Anatomic relationship between impacted third mandibular molar and the mandibular canal as the risk factor of inferior alveolar nerve injury.

Our aim was to explore the relation between the site of the mandibular canal and neurosensory impairment after extraction of impacted mandibular third molars. We organised a retrospective study of 537 extractions in 318 patients in which the affected tooth was intersected by the mandibular canal. This was verified by cone-beam computed tomography (CBCT), and we analysed the relation between the site of the canal and the likelihood of injury to the inferior alveolar nerve (IAN) after extraction of the third molar. The relation between the position of the root of the tooth and the mandibular canal was categorised into 4 groups: I=root above the canal; II=on the buccal side; III=on the lingual side; and IV=between the roots. The overall rate of neurosensory impairment after extraction was 6% (33/537). It occurred in 9/272 patients (3%) in group 1, 16/86 (19%) in group II, and in 8/172 (5%) in group III. There was no neurosensory impairment in group IV where the canal was between the roots. There were significant differences between group II and groups I and III (p<0.01), but not between groups I and III (p=0.32). The risk of damage to the inferior alveolar nerve is increased if third molars intersect with the mandibular canal, particularly on its buccal side.