Objective: To separately investigate the prevalence and risk factors of external root resorption (ERR) in maxillary and mandibular second molars (M2s) adjacent to impacted third molars (M3s).
Materials and methods: CBCT scans involving 184 maxillary and 323 mandibular impacted M3s were included. Age, gender, the impaction status of M3, the presence, severity, and location of ERR in M2 were assessed. Risk factors were identified by multivariate logistic regression analyses.
Results: The prevalence of ERR was 32.6% in maxillary and 52.9% in mandibular M2s (P < 0.001). Mesio-angulated and deeply impacted M3s were identified as risk factors for both ERR in maxillary and mandibular M2s (P < 0.05). Besides, age over 25 increased the risk of ERR in maxillary M2s (P < 0.05). ERR in maxillary M2s was overall more severe than that in mandibular M2s (P < 0.001). For maxillary M2s, ERR mostly occurred at the apical third, while the mandibular M2s ERR was most frequently detected at the cervical third.
Conclusions: ERR occurring in M2s adjacent to impacted M3s is common, especially in mandibular M2s. ERR in maxillary M2s cannot be neglected because of its relatively high severity. Mesial angulation and impaction depth of M3 are significantly associated with ERR in M2s. For ERR in maxillary M2s, age is another predictive parameter.
Clinical relevance: Considering the presence of ERR is associated with M3s' impaction, watchful monitoring or prophylactic removal of impacted M3s should be deliberated especially for the patients over 25 years and with mesially inclined and deeply positioned M3s.
Keywords: Cone-beam computed tomography; External root resorption; Impacted third molar; Second molar.