Diagnostic value of cone beam computed tomography and panoramic radiography in predicting mandibular nerve exposure during third molar surgery


The aim of this study was to evaluate the diagnostic accuracies of cone beam computed tomography (CBCT) and panoramic techniques in predicting inferior alveolar nerve (IAN) exposure. The sample size was determined based on a pilot study. This prospective clinical series study included 59 third molar extraction sites with any of seven previously suggested panoramic signs of IAN exposure. The diagnosis of nerve exposure was done on panoramic and CBCT images. Molars were extracted and nerve exposure was evaluated clinically. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of CBCT method, and sensitivity and PPV of panoramic method were estimated). The panoramic and CBCT methods correctly classified 67.7% and 93.3%, respectively, of 60 cases. This difference was statistically significant ( χ 2 = 13.333, P = 0.000). The sensitivity, specificity, PPV, and NPV for CBCT were 97.4%, 85.7%, 92.7%, and 94.7%, respectively. The sensitivity and PPV of panoramic radiography were 67.8% and 97.6%, respectively. The signs with the highest sensitivity were interruption of the mandibular canal border and abrupt canal narrowing. None of the Pell and Gregory criteria, molar angulations, or three-dimensional canal–apex relationships was significantly associated with clinically confirmed IAN exposure. Panoramic radiography may miss about one-third of exposure cases, but a positive panoramic diagnosis is most likely to be a real exposure and should be taken seriously.

The third molars show a high incidence of impaction, which may cause many conditions, often necessitating their surgical removal. The extraction of third molars is the most common procedure in maxillofacial surgery. This procedure may be followed by complications such as neurosensory deficits/dysesthesia, resulting from injury to the inferior alveolar nerve (IAN) bundle, in 0.4% to 8% of surgeries.

The gravity of this condition calls for preoperative radiographic examination. The most widely used technique is panoramic radiography. However, controversy exists over the efficacy of panoramic features, perhaps because of the inconsistent magnification of panoramic radiography and its limitations in demonstrating buccolingual depth. Unlike panoramic radiography, cone beam computed tomography (CBCT) can conveniently illustrate buccolingual depth without any distortions, and seems excellent for localizing the inferior alveolar canal (IAC) and impacted molars. Nevertheless, the only available study using clinically verified IAN exposure to investigate the diagnostic value of CBCT found this not to be different from the diagnostic value of panoramic radiography.

The present study was performed in view of the importance of the subject and the small number of studies that have included direct clinical examination. The aim was to assess the diagnostic value of CBCT and panoramic radiography in predicting IAN exposure by clinically verifying the IAN exposure.

Materials and methods


In this prospective clinical series study, 60 extraction sites were selected in patients attending the department of maxillofacial surgery of a university dental center and a private surgery clinic. The research committee of the university approved the protocol in accordance with the Declaration of Helsinki, and the patients provided signed written consent. The inclusion criteria comprised (1) indication for the extraction of an impacted mandibular third molar and (2) patient age between 16 and 40 years. The exclusion criteria were the absence of any panoramic features suggesting IAN exposure, any systemic or local diseases/cysts, the occurrence of uncontrollable post-extraction bleeding, or any other surgical events or problems.

Radiographic evaluation

Patients with an indication for the extraction of an impacted mandibular wisdom tooth were referred to a single radiology center for panoramic radiography (Proline XC; Planmeca, Helsinki, Finland). The radiographs were evaluated by a maxillofacial radiologist with more than 15 years of experience. If the radiograph showed signs of proximity of the IAC and molar root, the patient was referred to a single radiology center to obtain CBCT volumes. The CBCT images were obtained with a Promax-3D unit (Planmeca) with a flat receiver of 12 × 12 cm 2 and a field of view of 8 × 8 cm 2 , emitting pulsated rays (20 m/s each pulse) for 18 s. The same radiologist examined the CBCT scans.

Radiographic diagnosis of IAN exposure

All panoramic and CBCT images were scrutinized independently regarding the topographic relationship between the impacted third molar and the IAC. The absence or presence of direct contact on the panoramic radiographs was evaluated through consideration of the following seven features, all of which have been reported to be suggestive of close contact between the tooth root and the mandibular canal: (1) interruption of one or both borders of the IAC, (2) darkening of the root, (3) diversion of the IAC, (4) abrupt root dilaceration, (5) narrowing of the root, (6) darkening and divarication of the apex, and (7) narrowing of the canal. The diagnosis of canal exposure was performed by an experienced radiologist, based on the above criteria.

These criteria (excluding root/apex darkening) were also used and documented when assessing the CBCT scans for the diagnosis of IAC exposure, as well as the lack of cortication between the IAC and dental socket. The diagnosis was made by the same radiologist through an evaluation of volumetric reconstructions, as well as axial, panoramic, and cross-sectional images prepared from the tomography volumes.

Impaction status

The depth of impaction (classes A, B, and C as the deepest) and molar horizontal position (classes I, II, and III as the most posterior) were determined on panoramic images according to the criteria of Pell and Gregory. The angle of impaction (mesioangular, distoangular, vertical, horizontal) was also documented based on panoramic images.

Canal position relative to the tooth

CBCT images were used to determine the position of the canal in relation to the root: buccal, lingual, or none (underneath the tooth).

Clinical diagnosis of IAN exposure

The mandibular third molar was removed surgically by a maxillofacial surgeon who has been in practice for more than 15 years. The patients were treated under local anesthesia (lidocaine 2% and 1:100,000 epinephrine) administered to the inferior alveolar, long buccal, and lingual nerves. Each patient received two carpules of anesthesia. The patient could receive additional dose(s) according to their need or upon request, to ensure a painless surgery. A mucoperiosteal pocket was incised and everted to gain access to the third molar; osteotomy and dental sectioning (using rotary instruments) were performed when necessary. In the case of bleeding that was not easily controllable, the patient was excluded from the study, treated appropriately, and replaced with a new participant. IAN exposure was evaluated by the maxillofacial surgeon after surgically removing the mandibular third molar, irrigating the socket, and inspecting all socket walls/floor. Following this, the bone edges were smoothed and the flap was sutured. Patients were given ibuprofen 400 mg for pain control and were instructed to take one dose every 4 h plus additional doses if the pain persisted. Patients were monitored for 3–4 days for any signs of infection. Their sutures were removed at 1 week post-surgery.

Study variables and statistical analyses

The main predictor was the indication of IAN exposure according to CBCT and panoramic imaging, diagnosed on the basis of the criteria given above. Other independent variables were the Pell and Gregory depth of impaction, horizontal position of the molar, angle of impaction, and three-dimensional (3D) position of the canal in relation to the root. The outcome variable was the direct visualization of the neurovascular bundle at the time of molar extraction. The sample size was predetermined based on a pilot study; a minimum of 40 extraction sockets with clinically confirmed IAN exposure was required to obtain a test power above 90% (effect size = 40%, α = 0.05). The specificity, sensitivity, negative predictive value (NPV), and positive predictive value (PPV) of panoramic radiography and CBCT features were calculated. The data were analyzed by χ 2 and χ 2 goodness-of-fit tests using IBM SPSS Statistics version 20.0 software (IBM Corp., Armonk, NY, USA). The level of significance was set at 0.05.

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Dec 14, 2017 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Diagnostic value of cone beam computed tomography and panoramic radiography in predicting mandibular nerve exposure during third molar surgery

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