Anatomical Predictors of Second Molar External Root Resorption: A Cone-Beam Computed Tomography Study of Impacted Third Molars

Objectives: This study aimed to identify anatomical and radiographic risk factors associated with second molar (M2) external root resorption (ERR) induced by impacted third molars (M3) using cone-beam computed tomography, providing a strategic framework for clinical decision-making regarding prophylactic extraction. Methods: A retrospective analysis was conducted on cone-beam computed tomography scans of 513 impacted M3s. Anatomical parameters, including M3 angulation, impaction depth (Pell and Gregory classification), and skeletal constraints (M3 to the posterior border of the maxilla, distance from the M3 root to the posterior border of the mandible, etc.) were quantified. The severity and anatomical location of M2 ERR were evaluated using stratified statistical methods to identify key predictors of resorption. Results: In the univariate analysis, M2 ERR severity was significantly associated with M3 impaction depth and skeletal space constraints rather than angulation alone (P < .05). Class C impaction with apical-third involvement represented the highest risk profile; notably, 100% of severe maxillary ERR cases were localized to the M2 apical third. In the multivariate model, an increased distance from the M3 to the posterior border of the maxilla (OR = 1.26) and a reduced cervical width of the M2 were key predictors of severity. Furthermore, a distinct sexual dimorphism was observed, with female patients exhibiting a significantly higher prevalence of severe maxillary ERR, likely due to more restricted skeletal space. Conclusions: The severity of M2 ERR is primarily driven by impaction depth and anatomical constraints, particularly the limited space in the maxillary tuberosity and mandibular retromolar area. Class C impaction targeting the M2 apical third is the most critical risk factor for severe resorption. Clinical relevance: Clinicians should consider a proactive management strategy, prioritizing preventive M3 extraction in high-risk cases - specifically Class C impactions in female patients - to avoid irreversible pulp involvement and potential M2 loss. These findings support shifting from passive observation to early intervention based on specific anatomical risk profiles.