Is cone beam computed tomography accurate in predicting inferior alveolar nerve exposure during mandibular third molar extraction?

Objectives This study aims to evaluate the accuracy of cone beam computed tomography (CBCT) in predicting the exposure of inferior alveolar nerve (IAN) during complicated mandibular third molars (M3M) extraction. Methods 115 M3Ms with canal cortical defect signs on preoperative CBCT were extracted. Candidate variables included sex, age, types of CBCT machine, the Winter classification of M3Ms, the direct contact size between IAN and tooth root on CBCT, the size of cortical defect on CBCT. The primary outcome was the exposure of IAN and the exposed neurovascular bundle size which was recorded measured under endoscope. Bland-Altman analysis was performed to assess the agreement between the CBCT and endoscopic measurements. Results 85/115 (73.9%) M3Ms with canal cortical defect signs on preoperative CBCT had intraoperative exposure of IAN. The average length and width of the exposed IAN were 5.89 ± 1.72mm and 2.48 ± 0.79 mm, which were significantly smaller than the direct contact size between IAN and tooth root on CBCT (9.69 ± 3.05mm and 3.26 ± 0.87 mm, P < 0.001 ) but larger than the cortical defect size (5.06 ± 2.05mm and 2.10 ± 0.54 mm, P < 0.05 ). The probability of intraoperative IAN exposure was statistically different among different Winter classifications of M3M and the probability of IAN exposure was higher in non-horizontal impacted type. Conclusions Not all M3Ms with tooth-IAN contact signs on preoperative CBCT indicated intraoperative IAN exposure. The contact sizes on CBCT were always larger than the intraoperative endoscopic measurements. IAN exposure can be accurately predicted by the length of cortical defect on CBCT. Non-horizontal impaction predisposed the M3M to a higher risk of intraoperative IAN exposure. Clinical Relevance: Endoscope provides the possibility to observe and record the IAN exposure directly. IAN exposure can be accurately predicted by the length of cortical defect on CBCT instead of the direct contact size between IAN and tooth root. Non-horizontal impaction predisposed the M3M to a higher risk of intraoperative IAN exposure.