Differential premolar extractions.

The primary object of this research project was to establish the amount of incisor retraction to be expected during full fixed mechanotherapy and to generate regression models that could provide more accurate prediction of each of the three most popular combinations of symmetrical premolar extractions, where extraoral anchorage would not be used. Pretreatment and posttreatment records of 73 cases that had their four first premolars extracted (group 44), 74 cases with four second premolar extractions (group 55), and 59 cases with upper first and lower second premolar extractions (group 45) were selected. All these cases were treated by one orthodontist, who used the same edgewise technique throughout. The results indicate that, on average, maxillary retraction in relation to the facial plane (N Po) differed only slightly between group 55 (mean 4.2 +/- 2.4 mm) and group 44 (mean 4.7 +/- 2.3 mm), with relatively more retraction for group 45 (mean 6.6 +/- 2.5 mm; p < 0.05). In contrast, the mandibular incisors were retracted slightly more in group 44 than in the other two groups (p < 0.05). The regression models that were developed could be useful as an additional tool to assist the practitioner in the selection of which teeth to extract for a particular case.