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 Table of Contents  
Year : 2022  |  Volume : 34  |  Issue : 4  |  Page : 423-427

Evaluation of the spatial relationship of impacted mandibular third molar to mandibular canal using CBCT: A descriptive study

Department of Oral Medicine and Radiology, Government Dental College, Thiruvananthapuram, Kerala, India

Date of Submission27-Sep-2021
Date of Decision04-Jun-2022
Date of Acceptance21-Oct-2022
Date of Web Publication09-Dec-2022

Correspondence Address:
Jesmith Elsa Jacob
Thykoothattil House, Pullad P.O., Thiruvalla, Pathanamthitta Dist., Kerala
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jiaomr.jiaomr_282_21

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Introduction: Impaction is defined as the “lack of eruption of a tooth in the oral cavity within the time and physiological limits of the normal eruption process.” The prevalence of damage to the inferior alveolar nerve during surgical extraction of impacted mandibular third molars, resulting in transient/permanent dysesthesia or paresthesia, is around 0.4% to 13.4%. Aims and Objectives: The study's goal was to use cone beam computed tomography (CBCT) to examine the three-dimensional anatomic association between the impacted mandibular third molars and the mandibular canal and to develop a risk prediction model to screen inferior alveolar nerve injuries. Materials and Methods: DICOM files of 300 cases were retrieved and analyzed according to the parameters. Descriptive statistics and a Chi-square test were used to assess the association between different variables and logistic regression to develop a risk prediction model to screen inferior alveolar nerve injuries. Results: Inter-observer and intra-observer agreements were excellent for all CBCT variables. Direct contact of the third molar with the mandibular canal was seen in 61.7% of the cases. Statistically significant association was found for loss of corticalization (OR-5.95; P = 0.000), narrowing (OR-18.916; P = 0.000), and dilacerations of roots (OR-3.8; P = 0.004) with “direct contact” of the impacted mandibular third molar to the mandibular canal. Conclusion: Preoperative assessment with CBCT entails the opportunity to ascertain the “high-risk” impacted mandibular third molars at a cheaper cost, with less equipment maintenance and reduced exposure, compared to computed tomography (CT) imaging.

Keywords: Cone beam computed tomography, DICOM image, impacted third molar, mandibular canal, odds ratio (OR)

How to cite this article:
Jacob JE, Balan A, Bose C T, Nabeel A K, Girija K L, Ramachandran S. Evaluation of the spatial relationship of impacted mandibular third molar to mandibular canal using CBCT: A descriptive study. J Indian Acad Oral Med Radiol 2022;34:423-7

How to cite this URL:
Jacob JE, Balan A, Bose C T, Nabeel A K, Girija K L, Ramachandran S. Evaluation of the spatial relationship of impacted mandibular third molar to mandibular canal using CBCT: A descriptive study. J Indian Acad Oral Med Radiol [serial online] 2022 [cited 2023 Feb 3];34:423-7. Available from: http://www.jiaomr.in/text.asp?2022/34/4/423/363030

   Introduction Top

In cases of proximity between the mandibular third molar and the mandibular canal, inferior alveolar nerve injuries have been reported to increase by up to 30%.[1] To enhance surgical planning and to deter complications, proper delineation of the mandibular canal and its relationship with the surrounding structure is mandatory, using radiological investigations. Cone beam computed tomography (CBCT) is preferable to conventional imaging modalities for presurgical assessments, taking into consideration its reliable measurements and buccolingual assessment.[2] This study aims to radiographically analyze the proximity of the impacted mandibular third molar with the mandibular canal using CBCT among the South Kerala population.

   Materials and Methods Top

The research was authorized by the Institutional Ethics Committee (Institutional ethical clearance number: IEC/R/25/2015/DCT/dtd 1/12/2015) and conducted in compliance with Helsinki considerations. Good clinical practice guidelines were followed, and informed consent was obtained from every participant. The sample size was obtained using the formula 4 pq, where p is the proportion of patients with paresthesia following surgery, q is 100-p, and d is the precision taken as 20%. So the minimum sample size required was 300, within 1.5 years. Tomograms depicting pathologic alterations linked with impacted mandibular third molars were disregarded.

CBCT images were acquired using the Planmeca ProMax 3D (Finnish Planmeca Group) by Planmcea, Helsinki, Finland extraoral imaging system using a flat panel detector with a 12-bit sensor and 4096 gray values. The exposure specifications were as follows: kilovoltage peak: 90 kVp, tube current: 10 mA, time: 11.0 s, and isotropic voxel size varied between 0.2 and 0.4 mm, depending on the dynamic field of view. For the reconstruction and measurement of the scans in all the orthogonal planes, the Planmeca Romexis Viewer software was employed. Amending was done utilizing the adjustment tools for improving the visibility of the mandibular canal's course. The initial step was to obtain the panoramic reconstruction based on the CBCT data, followed by tracing of the right and left mandibular canals with a specific tool (” nerve tool” ), and the software automatically traced them on the other sections [Figure 1]. All images were reviewed by two oral and maxillofacial radiologists with CBCT competence. If the results were contentious, a senior professor intervened and the final results were validated by the three specialists. To account for the intra-observer variability, repeated analysis of a random sample of 30 images was done by the principal investigator after one month.
Figure 1: Mandibular canal traced in reconstructed panorex

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SPSS for Windows, Version 16.0 (SPSS Inc., Chicago, 2007) was used for statistical analysis. Kappa (k) values were computed to assess both the intra-examiner agreements and inter-examiner agreements. k value <0.40 was regarded as poor agreement, 0.40-0.59 was considered acceptable agreement, 0.60-0.74 was considered good agreement, and 0.75-1.00 was deemed great agreement. Association was tested statistically by the Chi-square test or Fisher's exact test. The predictive impact of parameters was evaluated by developing a multivariable logistic regression model with “direct contact” as the desired outcome. The model was attained using a stepwise backward elimination method with threshold values of 0.10 and 0.05 for elimination and incorporation of predictors.

   Results Top

Intra-observer agreement [Table 1] was excellent for all the CBCT variables and inter-observer reliability [Table 2] was also high. The detailed distribution of the patient's age, gender, and side is mentioned in [Table 3] and [Table 4]. Contact of the third molar with the mandibular canal was visible in 185 (61.7%), whereas contact between the third molar and mandibular canal was not apparent in 115 (38.3%) cases.
Table 1: Intra-observer agreement

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Table 2: Inter-observer reliability

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Table 3: Summary of primary outcome variables

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Table 4: Statistically significant independent variables on bivariate analysis

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   Discussion Top

A structured proforma was used to record the patient's socio-demographic data as well as tomographic findings. The minimum age identified in this study was 20 years [Figure 2], which is similar to the study by Gupta et al.[1] The present study found a modest male predominance (51.7%) [Figure 2] which is consistent with Gupta et al.[1] and in disparity with Mohanty.[3] This might be attributable to differences in sample sizes.
Figure 2: Distribution of age and gender among the study population

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In contrast to the study by Gupta et al.,[1] the majority of the impacted mandibular third molars (51%) were identified on the right side in the current investigation, which might potentially be related to differences in sample sizes. On the assessment of impacted mandibular third molars based on angulation, it was observed that the most prevalent was mesioangular (51.3%), followed by vertical (30%), horizontal (12.7%), distoangular (5.7%), and inverted (0.3%). Mesioangular impactions were the most often observed, which is consistent with research by Sedaghatfar et al.,[4] and Issrani et al.[5] This could be owing to the anterosuperior direction of its eruption. Mesioangular impactions were found to be in proximity to the inferior alveolar canal in research done by Miloro and DaBell, but vertical impaction was found to be more prevalent in a study done by Hugoson.[6]

According to the criteria outlined by Wang et al.,[7] the topographic association between root apex and lingual plate on cross-sectional CBCT images was categorized as non-contact, contact, and perforation. We found that 11.3% [Figure 3] and [Figure 4] of the teeth had contact with the lingual cortex, and 7.6% had a perforation of the lingual cortex. This was in accordance with the study done by Emes et al.[8] The lingual position of the mandibular canal was observed in a majority of subjects in the current study. One hundred and fifty-seven (52.3%) cases of the mandibular canal were lingual, 120 (40%) were buccal, and 23 (7.7%) were inter-radicular. This was in concordance with Ueda et al.,[9] Neugebauer et al.,[10] and in contrast to Saptadi et al.[11] Variations could be due to diverse ethnicities. The inter-radicular course was 7.7%, which was in consensus with the study by Maegawa et al.[12]
Figure 3: Topographic relationship between root apex and lingual plate

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Figure 4: Topographic relationship of root apex to the lingual plate and buccolingual position of the mandibular canal

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In the present study, 11 (3.7%) cases revealed ramifications of the mandibular canals, which was in congruence with the study by Guilherme et al.,[13] where ramifications were found in 30 (12%) cases.

Similar to the study carried out by Liu et al., this study categorized the course of the mandibular canal into linear, spoon-shaped, elliptic, and turning curve types. The most common course encountered in the present study was an elliptic curve (40%), followed by spoon-shaped (29%), linear curves (19.7%), and turning curves (11.3%). This was in agreement with the research conducted by Chung et al. and is contrary to the findings of Ozturk et al.[14]

Bivariate analyses were done for all the above-mentioned CBCT variables to determine which CBCT parameters were more likely to have associated with “direct contact,” which is used as the diagnostic criterion in expecting inferior alveolar nerve exposure in our study. Statistically, a significant association was seen between “direct contact” and gender, Winters, Pell, and Gregory classification, corticalization, narrowing, the shape of the mandibular canal, the topographical relationship of root apex to the lingual plate, dilaceration of roots on bivariate analysis [Table 4].

After adjusting for the data source, the multiple logistic regression model revealed that loss of canal corticalization (Adjusted OR-5.95; P = 0.000), canal narrowing (Adjusted OR-18.916; P = 0.000), and dilaceration of the roots (Adjusted OR-3.8; P = 0.004) were the three independent risk factors for “direct contact.” Among the three, narrowing of the mandibular canal has shown the maximum peril due to its highest adjusted odds ratio and highly significant association with the outcome variable in this study. Loss of corticalization was associated with a 5.9-fold increased risk, and dilaceration of the roots was associated with a 3.8-fold risk of postoperative injury. The notification of the cortical status is fundamental, as per the studies done by Tantanapornkul et al.[15] and Nakamori et al.[16]

Limitations of the study

  • The major drawback of this research was the utilization of a surrogate risk predictor, the “direct contact” in CBCT images, for predicting inferior alveolar nerve injury. Intra-operative examination for the proximity of the inferior alveolar nerve to the impacted mandibular third molars should have been the true risk predictor.
  • We reported a single-center experience, which might limit the generalization of our data.

Strengths of the study

  • To the finest of our knowledge, it was the first study conducted in the South Kerala population to evaluate the proximity of the impacted mandibular third molar to the mandibular canal using CBCT.
  • All outcome measures were assessed by two independent oral and maxillofacial radiologists with experience in the field of CBCT to ensure good inter-observer agreement, and repeated analysis of a random sample of 30 CBCT images was done by the principal investigator after one month to account for the intra-observer variability. All these procedures avoided measurement bias and guaranteed good reliability for the study.
  • Our study sample was large and heterogeneous (51.7% men and 48.3% women with an age range of 20 to 71 years).

Future prospects

Further prospective research with larger sample size and the association of nerve exposure during the surgical procedure with an assessment of probable neurological complications are required as the “Gold Standard” to substantiate the clinical versatility of CBCT in preoperative evaluation of impacted mandibular third molars.

   Conclusion Top

In conclusion, the results confirm that specific variables such as dilaceration of roots, loss of corticalization, and narrowing are linked to an increased risk of injury to the inferior alveolar nerve. This study could recommend CBCT as a preoperative diagnostic tool for revealing the three-dimensional anatomical relationship between the mandibular third molars and mandibular canal, thereby prudent planning of the surgical procedure.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Key message

CBCT provides the exact position, anatomical variation, and course of the canal, thereby allowing the surgeon to modify the surgical strategy or for an alternative approach.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

Gupta S, Bhowate RR, Nigam N, Saxena S. Evaluation of impacted mandibular third molars by panoramic radiography. ISRN Dent 2011;2011:406714.  Back to cited text no. 1
Sandhu A, Sandhu S, Kaur T. Comparison of two different flap designs in the surgical removal of bilateral impacted mandibular third molars. Int J Oral Maxillofac Surg 2010;39:1091-6.  Back to cited text no. 2
Mohanty R, Rout P, Singh V. Preoperative anatomic evaluation of the relationship between inferior alveolar nerve canal and impacted mandibular third molar in a population of Bhubaneswar, Odisha, using CBCT: A hospital-based study. J Maxillofac Oral Surg 2020;19:257-62.  Back to cited text no. 3
Sedaghatfar M, August MA, Dodson TB. Panoramic radiographic findings as predictors of inferior alveolar nerve exposure following third molar extraction. J. Oral Maxillofac Surg 2005;63:3-7.  Back to cited text no. 4
Issrani R, Prabhu N, Sghaireen M. Comparison of digital OPG and CBCT in assessment of risk factors associated with inferior nerve injury during mandibular third molar surgery. Diagnostics (Basel) 2021;11:2282.  Back to cited text no. 5
Hugoson A, Kugelberg CF. The prevalence of third molars in a Swedish population. An epidemiological study. Community Dent Health 1988;5:121-38.  Back to cited text no. 6
Wang D, He, X, Wang Y. Topographic relationship between root apex of mesially and horizontally impacted mandibular third molar and lingual plate: Cross-sectional analysis using CBCT. Sci Rep 2016;6:39268.  Back to cited text no. 7
Emes Y, Oncu B, Aybar B, Al-Badri N, Işsever H, Atalay B, et al. Measurement of the lingual position of the lower third molar roots using cone-beam computed tomography. J Oral Maxillofac Surg 2015;73:13-7.  Back to cited text no. 8
Ueda M., Nakamori K., Shiratori K., Igarashi T, Sasaki T, Anbo N, et al. Clinical significance of computed tomographic assessment and anatomic features of the inferior alveolar canal as risk factors for injury of the inferior alveolar nerve at third molar surgery. J Oral Maxillofac Surg 2012;70:514-20.  Back to cited text no. 9
Neugebauer J, Shirani R, Mischkowski RA. Comparison of cone-beam volumetric imaging and combined plain radiographs for localization of the mandibular canal before removal of impacted lower third molars. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008;105:633-42.  Back to cited text no. 10
Saptadi A, Lilies DS, Menik P, Latief BS. Evaluation of impacted mandibular third molar position in relation to mandibular canal on panoramic radiography compared to cone-beam computed tomography. J Int Dent Med Res 2019;12:591-6.  Back to cited text no. 11
Maegawa H, Sano K, Kitagawa Y. Preoperative assessment of the relationship between the mandibular third molar and the mandibular canal by axial computed tomography with coronal and sagittal reconstruction. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2003;96:639-46.  Back to cited text no. 12
Leite GM, Lana JP, de Carvalho Machado V, Manzi FR, Souza PE, Horta MC. Anatomic variations and lesions of the mandibular canal detected by cone beam computed tomography. Surg Radiol Anat 2014;36:795-804.  Back to cited text no. 13
Ozturk A, Potluri A, Vieira AR. Position and course of the mandibular canal in skulls. Oral Surg Oral Med Oral Pathol Oral Radiol 2012;113:453-8.  Back to cited text no. 14
Tantanapornkul W, Okouchi K, Fujiwara Y, Yamashiro M, Maruoka Y, Ohbayashi N, et al. A comparative study of cone-beam computed tomography and conventional panoramic radiography in assessing the topographic relationship between the mandibular canal and impacted third molars. Oral Surgery Oral Med Oral Pathol Oral Radiol Endodontol 2007;103:253-9.  Back to cited text no. 15
Nakamori K, Fujiwara K, Miyazaki A, Tomihara K, Tsuji M, Nakai M, et al. Clinical assessment of the relationship between the third molar and the inferior alveolar canal using panoramic images and computed tomography. J. Oral Maxillofac Surg 2008;66:2308-13.  Back to cited text no. 16


  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3], [Table 4]


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