Clinical and radiographic outcomes of second molars with external root resorption following impacted third molar removal: a retrospective CBCT-based cohort study

To evaluate the clinical and radiographic outcomes of second molars (M2s) affected by external root resorption (ERR) following impacted third molar (M3) removal, and to assess the influence of adjunctive guided bone regeneration (GBR). This retrospective cohort study included patients presenting with ERR of M2s associated with impacted M3s who underwent M3 extraction with or without concurrent GBR. Cone-beam computed tomography (CBCT) was used to assess ERR progression and distal alveolar bone levels before surgery and at least 6 months postoperatively. Pulp sensibility was also evaluated. Changes in distal bone height were compared between groups. Generalized estimating equations (GEE) were used to evaluate the association between treatment modality and bone gain while adjusting for baseline defect severity and potential confounders. Interaction analysis was performed to assess whether the effect of GBR varied according to baseline defect severity. The Wilcoxon signed-rank test revealed no significant pre- to post-treatment change in root length among teeth with root length alteration (Z = 0.110, 95% CI − 0.185 to 0.380, P = 0.457), nor in the vertical dimension of the defect among teeth without root length alteration (Z = 0.033, 95% CI − 0.248 to 0.283, P = 0.785), supporting the radiographic stability of the resorptive defect following M3 removal. Regarding pulpal status, excluding one tooth that had undergone preoperative root canal treatment, all remaining teeth (64/64, 100%) maintained positive pulp sensibility at the final follow-up, as assessed by cold testing. The between-group difference was 2.03 mm (Hodges-Lehmann estimate, 95% CI, 0.56–3.82 mm), with significantly greater bone gain in the GBR group (6.32 ± 2.67 mm) compared with the Extraction-Only group (4.08 ± 3.50 mm), P = 0.007. After adjustment for baseline defect severity and other covariates, treatment modality remained a significant predictor of distal bone gain in the GEE model with exchangeable correlation structure (QIC = 1123.229), with GBR associated with greater bone gain compared with extraction alone (β = 2.244, 95% CI, 0.612–3.875, P = 0.007). To assess the robustness of this estimate to potential unmeasured confounding, the E-value was calculated: the E-value for the point estimate was 3.25, while the E-value for the lower bound of the 95% CI was 1.68. The interaction term between treatment modality and baseline distal bone height was not statistically significant (β = 0.008, 95% CI (-0.229, 0.245), P = 0.945), suggesting that the effect of GBR on distal bone gain did not vary significantly across the range of baseline defect severities observed. Among M2s with available follow-up, no radiographic progression of ERR was detected following M3 removal. The findings do not support routine prophylactic root canal treatment in asymptomatic M2s with preserved pulp sensibility. However, spontaneous periodontal recovery of ERR-associated distal defects appeared incomplete. Adjunctive GBR was associated with enhanced distal bone regeneration. External root resorption of second molars associated with impacted third molars appears to stabilize after extraction of the third molar, with preservation of pulp sensibility. However, the distal bone defect frequently persists radiographically without full resolution. Guided bone regeneration was associated with greater radiographic bone gain in affected second molars.