Comparison of incisive canal remodeling and root resorption in extraction vs. non-extraction fixed orthodontic retraction: a CBCT study.

Background: This study evaluated three-dimensional changes in incisive canal (IC) morphology, root-IC proximity, and apical root resorption following fixed orthodontic retraction, comparing extraction and non-extraction protocols. Methods: CBCT scans of 86 patients (172 maxillary central incisors; mean age 22.3 ± 5.7 years) were analyzed before (T1) and immediately after treatment (T2). Participants were assigned to extraction (n = 42) or non-extraction (n = 44) groups. Linear measurements (IC width, cortical bone width, root-IC distance, U1 length/width, IC height) were recorded at three vertical levels (H1-H3). IC and U1 volumes and surface areas were quantified using standardized 3D segmentation. Continuous group comparisons were performed using patient-averaged data, whereas incisor-level categorical outcomes were analyzed using cluster-adjusted statistical models. Root-IC proximity patterns were evaluated using Generalized Estimating Equations (GEE), and apical root resorption and volumetric changes were assessed using Linear Mixed Models (LMM). Predictors of root-IC contact/invasion and predictors of root-IC distance reduction were examined using multivariable GEE and LMM, respectively. Multiplicity was controlled using Holm-Bonferroni correction. Results: IC width and cortical bone width decreased at several levels in both groups, more prominently in extraction cases (P < 0.05). Root-IC distance decreased in all patients, with an adjusted overall mean reduction of 1.33 mm (95% CI, 1.28-1.37). LMM showed no independent effect of extraction status on root-IC distance change (B = 0.08, P = 0.079). Each millimeter of U1 retraction produced an additional 0.40 mm reduction in root-IC distance (95% CI, 0.37-0.43; P < 0.001). GEE demonstrated that each millimeter of U1 movement increased the odds of root-IC contact or invasion by 1.76-fold (95% CI, 1.21-2.56; adjusted P = 0.030). Apical root resorption was significantly higher in teeth showing canal contact or invasion, with an average 0.38 mm greater shortening compared with separated roots (95% CI, 0.08-0.69). Conclusion: Changes in root-IC proximity during orthodontic retraction are driven primarily by the magnitude of tooth movement, not extraction status. Greater retraction increases both canal approximation and the likelihood of contact/invasion, which in turn intensifies apical root resorption. Pre-treatment CBCT assessment of IC morphology and careful force and torque control are essential to minimize biomechanical overload and reduce iatrogenic risk during orthodontic retraction.