The effect of extraction of the lower first permanent molar on the developing third molar in children

OBJECTIVE: To find the effect of extraction of the lower first permanent molar in children (aged 8–11 years) on the position and angle of the developing third molar. DESIGN: Retrospective radiographic analysis. PARTICIPANTS: Two cohorts of participants were identified: an extraction group, who had extraction of one or more first permanent molars aged 8–11 years; and a non-extraction group, who retained all mandibular teeth. Both cohorts previously had panoramic radiographs taken at mean ages of 9.7 years (T1), before extraction, and 12.12 years (T2). In total, there were 61 third molars with an associated extracted first permanent molar and 60 third molars with an associated retained first permanent molar. METHODS: A digital radiographic analysis was carried out on the panoramic radiographs to measure the movement of the third molar, vertically and horizontally, and its angle. The magnification of the T1 and T2 radiographs was calibrated. Reliability of the radiographic analysis was confirmed via intra- and inter-rater reliability tests. The extraction and non-extraction groups were compared via independent sample tests RESULTS: The third molar moved significantly more mesial in the extraction group (P < 0.001) and the angle uprighted significantly more than the non-extraction group (P < 0.001). Vertically, the third molar moved inferiorly in both cohorts with no significant difference. CONCLUSION: In the developing dentition, extraction of the lower first permanent molar encouraged mesial movement and uprighting of the developing third molar. This may improve the likelihood of future eruption of the third molar.