Clinical risk factors caused by third molar levelling following extraction of a mandibular second molar

Mandibular second molar (MdM2) is often lost, and its space is filled with a bridge or implant. MdM2 extraction followed by orthodontic treatment protracting mandibular third molar (MdM3) towards the MdM2 position may overcome the missing of MdM2. The objectives of our study were to describe the outcome of the procedure and examined clinical risk factors such as external apical root resorption (EARR) and alveolar bone loss (ABL), as the indicators of poor orthodontic treatment outcomes. This retrospective study included 70 cases in 56 patients who received orthodontic treatment at Tokyo Medical and Dental University Hospital between 2007 and 2018. Multi-bracket appliances were used in all patients for MdM3 protraction. Using linear mixed effects models, EARR and ABL were regressed on various factors, including panoramic and cephalometric variables. With the mean treatment duration of 1040.4 ± 441.8 days, MdM2 space closure was achieved in 92.8% (65 cases). The ANB angle (P = .023) and the use of temporary anchorage devices (TADs) (P = .021) were significantly associated with the greater EARR, while the mandibular plane angle (P = .033) was associated with the greater ABL. MdM3 protraction using the fixed appliances resulted in the closure of MdM2 space in > 90% of cases without evident root resorption. There is a possibility of residual confounding due to the nature of observational study. Orthodontic treatment of MdM3 protraction may be a feasible strategy to close the space of the missing MdM2.