Apical root displacement is a critical risk factor for apical root resorption after orthodontic treatment.

Objectives: To identify risk factors for apical root resorption (ARR) of maxillary and mandibular incisors using mathematical quantification of apical root displacement (ARD) and multiple linear mixed-effects modeling. Materials and Methods: Periapical radiographs of maxillary and mandibular incisors and lateral cephalograms of 135 adults were taken before and after orthodontic treatment. ARR was measured on the periapical radiographs, and movement of central incisors was evaluated on the superimposed pre- and posttreatment lateral cephalograms. ARD was mathematically calculated from pretreatment tooth length, inclination change, and movement of the incisal edge. Linear mixed-effects model analysis was performed to identify risk factors for ARR, and standardized coefficients (SCs) were calculated to investigate the relative contribution of the risk factors to ARR. Results: Vertical ARD showed the highest SCs for both maxillary and mandibular incisors. Horizontal ARD showed the second highest SC for mandibular incisors but was not significantly correlated with the ARR of maxillary incisors. When horizontal and vertical ARDs were included in the mixed-effects model, the use of self-ligating brackets was significantly correlated with increased ARR of mandibular incisors. Conclusions: ARD is a critical factor for ARR after orthodontic treatment. Careful monitoring of ARR is recommended for patients requiring significant ARD of incisors.