The horizontal inclination angle is associated with the risk of inferior alveolar nerve injury during the extraction of mandibular third molars.

The extraction of mandibular third molars can lead to injury to the inferior alveolar nerve. Hence, it is important to assess the proximity of the root to the inferior alveolar canal before extraction. The classification system of Pell and Gregory and the Winter classification are commonly used to evaluate the positional relationship of the third molar based on radiographs. This retrospective study involving 105 mandibular third molars was performed to assess whether these systems reflect the proximity of the root to the canal (based on computed tomography images), and to identify risk factors for nerve injury. Regarding the prediction of computed tomography-verified canal invasion, the sensitivity, specificity, and positive and negative predictive values were high for each Pell and Gregory category when there was radiographic evidence. The mean distance of invasion was significantly greater in class III than in class I. However, there were no significant differences between the Winter inclination categories. The mean distance differed significantly between a horizontal inclination angle to the buccal side of >5° and an angle of ≤5°. Thus, a horizontal inclination angle >5° represents a novel risk factor for nerve injury.