UCSF

Objective: To compare the difference in type and quantity of asymmetry between Class I, Class II, and Class III sagittal skeletal patterns using geometric morphometrics.

Methods: Surface models were constructed from pre-treatment three-dimensional CBCT scans of 144 patients (41 males, 103 females) randomly selected from the database at the University of California, San Francisco, Division of Orthodontics. There were a total of 62 skeletal Class I (0≤ANB≤4), 63 skeletal Class II (ANB>4), and 19 skeletal Class III (ANB<0) subjects. The surface models were constructed using Amira software (Mercury Computer Systems GmbH, Berlin, Germany)) for optimal viewing of various structures, including the maxilla, zygomatic arches, condyles, and mandibular body. Landmark placement (n=183) was completed using Landmark software (Institute for Data Analysis and Visualization (IDAV), UCDavis). Landmarks were identified for each individual and after Procrustes superimposition of the raw coordinates and deviations from bilateral symmetry were analyzed by Principal Components Analysis (PCA).

Results: Permutation tests of the Procrustes distance showed that there was a statistically significant difference between Class I and Class III groups and between the Class II and Class III groups, but no difference between Class I and Class II groups. Principal component 1 (PC1) was significant for anterior mandibular deviations to the left with compensation of the remaining craniofacial structures. The average PC1 score for the Class III group was significantly different than that of the Class I and Class II groups. Hartigan's dip test showed that asymmetries within Class III population may have a bimodal distribution with a predilection for left side deviations.

Conclusions: Asymmetries are more likely seen in skeletal Class III patients than in Class I and Class II patients. This asymmetry tends to be localized in the anterior mandible and is more often deviated to the left than right side.