Combined preoperative measurement of three inferior alveolar canal factors using computed tomography predicts the risk of inferior alveolar nerve injury during lower third molar extraction


A retrospective cohort study was performed to assess the clinical usefulness of combination assessment using computed tomography (CT) images in patients undergoing third molar extraction. This study included 85 patients (124 extraction sites). The relationship between cortication status, buccolingual position, and shape of the inferior alveolar canal (IAC) on CT images and the incidence of inferior alveolar nerve (IAN) injury after third molar extraction was evaluated. IAN injury was observed at eight of the 124 sites (6.5%), and in five of 19 sites (26.3%) in which cortication was absent + the IAC had a lingual position + the IAC had a dumbbell shape. Significant relationships were found between IAN injury and the three IAC factors (cortication status, IAC position, and IAC shape; P = 0.0001). In patients with the three IAC factors, logistic regression analysis indicated a strong association between these factors and IAN injury ( P = 0.007). An absence of cortication, a lingually positioned IAC, and a dumbbell-shaped IAC are considered to indicate a high risk of IAN injury according to the logistic regression analysis ( P = 0.007). These results suggest that a combined assessment of these three IAC factors could be useful for the improved prediction of IAN injury.

Extraction of the lower third molar (LM3) is a commonly performed procedure in oral surgery . However, injury to the inferior alveolar nerve (IAN) often occurs as a complication of mandibular third molar extraction, with a frequency ranging from 0.5% to 8% . Therefore, a reduction in the risk of IAN injury is the most important issue associated with LM3 extraction.

Paresthesia of the IAN has been associated with factors such as age, sex, type of anaesthesia, and the experience of the surgeon . However, several studies have reported the predictive value of computed tomography (CT) assessment for IAN injuries . On the basis of these studies, the combined assessment of three factors, namely the absence of cortication between the inferior alveolar canal (IAC) and the LM3 , the shape of the IAC , and the position of the IAC on CT Images , could reliably improve the prediction of IAN injury ( Fig. 1 ). Moreover, combination assessment using these factors could predict IAN injury with greater accuracy.

Fig. 1
(A) Representative panoramic radiograph showing an overlap of the right lower third molar and the inferior alveolar canal (IAC). (B) Representative computed tomography images demonstrating the absence of cortication between the IAC and the lower third molar, the shape of the IAC, and the position of the IAC.

The specific aim of this study was to clarify the clinical usefulness of such a combination assessment using CT images for LM3 extraction in those patients for whom panoramic radiography has identified known risk factors.

Materials and methods

Study design and sample

This retrospective cohort study included 85 consecutive patients who presented to the Department of Oral and Maxillofacial Surgery, University Hospital of Toyama (Toyama, Japan) for the management of lower third molars, between January 2012 and December 2013. Forty-seven of these patients were male and 38 were female, and their mean age was 31.46 years (range 17–90 years).

The extraction of the LM3 was performed under general or local anaesthesia by various surgeons. The criterion for inclusion in the study sample was the presence of a close relationship between the LM3 and the IAC on panoramic radiographs. The 85 patients (124 LM3) underwent a CT examination following panoramic radiography to allow the anatomical relationship to be studied in greater detail. The CT images were obtained using a Somatom Definition AS+ 128-slice CT machine (Siemens Germany Ltd, Munich, Germany). Axial sections were acquired with the plane of data acquisition parallel to the hard palate, with a slice thickness of 3 mm, and reconstructions at 0.75 mm in the coronal and sagittal planes. The scanning parameters were 190 mAs, 120 kV, and tube rotation time of 0.5 s.

Patients were excluded from the study if they had another impacted tooth to be extracted (e.g., an impacted premolar), a lesion around the third molar (e.g., a tumour or cyst), an immature LM3, mental nerve paralysis at the simultaneous extraction of the other teeth, or if they were lost to follow-up. This study was approved by the Institutional Review Board for Human Studies at the University Hospital of Toyama.

Study variables

The study variables were categorized into demographic, panoramic radiography, and CT finding factors. The demographic factors were the patient’s age and sex. The panoramic radiography factors included tooth angulation (from the panoramic radiographs). The CT finding factors consisted of the cortication status, position, and shape of the IAC. IAC shapes were evaluated based on a previously reported classification system . The outcome variable was IAN injury, that is, paresthesia of the lower lip and/or mental region. To assess the presence of paresthesia, touch-pressure thresholds were measured using a Semmes–Weinstein monofilament (Sakai Medical Co. Ltd, Tokyo, Japan). Cases with a loss of light touch sensation (<2.44 mg filament) were considered to be exhibiting paresthesia. Self-reported sensations of the lower lip or mental region were also recorded.

Data collection methods

The cortication status, position, and shape of the IAC were assessed by two examiners (a specialist oral and maxillofacial surgeon and a senior resident) on cross-sectional CT images.

Summary of operative methods

Surgical removal of the LM3 was performed under general or local anaesthesia by various surgeons (a specialist oral and maxillofacial surgeon, senior resident, and trainee). Alveolar bone removal and tooth sectioning were performed if necessary. If the surgical procedure was complicated for technical reasons, a senior surgeon provided support or completed the surgical procedure.

Data analysis

The statistical analysis was performed using the JMP 11.2 software program (JMP 11.2.0 for Windows; SAS Institute, Cary, NC, USA). The relationships among cortication status, IAC position, IAC shape, and IAN injury were assessed using the χ 2 test. Primary multivariate logistic regression was performed using demographic, anatomical, and radiographic factors.

The dependent variable of IAN injury was analyzed using a logistic regression model, with the following predictor variables: age, sex, type of anaesthesia, experience of the surgeon, angulations of the tooth, panoramic radiograph findings (separate, contiguous, or overlapping relationship with narrowing of the canal), and CT image findings (IAC position, IAC shape, and status of cortication). To construct a logistic regression model, backward selection stepwise regression was used. P < 0.05 was considered statistically significant.


Panoramic radiographs of 124 LM3 (39 on the right side and 85 on the left) were analyzed to determine tooth angulation (mesioangular, horizontal, vertical, distoangular, or transverse) and the relationship between the IAC and LM3. A distance between the IAC and LM3 of less than −0.5 mm was noted in 70 cases, and a contiguous relationship (a distance from the IAC to the LM3 between −0.5 and 0.5 mm) was found in 51 cases ( Table 1 ).

Dec 14, 2017 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Combined preoperative measurement of three inferior alveolar canal factors using computed tomography predicts the risk of inferior alveolar nerve injury during lower third molar extraction
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