Determination of Proximity of Mandibular Third Molar to Mandibular Canal Using Panoramic Radiography and Cone-beam Computed Tomography

Darshana S Nayak; Shubhasini A Raghavan; Praveen Birur; Shubha Gurudath; Gurushanth Keerthi

doi:10.4103/jiaomr.jiaomr_53_17

ORIGINAL ARTICLE

Year : 2017 | Volume : 29 | Issue : 4 | Page : 273-277

Determination of Proximity of Mandibular Third Molar to Mandibular Canal Using Panoramic Radiography and Cone-beam Computed Tomography

Darshana S Nayak¹, Shubhasini A Raghavan², Praveen Birur², Shubha Gurudath², Gurushanth Keerthi²
¹ Department of Oral Medicine and Radiology, Dayanand Sagar College of Dental Sciences, Bengaluru, Karnataka, India
² Department of Oral Medicine and Radiology, Karnataka Lingayat Education Society's Institute of Dental Sciences, Bengaluru, Karnataka, India

Date of Submission	07-Jun-2017
Date of Acceptance	10-Jan-2018
Date of Web Publication	15-Feb-2018

Correspondence Address:
Dr. Praveen Birur
Department of Oral Medicine and Radiology, Karnataka Lingayat Education Society's Institute of Dental Sciences, Bengaluru, Karnataka
India

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/jiaomr.jiaomr_53_17

Abstract

Objectives: Position of inferior alveolar canal with respect to an impacted third molar reveals certain radiographic signs, but three-dimensional relationship to the canal can be provided with cone-beam computed tomography (CBCT). The purpose of this study was to determine which radiographic signs on panoramic radiography indicate a true relationship on CBCT. Materials and Methods: Forty samples with signs or symptoms of impacted mandibular third molar and panoramic radiograph showing signs of a close relationship with the mandibular canal as described by Félez-Gutiérrez et al. were included in the study and subjected to CBCT. Radiographic signs on panoramic radiography were compared with the relationship on CBCT. Statistical analysis was done using Chi-square test. Results: Twenty-one samples (52.5%) showed darkening of the apex, which was the most frequent type of radiographic sign of a close relationship on panoramic radiography. Twenty-three samples (57.5%) revealed a true relationship on CBCT. Darkening of the apex and narrowing of the canal were the signs most frequently associated with a true relationship. On CBCT, coronal and axial sections better predicted a true relationship. Conclusion: This study showed that the presence of any of the radiographic signs cannot definitely predict a true relationship; however, the presence of a close sign on panoramic radiography is often associated with a true relationship to the canal.

Keywords: Cone-beam computed tomography, impacted third molar, mandibular canal, panoramic radiography

How to cite this article:
Nayak DS, Raghavan SA, Birur P, Gurudath S, Keerthi G. Determination of Proximity of Mandibular Third Molar to Mandibular Canal Using Panoramic Radiography and Cone-beam Computed Tomography. J Indian Acad Oral Med Radiol 2017;29:273-7

How to cite this URL:
Nayak DS, Raghavan SA, Birur P, Gurudath S, Keerthi G. Determination of Proximity of Mandibular Third Molar to Mandibular Canal Using Panoramic Radiography and Cone-beam Computed Tomography. J Indian Acad Oral Med Radiol [serial online] 2017 [cited 2021 Jan 25];29:273-7. Available from: https://www.jiaomr.in/text.asp?2017/29/4/273/225558

Introduction

Extraction of impacted mandibular third molars might result in inferior alveolar nerve (IAN) injuries causing paresthesia of lower lip.^[1],[2] The precise localization of the mandibular canal is important in preventing injury to IAN.^[3] Panoramic radiography, which provides a two-dimensional image, is commonly performed before third molar surgery.^[3] Cone-beam computed tomography (CBCT) provides a higher quality image in three dimensions (3D).^[4] Gomes et al. described seven radiologic features on panoramic radiography that represent close relationship between mandibular third molar and mandibular canal.^[5] The purpose of this study was to determine which of these represents a true relationship on CBCT.

Materials and Methods

The present study was carried out following approval of Ethical Committee and informed consent was obtained from all individual participants included in the study. Sixty-five patients (having 40 impacted third molars) with signs or symptoms of impacted mandibular third molar were examined and a detailed case history was recorded. The patients were then subjected to panoramic radiography in standard head position. Close relationship between impacted mandibular third molar and mandibular canal was evaluated on panoramic radiography according to the radiographic signs given by Félez-Gutiérrez et al., modified by Gomes et al.,^[5] i.e., (i) Darkening of the root, (ii) deflection of the apex, (iii) narrowing of the apex, (iv) dark and bifid apexes, (v) deviation of the mandibular canal, (vi) narrowing of the mandibular canal, and (vii) island-shaped apex, as shown in [Figure 1]. The type of impaction was recorded on panoramic radiography based on Winter's classification.^[6]

Figure 1: Cropped panoramic radiographs showing (a) darkening of the apex, (b) deflection of the root apex, (c) narrowing of the apex, (d) deviation of mandibular canal, (e) narrowing of the canal, and (f) island-shaped apex

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Twenty-four patients having 40 impacted third molars with a sign of close relationship were then subjected to CBCT (Planmeca Promax3D, Helsinki, Finland) with parameters of voltage set at 96 kV and current 10 mA. The images were evaluated using Planmeca Romexis Software. Scans with slice thickness of 1 mm were obtained in axial, sagittal, and coronal planes to determine true relationship. The relationship between the mandibular canal and the impacted third molar was observed in the three planes and any direct contact between the canal and the root was noted.

The root of lower third molar was considered to be in contact with the neurovascular bundle in the mandibular canal when loss of the cortical lining of the mandibular canal was observed [Figure 2]. The corresponding panoramic radiographic images were verified to determine which radiographic signs represent a true relationship. Statistical analysis was carried out using Chi-square test of significance. A P value of <0.05 was set as statistically significant. Data analysis was carried out using Statistical Package for the Social Sciences (SPSS, version 10.5).

Figure 2: CBCT sections showing breach in cortex of the canal; (a) coronal section; (b) sagittal section; and (c) axial section

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Results

Out of 24 patients (40 samples), 19 were females (79.2%) and 5 were males (20.8%), with age range between 18 and 25 years (mean, 20.3 years). Darkening of the apex was the most frequent type of radiographic sign of a close relationship on panoramic radiography and was seen in 21 samples (52.5%) [Table 1]. None of our samples had dark and bifid apices. On CBCT, 23 out of 40 samples (57.5%) showed a true relationship with the mandibular canal. None of the signs on panoramic radiography could significantly predict a true relationship on CBCT; however, darkening of the apex (n = 13, 56.5%) and narrowing of the canal (n = 4, 17.4%) were the signs most frequently associated with a true relationship on CBCT (P value = 0.696) [Table 2].

Table 1: Type of radiographic sign of a close relationship on panoramic radiography

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Table 2: Prediction of true relationship on CBCT by signs on panoramic radiography

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In the coronal section, out of 27 samples with loss of cortical integrity, only four samples (23.5%) did not show a true relationship on CBCT, thereby indicating an ability to predict a true relationship, with P value < 0.001. In the sagittal section, out of 33 samples with loss of cortical integrity, 10 samples (58.8%) did not reveal true relationship on CBCT (P value = 0.001). In the axial section, out of 30 samples with loss of cortical integrity, seven samples (41.2%) did not reveal true relationship on CBCT, with P value < 0.001 [Table 3].

Table 3: Prediction of true relation on CBCT by different sections

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Discussion

Damage to the IAN is a serious complication of the extraction of mandibular third molars. The risk factors include the surgeon's experience, the age and sex of the patient, and the degree of operative tissue damage. The most important factor, however, is the anatomical relation between the impacted third molar and the mandibular canal.^[7] Accurate preoperative imaging to confirm the relation is therefore important for predicting the degree of difficulty and the risk of damage.^[8] The present study focuses on the close relationship between the apexes of the mandibular third molar and the mandibular canal, as the existence of a positive radiographic sign is a risk factor for the appearance of nerve injury.

Panoramic radiography is the most frequently used radiographic investigation of choice prior to extraction. Félez-Gutiérrez et al. showed that the existence of a positive radiographic sign on panoramic radiography may indicate a true relationship between the impacted mandibular third molar and the mandibular canal.^[3] However, the image obtained on panoramic radiography is two-dimensional, and does not permit recording in the buccolingual direction. When the surgical exposure of the neurovascular bundle occurs intraoperatively during the removal of the tooth, the relationship is regarded as a true one, and in such cases there is a contribution to the appearance of nerve injury.^[3],[9],[10] To obtain such a true relationship, CBCT is the best and most accurate method for the anatomic identification of the mandibular canal.^[8]

The type of radiographic sign as described by Félez-Gutiérrez et al., modified by Gomes et al., was considered as a parameter in the present study.^[5] The darkening of the apex represents decreased amount of dental structures or cortical lining of IAN canal. On panoramic radiography, deflection of the apex is seen as a deviation of the root buccally, lingually, or both. When it reaches the canal, it may even be deflected to the mesial or distal aspect. Narrowing of the apex denotes the presence of deep grooving or perforation of the root or the involvement of the greater diameter of the root by the canal in some form. Bifid apex shows that when the canal crosses the root apex, it can be identified by the double periodontal membrane shadow of the bifid root apex. Deviation of the canal is due to an upward displacement of the canal passing through the root. Narrowing of the canal occurs when the canal crosses the root apex, and there is reduction in its diameter. It is due to the displacement of the roof and floor of the canal toward each other. This appearance indicates partial encirclement of the canal. Island-shaped apex is due to the disappearance of the two corticals from the mandibular canal.^[11] The type of radiographic sign encountered most frequently in this study was the darkening of the root apex, which is in accordance with the studies by Albert de Melo et al.^[3] Other studies concluded that darkening of the third molar roots on the panoramic radiographs was one of the strongest classic specific signs, indicating close relationship between the third molar roots and IAN canal.^[2],[12]

CBCT images provide a reliable insight in the buccolingual relationship between the third molar root and the mandibular canal, which cannot be achieved with panoramic radiography. On CBCT, the presence or absence of direct contact between the tooth root and the canal contents was three dimensionally evaluated. It was considered that direct contact was present when loss of bone tissue between the two structures was observed on all three sections, i.e., coronal, sagittal, and axial. CBCT is the best way of displaying the mandibular canal from different directions, coronal, axial, and sagittal.^[8]

The incidence of IAN injury after surgical removal of mandibular third molars has been reported to be between 0.4 and 5.2%.^{[13],[14],[15]} In our study the roots of lower third molar were considered to be in contact with the neurovascular bundle in the mandibular canal when loss of the cortical lining of the mandibular canal was observed on the coronal, sagittal, and axial sections. A true relationship was confirmed in 23 samples (57.5%). Coronal and axial sections showed a significantly better ability to predict a true relationship.

Axial, coronal, and sagittal images on CBCT provide surgeons with useful information, and such images are also beneficial for a preoperative explanation of the surgical procedure because of the high-resolution quality of the CBCT images.^[12] Xu et al.^[8] showed that CBCT in the coronal plane can accurately show the relation between the impacted third molar and the mandibular canal, which can help the surgeons make precise surgical plans and reduce the incidence of complications, particularly damage to the IAN.

None of the signs on panoramic radiography showed a significant correlation with CBCT findings. However, cases with darkening of apex and narrowing of canal showed a greater association with CBCT. Statistically significant results for the darkening of the root apex,^{[7],[16],[17],[18]} interruption of the white lines, and diversion of the mandibular canal ^{[16],[18],[19],[20],[21],[22],[23]} have been reported. Sedaghatfar et al.^[7] reported that darkening of the root, interruption of the white lines of the mandibular canal, diversion of the mandibular canal, and narrowing of the root were significantly associated with IAN exposure at surgery.

Our study population was small and limited, and we suggest that a larger population analysis of multiple factors is necessary because of the low frequency of IAN injury. Further studies are recommended to assess to what extent radiographic findings of nerve proximity can predict postsurgical complications.

Conclusion

The presence of any of panoramic radiographic signs cannot definitely predict a true relationship; however, the presence of a close sign on panoramic radiography suggests the possibility of a true relationship to the canal. Hence, all patients with a close relationship on panoramic radiography should be referred for CBCT. Of all the sections in CBCT, the coronal and axial sections should be carefully examined as they are better at predicting true relationship.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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