RADIOGRAPHIC ASSESSMENT OF THE COURSE AND VISIBILITY OF THE MANDIBULAR CANAL BY PANORAMIC RADIOGRAPHY

Background and Objectives:  The mandibular canal is a canal inside the mandible that contains the inferior alveolar artery, the inferior alveolar nerve and the inferior alveolar vein. The anatomy and variations of the human mandible are very important for planning various surgical measures such as extraction of the third molar, dental implants and mandibular reconstruction. This study was conducted to determine the frequency of the different courses of the mandibular canal  and to investigate the visibility of the mandibular canal by panoramic radiography, among a sample of Yemeni adults. 
Subjects and Methods: A retrospective cross-sectional study was conducted in Sana'a city on a sample of Yemeni adults. The study consisted of panoramic radiographs of 928 patients; the number of males was 340 (36.6%) and 588 females (63.4%). The mandibular canal course, as shown in the panoramic radiographs, was classified into four types: elliptical, linear, turning curves and spoon-shaped. The visibility of this canal from the first to the third molar region was assessed through visual determination of whether the lower canal was clearly visible, possibly visible, or invisible. 
Results:   Elliptical curves were the most observed along the mandibular canal course at 62.2% of the total, followed by the linear curve (24.8%) while the lower rate curve spoon (7.9%) and the turning curve (5.2%). There were no statistically considerable differences between the distribution of the mandibular canal courses between males and females except in the spoon curve where the ratio was 10.8% in males versus 6.1% in females (p <0.05). The percentage of clearly visible mandibular canals was the highest among the spoon-shaped curves (49.9% in the first molar, 63% in the second, 78.1% in the third molar) and the lowest among the linear curves (10.9% in the first, 18.7% in the second and 33.5% in the third molar). On panoramic radiographs, the invisible mandibular canals are found in 198 (21.3%) of the examined sites in the first molar region, in 85 (11%) in the second molar region, and in only 6 (0.64%) in the third molar region. 
Conclusion:  It was observed that elliptical curves along the path of the mandibular canal were the most present, while a decrease in the ratio of the two types of spoon curve and turning curve was observed. It was found that spoon curve was significantly more common among males than females. It was also found that the visibility of  madibular canal is fewer in the first molar region than in the third molar region. 
Peer Review History: 
Received: 21 September 2020; Revised: 8 October; Accepted: 29 October, Available online: 15 November 2020 
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Received file 
 
Average Peer review marks at initial stage: 6.5/10 
Average Peer review marks at publication stage: 8.0/10 
Reviewer(s) detail: 
Dr. Sunita Singh, Baylor College of Medicine, Houston, Texas, USA, sunita.nccs@gmail.com 
Dr. Ali Abdullah A. Al-Mehdar, Thamar University, Yemen,  alialmehdar2006@yahoo.com 
  
Comments of reviewer(s): 
 
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INTRODUCTION
Inside the mandible is the mandibular canal, which is a canal that contains the inferior alveolar artery, the inferior alveolar vein and the inferior alveolar nerve. It runs obliquely down and forward to the branch, then forward laterally to the body, where it is placed under the alveoli and communicates with it all through small openings. On reaching the incisive teeth, it turns back to connect with the mental foramen, giving rise to a small canal known as the mandibular incisive canal, which extends into the cavities containing the incisor teeth 1 . The mandibular canal is fairly close to the tops of the second molar in 50% of the radiographs. In 40% the canal is distant from the root apices, and in only 10% of the radiographs, the root crests appeared to penetrate the canal. In root canal treatment of the second molar, care must be taken to extend the perforator expander or root canal filling material because there is a potential risk of injury to the inferior alveolar nerve 1-4 . The mandibular canal is an important milestone to consider before performing any posterior mandibular surgery 2-4 . The precise location of the mandibular canal can be provided by radiological diagnosis before surgery and therefore prevent complications during any surgical procedures 5 . The mandibular canal pathway has been investigated in several studies, and frequent anatomical differences have been found in the intrabony course of the inferior alveolar nerve 6-8 . Moreover, the radial profile of the mandibular canal can be alterable 9 . The mandibular canal visibility may vary significantly 10-12 . Wadu et al., discovered that in an adequate number of cases, the radiopaque boundaries are disrupted in radiographs, and it is invisible in some other cases. The mandibular canal typically consists of a thin trabecular bone including many circumferential spaces, and there is a fine layer of cortical bone in just a few of the mandibles 13, 14 . Recently, Ozturk and colleagues' study 15 and Liu colleagues' 16 classified the mandibular canal pathway into linear, spoon-shaped, elliptic, and turning curve types (Figure 1). In panoramas, the mandibular canal is usually observed as a definite radiopaque area, lined with a radiopaque superior and inferior boundaries 5 . Understanding the anatomical contrast of the pathway and the visibility of the mandibular canal will be helpful in planning treatment for procedures concerning the posterior mandible 15 . This study was conducted to determine the frequency of the different courses of the mandibular canal and to investigate the visibility of the mandibular canal by panoramic radiography, among a sample of Yemeni adults. canal curve is approximately a straight line; B-Elliptic curve, the curve is approximately symmetrical; C-Spoonshaped curve, the canal has an approximate spoon shape that is similar to an asymmetric elliptic arc; and D-Turning curve, the course is unsmooth and has a turning point 16 .

SUBJECTS AND METHODS
The samples in this retrospective study were randomly assigned from patients who visited Ammar Dental Clinics and undertaken panoramic radiography in 2019, up to September 2020. The panoramic radiograph was done as part of the clinical diagnostic process for various causes, as extraction of the third molar and implant placement and or, orthodontic treatment planning. The number of subjects was 588 females and 340 males, with a mean age of 27.3 years and the age range was 20-58 years. Patients with mandibular disease were excluded from the study. Using Proline XC panoramic radiographs were taken for all. These images were carefully examined by the first investigator. The mandibular canal course was classified, as in the published research by Liu et al., 16 where they classified them into four types: elliptic, linear turning curves and spoon-shaped (Figure 1

RESULTS
Elliptical curves (Figure 1 There were no statistically significant differences between the distribution of the mandibular canal courses between males and females except in the spoon curve where the ratio was 10.8% in males versus 6.1% in females (p <0.05). The percentage of clearly visible mandibular canals was the highest among the spoon-shaped curves (49.9% in the first molar, 63% in the second, 78.1% in the third molar) and the lowest among the linear curves (10.9% in the first, 18.7% in the second and 33.5% in the third molar). In panoramic radiographs, invisible mandibular canals were found in 198 (21.3%) of the examined sites in the first molar region, in 85 (11%) in the second molar region, and in only 6 (0.64%) in the third molar region.   16 . The linear curve (24.8%) in the current study was completely different from the straight projection (12.2%) observed by Ozturk et al., in 2012, and the other curves were not similar to their findings 15 . In the study by Liu et al., 16 elliptic curves (48.5%) were the most frequent, and spoon-shaped curves (29.3%) were the subsequently most frequent pathway. The most common pathway found in the current study was the elliptical curve (62.2%), followed by the linear curves (24.8%) and the spoon-shaped curves (7.9%). In the study conducted by Jung and Cho in Korea in 2014 18 , elliptical curves (64.7%) were the most common, followed by linear curves (22.9%) and spoon curves (6.9%). This result is nearly similar to the current study in which the elliptic curve was 62.2%, followed by the linear curves (24.8%) and the spoon-shaped curves (7.9%). These results indicated that in Yemeni patients there is more space for implant placement, because the elliptical and spatula curves can provide more space for implant placement than the linear curves 18 . In the current study, invisible mandibular canals were discovered in 198 (21.3%) of the examined sites in the first molar region, in 85 (11%) in the second molar region, and in only 6 (0.64%) in the third molar region. This finding is consistent with the fact that the visibility of the mandibular canal may differ between patients and between different areas of the lower jaw as described by Oliveira Santos et al., 2 . The current incidence of invisible mandibular canals (21.3%) was lower than that reported by Klinge et al.,19 as the mandibular canal were not visible in 36.1% of the panoramic radiographs. Current study rate (21.3%) is also lower than that of Naitoh et al., 20 who reported that in panoramic images, the canal was completely invisible in 32% of the inspected sites in the molar region. While current study rate was roughly similar to that reported by Jung and Cho 18 in that the mandibular canal was not invisible in 22.7% of the sites examined in the first molar region on panoramic radiographs. Additionally, Lindh et al., 21 in Sweden accounted that the mandibular canal of the specimen cadavers was obviously visible in 25% of the panoramic radiographs. In this study, on the panoramic radiographs, the mandibular canal was obviously visible in 13.5% of the examined positions in the first molar region, 19.3% of the positions examined in the second molar region and 47% of the sites inspected in the third molar region. Imaging of the mandibular canal wall on the panoramic images was linked to the bone density in the alveolar region 20 . Decreased visibility of the mandibular canal wall on a panoramic radiograph may reveal decreased integrity of the mandibular canal wall and reduced bone trabeculation. Moreover, the extent of mandibular canal vision differed according to the the mandibular canal pathway. The percentages of clearly visible mandibular canals were highest in spoon-shaped curves (47.9% in first, 37% in second, 78.1% in third molar) and lowest in linear curves (10.9% in first, 18.7% in second, and 33.5% in the third molar). The current study result is comparable to that reported by Jung and Cho 18 . What's more the current study results showed that visibility of the mandibular canal in the third molar region was better than that in the first molar region ( Table 2). This result is similar to that reported previously 18-21 .

CONCLUSION
In conclusion the elliptical curves along the path of the mandibular canal were the most present, while a decrease in the ratio of the two types of spoon curve and turning curve was observed. It was found that spoon curve was significantly more common among males than females. It was also found that the visibility of this canal is less in the first molar region than in the third molar region, that's mean the visibility of the mandibular canal increased in more distal regions of the canal.

AUTHOR'S CONTRIBUTION
This research work is part of a research work under the direction of Al-Kasem Mohammed Abbas. The panoramic radiography images were carefully examined by Nashwan Yahya Al-Shamahi, the professor of radiology, with the help of other authors. All authors assisted with editing the manuscript.