Prevalence of third molar impaction in orthodontic patients treated nonextraction and with extraction of 4 premolars.

The purposes of this study were to confirm that premolar extraction treatment is associated with mesial movement of the molars concomitant with an increase in the eruption space for the third molars and to test the hypothesis that such treatment reduces the frequency of third molar impaction. Lateral cephalograms, panoramic or periapical radiographs, and study models made before (T1) and after (T2) treatment and a minimum of 10 years postretention (T3) of 157 patients were selected from the postretention sample at the Department of Orthodontics of the University of Washington, Seattle. Treatment for 105 patients included the extraction of 4 premolars; the other 53 were treated nonextraction. These patients represented all the extraction and nonextraction patients in the sample who had at least 1 third molar at T1 or T2 and who showed evidence of full eruption or closure of the root apex at T2 or T3. Student t tests showed higher scores for third molar impaction (P <.01), less mesial movement of the molars from T1 to T2 (P <.01), and smaller retromolar space at T2 (P <.001) in both arches of the nonex patients than in the ex patients. Similarly, molar movement was more mesial from T1 to T2 in the maxilla (P <.01) and in the mandible (P <.05), and the retromolar space was larger in both arches (P <.001) of the patients with eruption than in those with impaction of the third molars. Our results suggest that premolar extraction therapy reduces the frequency of third molar impaction because of increased eruption space concomitant with mesial movement of the molars during space closure.