Radiographic Progression of External Root Resorption in Second Molars With Retained Third Molars: A Multicentre Cone-Beam Computed Tomography Study

Objectives To characterize radiographic progression of external root resorption (ERR) in second molars associated with retained third molars and identify predictors. Methods This multicentre longitudinal cone-beam computed tomography study included 451 second molars from 341 patients with at least two examinations ≥12 months apart. ERR depth, location, and affected-root number were assessed at baseline and follow-up. The primary outcome was radiographic progression, defined as increased ERR depth and/or additional affected roots. Root canal treatment and extraction were reported separately as secondary clinical intervention outcomes. Patient-clustered generalized estimating equation models evaluated predictors. Results Median follow-up was 24 months. Six teeth underwent root canal treatment, and five were extracted. Among 440 teeth, 386 (87.7%) remained stable, and 54 (12.3%) progressed. ERR depth progression occurred in 45 teeth (10.2%), affected-root-number progression in 12 (2.7%), and both in 3. Depth progression was more frequent in mandibular than maxillary teeth (38/335, 11.3% vs 7/105, 6.7%), whereas affected-root-number progression was more frequent in maxillary teeth (9/105, 8.6% vs 3/335, 0.9%). Multivariable generalized estimating equation identified baseline Grade V, mandibular location, and apical-third location as predictors of depth progression. Mandibular location was associated with lower odds of affected-root-number progression, whereas Grade IV to V lesions showed higher odds. Conclusions Most ERR lesions remained radiographically stable over 24 months. Baseline ERR depth was the most consistent predictor, and component-specific patterns suggested potential jaw-related heterogeneity. Clinical relevance Baseline cone-beam computed tomography assessment of ERR depth and progression components may support risk-stratified decision-making.