusion and a decrease anterior facial height and deep overbite. Cluster 4 represents patients with a smaller mandible and decreased ramus height, a very steep mandibular plane and a tendency to an anterior open bite. Finally, Cluster 5 represents patients with a very flat mandibular plane angle, short anterior face height, protrusive upper incisors, large overjet and deep overbite.