Assessment of the Relationship Between Impacted Mandibular Third Molars and Inferior Alveolar Nerve With Dental 3-Dimensional Computed Tomography

Department of Oral and Maxillofacial Surgery, Fukuoka Dental College, Fukuoka, Japan.
Journal of oral and maxillofacial surgery: official journal of the American Association of Oral and Maxillofacial Surgeons (Impact Factor: 1.43). 12/2009; 67(12):2587-91. DOI: 10.1016/j.joms.2009.07.017
Source: PubMed


The purpose of this study was to assess the capacity of dental 3-dimensional computed tomography (3D-CT; limited cone-beam CT) to predict the exposure and injury of the inferior alveolar nerve (IAN) after mandibular third molar extractions.
This study was a retrospective case series of patients who presented for extraction of mandibular third molars. Subjects eligible for study enrollment were those who underwent preoperative dental 3D-CT because the mandibular third molars were determined to be extremely close to the IAN on panoramic radiogram. The predictive variable was the anatomic relation of the IAN and third molar apices and was a binary variable, contact or noncontact. The primary outcome variable was IAN exposure, and the secondary outcome variable was IAN injury.
From January 2006 to August 2007, 1,853 mandibular third molars in 1,539 patients were extracted. Among them, dental 3D-CT was performed on 53 third molars in 47 patients. The mandibular third molars were judged to make contact with the mandibular canal on dental 3D-CT images in 35 cases (66%). Intraoperative IAN exposure was observed in 17 (49%) contact cases and 2 (11%) noncontact cases on dental 3D-CT images. Of 53 cases extracted after dental 3D-CT examinations, IAN injury occurred in 8 cases (15%). IAN exposure led to IAN injury in 36.8% of cases, whereas IAN injury occurred in only 2.9% of cases without IAN exposure. Although the incidence of IAN injury in the molar-canal contact cases was 23%, all 8 cases with IAN injury (100%) were included in these contact cases.
When viewing the anatomic relation between the IAN and mandibular third molar root apices using dental 3D-CT, contact of the 2 anatomic structures results in an increased risk for IAN exposure or injury.

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    • "In the literature, several signs have been defined on panoramic radiographs, which indicate the close proximity of the mandibular canal to the third molar. Notably, four radiographic signs - darkening of the root, interruption of the cortical lines, diversion of the mandibular canal, and narrowing of the root - are recognized to be associated with inferior alveolar nerve proximity to the mandibular third molar on panoramic radiography.2 "
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    ABSTRACT: To evaluate the association between computed tomographic (CT) assessment of inferior alveolar nerve (IAN) canal cortical integrity and intraoperative IAN exposure. This was a retrospective cohort study. The study sample included patients considered at high risk for IAN injury based on panoramic findings. The primary predictor variable was IAN canal integrity (intact or interrupted) assessed on coronal CT images. The secondary predictor variable was length of the cortical defect, in millimeters. The primary outcome variable was intraoperative visualization of the IAN. Other variables were demographic and operative parameters. Bivariate and multiple logistic regression analyses were used to evaluate the unadjusted and adjusted associations between the cortical integrity and IAN exposure. Diagnostic test characteristics were computed for cortical integrity and threshold cortical defect size. A P value < or = 0.05 was statistically significant. The sample consisted of 51 subjects (57% female) with a mean age of 35.2 +/- 12.8 years. Of the 80 third molars available for evaluation, 52 third molars (64.1%) had evidence of loss of cortical integrity. The mean cortical defect length was 2.9 +/- 2.6 mm. Loss of cortical integrity had a high sensitivity (> or = 0.88) but low specificity (< or = 0.49) as a diagnostic test for IAN visualization. A cortical defect size > or = 3 mm was associated with an increased risk for intraoperative IAN visualization with a high sensitivity and specificity (> or = 0.82). Cortical defect size on a maxillofacial CT has a high sensitivity and specificity for predicting intraoperative IAN exposure during third molar removal.
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