DISCUSSION

The results of this study on the distance of MC from posterior teeth roots showed that the distal root of third molar was the closest root to canal, so that, the average distance between the left and right sides was 2.88 and 2.49 mm, respectively. By moving towards the anterior area, the canal gets farther away from posterior teeth apex, so that, the average distance of mesial root of the first molar on the left and right sides was, 3.96 to 4.64 mm, respectively. Fewer studies were performed to examine the distance of the third molars roots from MC, and most studies in this field only tried to examine the canal course in the impacted and unerupted third molar area (19, 20). Chong et al. on 272 second mandibular molars, reported that in 55% of cases, the distance between the root apex and inferior alveolar nerve was less and equal to 3 mm, which is close to the results of this study (21). Simonton in a study reported that the distance of MC from mesial root of first molar was 4.9 mm in women and 6.2 mm in men, which is closely consistent with the results of this study (22).

In this study, MC distance from the inferior mandibular cortex in the distal root of third molar area was 7.52 to 8.41 mm on the right and left sides respectively, and this distance decreased gradually by moving forward to the mesial root of first molar and increased again in the premolar area. Rajchel et al. in a study on cadavers reported that this distance was mm10 in the third molar area (23); with respect to the fact that mandibular form vary in different people and in different age ranges, so the differences in measurement seems normal. Also in this study, it was observed that MC was closer to the apex of posterior teeth rather than the inferior mandibular cortex. Sato in a study on panoramic images indicated that the MC course in the vertical dimension was closer to the apex of first and second molars rather than inferior mandibular cortex (24).

The MC distance from buccal and lingual cortical borders, it was observed that distal root of third molar was the closest root to lingual plate and the second premolar tooth was the closest root to buccal plate. The average distance of MC to the lingual cortical plate in the distal root of third molar was 1.64 and 1.98 mm on the right and left sides, respectively. In Rajchel’s study, the canal in the third molar area had approximately 2mm distance from the lingual plate, which is very close to our results (23). In the present study the average distance of MC from buccal cortex in the mesial root of the first molar was 4.44 and 4.53 mm on the right and left sides, respectively. Leith et al. in a study on 157 CBCT images of patients with a mean age of 48 years, this distance was 4.4 mm in 75% of cases, which is very close to the results of this study (5).

For the MC diameter, it was observed that the average minimum and maximum canal diameter was 1.80 and 2.75mm in second premolar and the distal root of third molar, respectively. Canal diameter from the posterior to the anterior decreased with a gentle slope. Rajechel demonstrated that when proximal to the third molar, MC diameter was 2 to 2.4 mm. on measurements obtained from 105 mandibular cadavers; Obradovic et al. also found that the mean MC diameter in its horizontal part was 2.6 mm, which is closely consistent with these results (23).
One of the common but inadvertant complications in the anterior mandible during implant placement is neurosensory alteration. Mental foramen shows many anatomical variations in shape, size and position. In the present study, 95.2% of patients had anterior loop and the area between premolars on both sides was the most common site for that. Investigations that compared radiographic and cadaveric dissection data with respect to identifying the anterior loop reported that radiographic assessments result in a high percentage of false-positive and false-negative findings (25). Perhaps these varied results may be attributed to different criteria used to define the anterior loop and dissimilar diagnostic techniques. Arzouman showed 92 to 96% of direct measurements on cadavers had detected anterior loop, while only 56 to 76 % of the panoramic machines showed the loop (25). With regard to the mental foramen, apex of the second premolars or the area between premolars have been reported as the most common site for that. In the study by Haqhanifar et al. on panoramic images, the area between premolars was the most common area for mental foramen, which is consistent with the results of this study (14).

The mean incisive canal diameter in the last visible point was 1.12±0.31mm on the right and 1.06±0.28mm on the left side. Jacobs et al. examined 230 spiral CT where the incisive canal was identified in 93% of the cases, and they reported the average inner diameter was 1.1 mm, which is consistent with our results (25).

For assessing the amount of incisive branch progression, an area between the first premolar and canine teeth was observed as the most common visible area for that on both sides. Most studies have investigated quantitative measures of incisive nerve length and there is no study that has tried to investigate the progression level of the canal compared to other surrounding anatomic structures. Mardinger et al. have examined anatomical and radiographic course of incisive canal in 46 cadaver mandibles, they found that the canal walls in some cases were complete, some incomplete and in others without corticated limits. They concluded that there are correlations between the anatomical structure and visible radiographic limits(26). The visibility or invisibility of incisive canal largely depends on racial differences, radiologists’ experiences and radiographic technique. Pieres et al. showed that the incisive canal is better seen in CBCT images rather than panoramic radiography. They reported the average length of incisive canal was about 7±3.8mm (27). This distance is almost where the mandibular canine apex can be placed. The results of this study are very close to our results.

For the assessment of gender effect on the measured distances, it was found that the overall pattern of MC course was similar in both genders, but in general women have lesser distances than men, which is consistent with

results of other studies in this domain (22, 28). About the influence of age on the measured points, it was observed that the average distance of MC from root apex and from the lower mandibular border was significantly less in under 30 years than other age groups; Given that skeletal growth in these patients is not yet complete, this result is justified. It was also observed that in patients over age 50, bone width was slightly less than other age groups, and according to the first molar was the most missing tooth in this age group; reduced bone width was more evident in this area. Simonton et al. have also reported reduced bone width in patients in their 50s-60s (22). Perhaps the rationale reason is that older patients have generally less bone mass than the younger age groups.

It should be mentioned, CBCT images in horizontal and vertical planes can help in the examination of the MC course, because the canal can pass different courses in each view for different patients. Anderson et al. in a study on panoramic radiographs found that the MC may slowly come down from anterior to posterior or have a gentle progressive curve, or even a combination of these two (23). Also, in the horizontal plane the canal course extends from lingual to the buccal border, which in most cases, the canal in the first molar area is in the middle distance between the bucco-lingual plates (23). In the present study, the second premolar apex was located in the middle of bucco-lingual plates; given that in this study, the most common area for mental foramen was between premolar teeth, it is justified. As Simonton said that as the mental foramen became more distally positioned, the MC became more buccally located within the mandible, and in relation to the roots of the mandibular first molar (22).

This study was conducted on adult patients most images taken due to the replacement of single edentulous area and there are a few studies that tried to examine the relationship between canal and all mandibular posterior teeth by CBCT imaging, and this is one of the salient points of this study. However, given that in this study, measurements were performed on patients with partial and complete tooth, and classification of the age groups needed more details, this limitation cannot be forgotten. It is recommended to perform further investigation with a greater sample size with complete teeth and considering panoramic and CBCT images can have many clinical benefits during surgical procedures in this area. The appropriate sensitivity and specificity of CBCT in the detection of these alterations reinforces its use in oral and maxillofacial radiology, and since the bone dimensions are not fixed in one’s life, providing CBCT before surgery is necessary.