Association between impacted third molars and external root resorption on adjacent second molars based on cone-beam computed tomography: a systematic review and meta-analysis

BACKGROUND: External root resorption (ERR) on second molars (M2s) is an undesirable type of pathological damage associated with adjacent impacted third molars (iM3s). Owing to its limited detectability on conventional radiographs, ERR may remain undiagnosed until irreversible damage has occurred. This systematic review aimed to quantitatively evaluate the association between iM3s and ERR on M2s via cone-beam computed tomography (CBCT). METHODS: Six databases, including PubMed, Scopus, Web of Science, the Cochrane Library, Embase and ProQuest, were searched for relevant articles published prior to June 2025. Only human studies using CBCT to investigate iM3-related ERR on M2s were included. Methodological quality was assessed via the Joanna Briggs Institute Critical Appraisal Checklist. Meta-analyses were performed using random-effects model with heterogeneity assessed via RevMan software. Meta-regression analysis and the evaluation of publication bias were conducted using Stata software. RESULTS: Twenty-six articles were included in this meta-analysis: Twenty-four had a low risk of bias. The quantitative synthesis demonstrated a 31% (95% confidence interval: 26% to 36%) prevalence of iM3-associated ERR on M2s. A higher prevalence of ERR was associated with mesioangular, horizontal or transverse inclination of iM3s and direct contact with the apical third of the M2 roots. CONCLUSIONS: Findings indicate that ERR on M2s is related to the contact between iM3s and the apical third of M2 roots. Mesioangular, horizontal, or transverse iM3 inclinations are significantly associated with higher ERR prevalence on adjacent M2s. This study identified key associated factors for ERR on M2s with iM3s, providing evidence-based suggestions for treatment plans of asymptomatic iM3s. REGISTRATION: PROSPERO: CRD42022332920.