Abstract
Introduction and aims
Violations of the mandibular canal (MC) and mental foramen (MF) and subsequent injuries to their neurovascular bundle have been reported after surgical and nonsurgical dental procedures. Besides using advanced technologies such as cone-beam computed tomography (CBCT), clinicians should be aware of the anatomy and location of MC and MF in different populations. This study aims to describe the morphologic characteristics of the MF, MC, and its intrabony location in relation to the apices of mandibular posterior teeth in an Emirati subpopulation using CBCT.
Methods
A total of 3700 CBCT scans were screened, and 154 scans that met the inclusion and exclusion criteria were randomly selected. The scans were assessed using 3-dimensional multiplanar imaging for the following structures: the location of MF and the MC course, its intrabony location, and its relationship to the apices of the mandibular posterior teeth. The data were analysed statistically using SPSS software.
Results
The MC ran lingually and inferiorly at the posterior region and became more buccal and superior towards the MF. The distal root of the mandibular second molar was found to be the closest root to the MC (2.06 ± 1.83 mm). Moreover, the most common location of the MF was distal to the contact area between the 2 premolars (0.83 ± 1.84 mm) with a significant negative correlation to age (with and increase in age, the MF moves distally). The distance between the root apices and the MC was statistically significantly affected by age (positive correlation) and gender (male patients had a greater distance).
Conclusions
The common course of the MC is lingual and inferior posteriorly and becomes more buccal and superior towards the MF, which is located mostly between the mandibular first and second premolars. Furthermore, the distal root of the mandibular second molar is the closest to the MC and has a positive relationship with age.
Key words: Mandibular canal, Mental foramen, CBCT, UAE, Anatomy, Emirati population, Complication
Introduction
The mandibular canal (MC) is an essential intrabony structure within the mandible that houses the inferior alveolar nerve (IAN) and artery. It begins at the mandibular foramen in the middle third of the ascending ramus medially. The MC runs forward in the body of the mandible and ends at the mental foramen (MF) on the lateral surface.1 Complications as a consequence of violations of the MC and MF and subsequent injuries to their contents have been reported after surgical and nonsurgical dental procedures. Violations may occur during local anaesthesia administration,2 root canal preparation, irrigation,3 root canal intracanal medicaments and filling,4 surgical endodontic procedures, implant placement, and surgical extraction.5 One of the leading causes of the described complications is the close proximity of the MC to the root apices of mandibular posterior teeth and extensive variation in the MF location.5 Recent advancements in available diagnostic tools have been used to locate the MF and MC, including cone-beam computed tomography (CBCT).6,7 However, clinicians must be well versed in their knowledge of the anatomy of the MF and MC, including the intrabony location and course of the MC, the relation to adjacent anatomical structures, the termination point of the MC, anatomical variations, and the effect of ethnicity and gender on such variations.
Several methods have been used to study and describe the location and course of MC, such as sectioning dry mandibles, panoramic radiography, micro-computed tomography, and CBCT. The latter allows a 3-dimensional assessment of dental and maxillofacial structures noninvasively. Moreover, CBCT allows for correlating specific anatomical characteristics with age, gender, and ethnicity.6,7
Extensive review of the current literature has shown that limited studies, if any, have analysed the intrabony location and course of MC and MF within the Emirati population. Such data would highlight possible MC and MF anatomy variations in this population, leading to better treatment planning and decision-making processes before, during, and after surgical and nonsurgical dental procedures. In addition, dentists may be encouraged to use additional diagnostic tools such as CBCT during the treatment of mandibular posterior teeth and referral of complex cases to specialists to reduce possible violations of the MC and MF. Therefore, this retrospective study aims to evaluate the course and intrabony location of MC and MF in an Emirati subpopulation using CBCT.
Methods
Sample collection
Institutional review board approvals were obtained from the concerned committees of the Mohammed Bin Rashid University of Medicine and Health Sciences and Healthpoint Dental Center (HDC) to conduct this retrospective study. CBCT scans of patients treated at HDC between 2017 and 2018 were obtained and analysed. CBCT scans were acquired using the Orthophos SL 3D (Dentsply Sirona). The imaging protocol was as follows: field of view = 16 × 11 cm; tube peak potential = 85 kVp; tube current = 7 mA; time = 5 seconds; voxel size = 0.15 mm. The scans of patients who met the following inclusion criteria were included in the study: Emirati, age between 16 and 75 years, and presence of fully mature and erupted bilateral permanent mandibular first and second premolars and molars. CBCT scans showing the presence of impacted teeth, supernumerary teeth, large pathologic lesions, or a history of mandibular fracture or fractures treated with surgical plates were excluded from the study. Scans that met the inclusion and exclusion criteria were randomly selected, anonymised, and exported from the HDC database in Digital Imaging and Communication in Medicine (DICOM) format. The sample size of 154 scans was determined based on power analysis using Cochran test.
Radiographic evaluation
One endodontist evaluated all scans on an iMac computer (27-inch screen size with Retina 5K display, 5120 × 2880 resolution with support for 1 billion colours, 500 nits brightness; Apple) in a room with controlled lighting using the Horos DICOM viewer (horosproject.org).8 The MC course, intrabony location, and MF location were assessed using the 3-dimensional multiplanar reconstruction (3D MPR) tool. All images were examined in the axial, coronal, and sagittal planes by the evaluator (endodontic resident) who was trained before the evaluation process by an expert oral and maxillofacial radiologist on a sample of CBCT scans, exhibiting different courses and locations of MC and MF. Moreover, the evaluator has read 25% of the scans twice with a 3-month interval for intrarater reliability and data validation.
The evaluator recorded the following measurements: (1) the distance between the MC and the root apices; (2) the distance between the MC and buccal, lingual, and inferior mandibular outer cortex; and (3) the distance of the mesial border of MF to a vertical line that runs apically from the interproximal area between the mandibular premolars. When the MF is distal to this line, a negative value is assigned to this measurement, and when it is mesial the value is positive. Finally, the findings were tabulated and correlated with age and gender.
Statistical analysis
Data were analysed using SPSS for Windows version 25.0 (SPSS Inc.). Measurements were tested for normality using the Shapiro–Wilk test/Kolmogorov–Smirnov test. The measurement was described by means and standard deviations and with 95% confidence intervals (CIs); bars and error bars were used to graphically describe the MF location measurements. Where 2 continuous independent variables (MF locations) were examined, an independent t test was used if the measurement were normally distributed. In case of nonnormality of the measurements, the Mann–Whitney test for continuous related data was used. In addition, the correlation coefficient was used to test the association between age and the location of MF in relation to the premolars. Kappa test was used to test intrarater reliability. A P value <.05 was considered significant in all statistical analyses.
Results
Overall, 3700 CBCT scans were reviewed and 154 CBCT scans were selected based on inclusion/exclusion criteria and sample size calculation. The results of the intrarater reliability tests showed nearly perfect intrarater agreement (r = 0.99; P < .001).
Of the154 patients, 72 (46.8%) were female and 82 patients (53.2%) were male (Figure 1). The age of patients ranged from 16 to 71 years; more specifically, 26% were aged 30 or younger, 28.5% were between 31 and 40 years, and 45.5% were older than 40.
Fig. 1.
Patient distribution according to age and gender.
Overall, the distance from the mesial border of the MF to the line drawn from the contact point of the premolars showed that the mean distance for the left and right sides of the mandible was distal to the line by 0.63 ± 2.54 mm and 0.2 ± 2.42 mm, respectively. The cumulated overall mean distance was distal by 0.83 ± 1.84 mm. The most distal location of the MF was 8.6 mm, whilst the most mesial location was 5.2 mm (Figure 2).
Fig. 2.
Mean distance between the premolars and the mental foramen as well as mean distance of the apices of the roots of mandibular first and second molars and the mandibular canal.
The mean distance between the superior border of the MC and the root apices of the mandibular second premolar, mesial and distal roots of the first molar, and mesial and distal roots of the second molar were 4.02 ± 2.02 mm, 4.54 ± 1.96 mm, 4.07 ± 2.08 mm, 2.58 ± 1.79 mm, and 2.06 ± 1.83 mm, respectively (Table; Figure 1). Comparing the different distances shows that both the mesial and distal roots of the mandibular second molar were significantly closer to the MC when compared to their respective roots of the mandibular first molar (P < .001). Moreover, the distal root of the mandibular second molar was significantly closer to the MC than the mesial root (P < .05; Figure 3). Furthermore, the different distances of the root apices to the MC have been categorised into 4 categories: (1) at the MC (distance = 0 mm), (2) within 0.5 mm, (3) within 1 mm, and (4) more than 1 mm from the MC. None of the apices of the mesial and distal root of the mandibular first molars were at the MC. In contrast, 1.9% and 3.9% of the mesial and distal root apices, respectively, of the mandibular second molars were at the MC. The distal root's apex of the mandibular second molars was within 0.5 mm of the MC in 22.1% of the cases compared to the mesial root's apex of 9.4%. Almost 30.5% of the distal root apices of the mandibular second molars were within 1 mm of the MC, whilst 69.5% were more than 1 mm from the MC. Most of the studied samples had the mesial and distal root apices of the mandibular first molars more than 1 mm from the MC (99% and 96.4%, respectively; Figure 4). Moreover, the mesial and distal roots’ apices of the mandibular first molar were within 2, 3, and 4 mm from the MC in 8% to 13%, 19% to 26%, and 35% to 43% of all studied samples, respectively. Furthermore, the mesial and distal roots’ apices of the mandibular second molar were within 2, 3, and 4 mm from the MC in 38% to 51%, 55% to 66%, and 71% to 80% of all studied samples, respectively.
Table.
Distances between the MC and the root apices of mandibular posterior teeth, buccal border, lingual border, and inferior border of the mandible with the MC.
| Root apices | Buccal border | Lingual border | Inferior border | |
|---|---|---|---|---|
| Second premolar | 4.02 ± 2.02 | 3.59 ± 1.03 | 2.70 ± 1.03 | 6.6 ± 1.61 |
| First molar MR | 4.54 ± 1.96 | 4.52 ± 1.13 | 1.92 ± 0.8 | 5.52 ± 1.41 |
| First molar DR | 4.07 ± 2.08 | 5.05 ± 1.21 | 1.68 ± 0.68 | 5.16 ± 1.24 |
| Second molar MR | 2.58 ± 1.79 | 5.11 ± 1.27 | 1.77 ± 0.69 | 5.11 ± 1.36 |
| Second molar DR | 2.06 ± 1.83 | 4.83 ± 1.36 | 1.73 ± 0.74 | 5.46 ± 1.51 |
Values are mean distance (mm) ± SD.
DR, distal root; MC, mandibular canal; MR, mesial root.
Fig. 3.
Mean distances between root apices of first and second molars and mandibular canal.
Fig. 4.
Distances between mesial root (MR) and distal root (DR) apices of first and second mandibular molars and mandibular canal (MC), based on the 4 categories: at the MC (distance = 0 mm), within 0.5 mm, within 1 mm, and more than 1 mm from the MC.
The mean distance from the buccal cortex of the mandible to the MC at the second premolar, mesial and distal roots of the first molar, and mesial and distal roots of the second molar were 3.59 ± 1.03 mm, 4.52 ± 1.13 mm, 5.05 ± 1.21 mm, 5.11 ± 1.27 mm, and 4.83 ± 1.36 mm, respectively. Moreover, the mean distance between the lingual cortex of the mandible and the MC at the second premolar, mesial and distal roots of the first molar, and mesial and distal roots of the second molar were 2.79 ± 1.03 mm, 1.92 ± 0.8 mm, 1.68 ± 0.68 mm, 1.77 ± 0.69 mm, and 1.73 ± 0.74 mm, respectively. Finally, the mean distance from the inferior cortex of the mandible to the MC at the second premolar, mesial and distal roots of the first molar, and mesial and distal roots of the second molar were 6.6 ± 1.61 mm, 5.52 ± 1.41 mm, 5.16 ± 1.24 mm, 5.11 ± 1.36 mm, and 5.46 ± 1.51 mm, respectively. All measurements are listed in the Table.
Statistical analysis showed no significant association between the MF location and gender (male patients, −0.3 ± 2.2 mm; female patients, −0.5 ± 2.4 mm; P = .635). However, the distances between root apices and the MC were significantly greater in male patients than female patients. In addition, most mean distances between the MC and different borders of the mandible were greater in male patients than female patients. Interestingly, the distance between the lingual cortex of the mandible and the MC in relation to the mesial root of the second molar was significantly greater in female patients compared to male patients (male patients, 1.4 ± 0.7 mm; female patients, 1.9 ± 0.7 mm; P < .001).
Pearson correlation test showed a negative relationship between age and location of the MF (P < .001). As age increases, the MF tends to be more distal to the line drawn between the premolars. The same test showed a positive relationship between the distance of the root apices of mandibular first and second molars to the MC and age (P < .001), in which as age increases, the distance between the root apices and the MC increases (Figure 5). No significant associations were found between age and distance from the MC to the buccal, lingual, and inferior cortex of the mandible.
Fig. 5.
Scatter plot showing a linear relationship between age and location of mental foramen (A) and distances between mandibular canal (MC) and first molar mesial root (B), first molar distal root (C), second molar mesial root (D), and second molar distal root (E).
Discussion
It is crucial to determine the location of the MF and the MC and its proximity to the mandibular teeth root apices to avoid potential injuries to the IAN bundle during surgical and nonsurgical dental procedures.2, 3, 4, 5 Multiple approaches have been suggested to facilitate the localisation of the MF and the MC, such as the acquisition of knowledge regarding the differences in the location of the MF and course of the MC in different racial and ethnic groups and using advanced clinical techniques such as CBCT. In this study, we examined the intrabony location of the MF and the MC and its relation to the root apices of mandibular posterior teeth in an Emirati subpopulation. The aim is to address the knowledge gap related to the location of the MF and course of the MC in this population and generate data that can supplement clinicians’ other diagnostic tools in planning their surgical and nonsurgical procedures. A summary of findings is illustrated in Figure 2.
Our results showed that the overall mean distance between the mesial border of the MF and the line drawn between the interproximal contact area of the mandibular first and second premolar was distal to the line by 0.83 ± 1.84 mm. These data denote that the MF was commonly located between the premolars and towards the mandibular second premolar. This finding is similar to those of several other studies that identified MF location between the mandibular premolar and closer to the apex of the mandibular second molar.9, 10, 11, 12, 13, 14, 15, 16
Our results show no significant association between the location of the MF and gender (male patients, −0.3 ± 2.2 mm; female patients, −0.5 ± 2.4 mm; P = .635). However, there was a negative correlation between age and the horizontal location of the MF (P < .001). As age increases, the MF tends to be towards the apex of the second premolar. This finding agrees with a study by Al-Khateeb et al, 17 which found that with advancing age, the MF tends to be in a more posterior position amongst a Jordanian population. On the other hand, a contradicting finding was found amongst Polish citizens, in which the MF was found to be in line with the second premolar in younger patients.18 This disparity in findings could be attributed to the difference in ethnicity. The result of this study regarding the location of the MF in an Emirati population will help clinicians working in this area in planning their surgical and nonsurgical dental procedures. For example, to ensure a successful mental nerve block, clinicians might consider the interproximal area between the premolars to be the ideal location to administer a suitable local anaesthetic agent. Furthermore, in surgical procedures involving a vertical incision, avoiding the interproximal area between mandibular premolars is highly recommended. Having the vertical incision mesial to the mandibular first premolar or distal to the mandibular second premolar will be safer. This will aid in preventing possible iatrogenic damage to the MF and its contents. Similarly, caution during incision and drainage, surgical extraction, or implant placement procedures at the mandibular premolar area may avert complications.
Regarding the distance between the MC and the root apices of mandibular posterior teeth, our findings show that the closest root to the MC was the distal root of the second molar, with a mean distance of 2.06 ± 1.83 mm. These findings concur with most studies within the literature, where the closest tooth to the MC was the mandibular second molar.19, 20, 21 When comparing the proximity of the root apices to the MC in terms of age and gender, we found that male patients had a greater distance than female patients for all roots of mandibular teeth except the second premolar (P < .001). Moreover, there was a significant positive relationship between the distance from the MC to root apices and age. Older individuals had a greater distance than younger Emiratis (P < .001). These findings are consistent with those observed by Kovisto et al,22 in which the mesial root of the second molar was closer to the nerve in female compared to male patients. Furthermore, they reported that the root apices in younger patients (<18 years) were generally closer to the MC than in older patients. Similar findings were also observed in Bürklein et al,21 in which individuals younger than 35 years had significantly shorter distances from the MC to the root apices compared with older individuals.
Knowledge about the distance between the MC and the root apices of mandibular teeth is fundamental, as it can help prevent possible procedural errors and damage to its contents. Data from our study show that the distance between the mandibular second molar's distal root apex and the MC is within 0.5 mm or 1 mm in 22.1% and 30.5% of patients, respectively. Furthermore, almost 4% of all studied samples had the distal root of the mandibular second molar at the MC. Consequently, extrusion of endodontic materials such as sodium hypochlorite, calcium hydroxide, root canal filling materials (such as gutta percha) or separated endodontic files through mandibular second molar may result in transient or permanent damage to the IAN. This finding has been reported in several other studies.4,23,24 Therefore, caution must be taken when performing endodontic procedures involving the mandibular second molars.
Moreover, our results showed that roots’ apices of the mandibular first molar are within 2 and 3 mm of the MC in 8% to 13% and 19% to 26% of patients, respectively. This information is of utmost importance for clinicians performing apicoectomy procedures. Vigilance and care should be taken during osteotomy to avoid potential violation of the MC, which might result in direct damage to the neurovascular bundle housed within the MC. Therefore, preoperative planning using CBCT and applying the concept of microsurgical endodontics (with conservative bony crypt preparation) is essential before performing any surgical endodontic procedure involving mandibular teeth.
We have also assessed the canal's course by measuring the distance between the MC and the buccal, lingual, and inferior borders of the mandible. The shortest distance between the MC and the buccal border of the mandible is at the area of the root of the second premolar (3.59 ± 1.03 mm). As for the distance between the MC and the lingual border of the mandible, the shortest was at the area of the distal root of the first molar (1.68 ± 0.68 mm). Our findings are in accordance with the CBCT study conducted by Ozturk et al20 in 2012. On the other hand, our results show that the MC is closest to the inferior border of the mandible at the area of the mesial root of the second molar (5.11 ± 1.36 mm) and farthest in the region of the second premolar (6.6 ± 1.61 mm). Liu et al9 reported similar findings. Therefore, our data suggest that the MC is located in the posterior part of the mandible close to the lingual and inferior borders of the mandible. As it extends anteriorly, it moves more towards the buccal wall of the mandible and away from the inferior border of the mandible as it opens as the MF. This information is significant concerning implant placement procedures.
Conclusions
Based on the results of this study, we can conclude that the most common location of the MF is distal to the contact area between the mandibular first and second premolars (the distance from the mesial border of the MF with the line drawn from the contact point of the premolars = 0.83 ± 1.84 mm), and this distance has a negative relationship with advancing age. We also conclude that the distal root of the mandibular second molar is the closest root to the MC (2.06 ± 1.83 mm). In addition, the distance between the root apices of the mandibular teeth and the MC has a positive relationship with advancing age and is affected by gender, as male patients have a greater distance. Last, the common course of the canal is more lingual and inferior posteriorly and becomes more buccal and superior towards the MF.
Conflict of interest
None disclosed.
Acknowledgments
Acknowledgements
We would like to acknowledge the support of Healthpoint Hospital for providing the support of collecting the data used for this study.
Author contributions
Abdulaziz Alazemi: concept/design, methodology, data collection, data interpretation, drafting article. Eman Al Muhairi: methodology, data collection, critical revision of article. Nouf Alharbi: methodology, data collection, critical revision of article. Farida Abdunabi: data analysis, drafting article, critical revision of article. Mohammed Mashyakhy: data analysis, critical revision of article. Jahanzeb Chaudhry: methodology, writing–review and editing. Keyvan Moharamzadeh: methodology, critical revision of article. Rashid El Abed: data collection, data analysis, writing–review and editing. Mohamed Jamal: concept/design, methodology, data analysis and interpretation, critical revision of article, editing and approving the final draft.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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