The association between orthodontic treatment with removal of premolars and the angulation of developing mandibular third molars over time

Abstract

To assess changes in mandibular third molar angulation during orthodontic treatment in subjects having either first or second premolars or neither removed. In a retrospective study approved by the institutional review board, right and left mandibular third molar angulations were compared to the vertical axis of adjacent second molars before and at the end of orthodontic treatment. The sample included 25 subjects with first premolars removed, 25 subjects with second premolars removed, and 24 subjects with no premolars removed. A decrease in angulation over time of at least 5°, so that the third molar became more vertical, was considered clinically favorable. Data were assessed by a linear mixed effect model and a proportional odds model with significance set at P < .05. Prior to treatment, the average mandibular third molar angulation did not differ significantly among the three study groups (P = .97). The average change during treatment was not significantly affected by group (P = .59), but a higher proportion of mandibular third molars were more vertical by at least 5° in the second premolar extraction group compared to the other two groups at the completion of treatment. Although creating space for third molars to erupt and function has intuitive appeal, clinicians should not assume that third molars will move upright to a vertical position even if premolar removal is performed as part of an orthodontic treatment plan.