Bilateral Asymmetry in the Tooth Relationships of Orthodontic Patients

Malocclusions have been labeled “biological conflicts,” where harmony of the developmental processes has been lost. One consequence can be increased left-right asymmetries—either in positions of the teeth themselves or of the supporting bones. Objective: The nature and extent of bilateral dentoalveolar asymmetries were quantified in a collection of routine adolescent orthodontic patients. Methods: Eight left-right pairs of occlusal dimensions were measured from dental casts (n = 211 subjects), with proportionate samples of Class I, II, and III malocclusions. Two kinds of asymmetry were assessed, directional asymmetry where one side is characteristically larger and fluctuating asymmetry where the magnitude of inter-side differences is the focus. Results: Directional asymmetry is a subtle but pervasive feature of the dental arches, with systematically larger dimensions on the left side. Prior studies attribute this sidedness to compensations for hemispheric laterality. Patient's sex did not influence the extent of asymmetry, but Class II cases exhibited significantly greater asymmetries, particularly in the anterior segment. Inspection suggests that this is due to the lack of coupling and guidance of the teeth between the jaws. There is a significant association between the severity of Class II buccal segment relationship and the extent of left-right dentoalveolar asymmetries. Conclusions: Molar and canine relationships tend to be worse on a person's left side—across all malocclusions—but Class II cases show the greatest magnitudes of dentoalveolar asymmetry because of their reduced interarch coupling of teeth. Clinically, these lateralities need to be anticipated, particularly in Class II malocclusions, and incorporated into the treatment plan. Supported by the UT Alumni Endowment for Research.