Effects of Unilateral Premolar Extraction Treatment on the Dental Arch Form of Class II Subdivisions

Hypothesis: Effects of unilateral maxillary premolar extraction treatment on dental arch form and midline deviation do not differ significantly from bilateral maxillary premolar extraction treatment. Distances of corresponding points between right and left sides of the dental arch are approximately the same within each group. Objective: A retrospective study evaluating post-treatment maxillary arch forms and midlines for asymmetry in Class II subdivisions treated with unilateral and bilateral maxillary premolar extractions. Methods: Using GeoMagicTM and MATLABTM softwares, best fit curves expressed as quadratic polynomials were generated from post-treatment digital models of fifteen Class II subdivisions treated with unilateral and twenty with bilateral maxillary premolar extractions. Transverse and sagittal measurements were recorded to analyze asymmetry in arch form. Midline deviation relative to the mid-palatal raphe was calculated. Dental models were superimposed on constructed reference planes to generate average post-treatment arches. Statistical comparisons were performed with significance level set at p ≤ 0.05. Results: Within the unilateral extraction group, anterior, anterior-middle and middle segments showed significant differences in arch form between right and left sides transversely, and all segments of the arch aside from the posterior segment in the sagittal dimension. Bilateral premolar extraction treatment did not show significant differences in arch form between right and left sides in either dimension. Significant differences were found between the groups in anterior and anterior-middle segments of the arch transversely, and in the middle and middle-posterior segments sagittally. A significant difference in midline deviation relative to the mid-sagittal plane was observed between the groups. Superimposed average arches showed similar results. Conclusions: Unilateral maxillary extraction treatment generally results in a narrower and more posteriorly displaced arch form on the extraction side, with a deviated maxillary midline from the mid-palatal raphe towards the extraction side of the arch.