Physiologic drift of the mandibular dentition following first premolar extractions.

A retrospective study of two groups of patients was conducted to evaluate the physiologic drift, or "driftodontics", of the mandibular teeth following the extraction of four first premolars. Group 1 included 32 patients who underwent early extraction in the mixed dentition stage at a mean age of 10.4 years; they were followed for approximately 2.5 years postextraction. Group 2 included 20 patients whose premolars were extracted after the permanent dentition had fully erupted. Their mean age at the time of extraction was 14.2 years, followed by a 0.8 year observation period. Except for the extractions, no other mandibular therapy was rendered. The results show no differences in rates of molar movements between groups. The molar apex moved mesially approximately 0.6 mm/yr; the molar cusp moved mesially approximately 1.2 mm/yr. In contrast, there were marked group differences in movements of the mandibular incisors and canines; rates of change were significantly greater in Group 2 than in Group 1. The canines drifted laterally and distally into the extraction sites while the incisors became more upright over basal bone and less crowded. Incisor irregularity decreased 1.3 mm/yr in Group 1 and 5.5 mm/yr in Group 2. The group differences in amounts of tooth movement were accounted for by changes in incisor crowding.