Many studies have been conducted on the incidence rate of RMC and RMF using panoramic radiograph which all have reported low incidence rate. Recently, high-resolution CBCT has become notably effective for confirming anatomical variation of mandibular canal that cannot be assessed using panoramic radiograph which all have reported a wide incidence rate ranging from 6.1 to 72% [21, 28].
Numerous studies have been conducted studying the incidence of retromolar canal in different populations, all with varying results. The studied populations included Americans, Canadians, Argentinians, Italian, Japanese, Turkish, Brazilian, and Indian. However, there has been no study conducted on the Egyptian population except for only one study that has been conducted on 11 dry mandibles [5, 22,23,24, 29]. Hence, the current study was aimed to identify the incidence of retromolar canal in the Egyptian population using CBCT.
The CBCT scans of 0.25 mm voxel size were selected which allowed us to evaluate and detect the RMC and RMF of 0.5 diameter or larger, identifying the course and the location of these canals and foramina with good precision and accuracy. Higher resolution scans of 0.125 mm voxel size were available from the same i-CAT next generation CBCT machine in our archive but were not used as they provide limited field of view (FOV) to 4 cm × 16 cm which will limit the height of the scan and will not be enough to evaluate the ramus and the mandibular foramen areas.
The Invivo dental software was used in our study as a reliable and accurate measuring software in accordance with Sabban et al. who evaluated the linear measurements at implant sites using the said software [30].
Previous osseous studies of dry mandibles or cadavers were done on different populations investigating the presence of RMF. These studies showed an incidence rate ranging from 3.2 to 72% [11, 17]. This wide range may be attributed to the lack of 3D visualization of the canal system. These studies were not associated with any RMC classification, as these studies only examined the bony surface without sectioning, and therefore, it can be postulated that some of these foramina were not connected to the RMCs.
It is quite obvious that there is a vast difference between the incidence rate reported in panoramic studies due to the limitation of being a 2D modality and CBCT studies; several studies have reclaimed incidence rates of 3–8% [14, 23, 28] and 7.3–75.4% [5, 27] in panoramic and CBCT studies, respectively. 11.2% was reported in this study which falls in the range of the previously reported studies.
Many studies have been done on the Japanese and Korean populations using CBCT with incidence rate ranging from 25.4 to 75.4% [5, 12, 31] and 8.5 to 17.4% [21, 32, 33], respectively; this wide range of incidence rate can be explained by many factors like the ethnic, genetic, and environmental factors [11, 34].
The wide range of incidence rate of RMC in different studies can be attributed also to the sample size difference in each study and the inclusion and exclusion criteria; a study with large sample size 300 CBCT scans which was done on the Brazilian population showed an incidence rate of 5 % only [27], while the study done on 84 CBCT scans only of the Chilean population showed an incidence rate of 23.8% [35].
Some researchers found a notably higher incidence rate than that found in this current study. For instance, Kawai et al. found a prevalence rate of 52% among the Japanese population [12]. Patil et al. found a prevalence rate of 75.4% as well [5]. This may be attributed to the smaller voxel size used in these studies, which was 0.1 mm and 0.08 mm, respectively. On the other hand, Han and Hwang found an incidence rate of 8.5% when using a 0.38-mm voxel size [21]. Thus, the voxel size of CBCT scans examined affects the quality of the image; a smaller voxel size allows the visualization of much narrower canals therefore affecting the incidence rate in each study [5, 11, 21].
Freitas et al. used a voxel size of 0.25 mm using an i-CAT CBCT Scanner in a protocol similar to ours and showed a comparable result of 7.33% incidence rate in the Brazilian population [27]. Also, a recent study done on the Turkish population showed comparable results [36, 37].
Although the number of RMCs found in males was greater than that found in females, there was no significant difference in gender, which agrees with the findings of Patil et al., Sawyer and Kiely, and Pyle et al. [5, 38, 39].
The number of unilateral canals was found to be greater than that of the bilateral canals, the majority of which were found in the left side of the mandible. However, there was no significant difference which also agrees with the findings of Patil et al., von Arx et al., and Sagne et al. [5, 14, 40].
In this study, we have adopted Patil et al.’s system for the classification of RMCs [5]. We found it to be the most convenient classification. Ossenberg’s [11], Park’s [41], and Jamalpour et al.’s [41] classifications did not include the canal type which starts from the RMF and ends in the radicular portion of the 3rd molar.
According to the course of the canals, the most common type in our study was type A (23/30), followed by type B (5/30), and type C was the rarest type with only 2/30. These results are different from Patil et al.’s study that found type B to be the most common. This can be explained by the voxel size used in their study (0.08 mm) which allowed them to investigate much narrower canals, and most of the type B canals in Patil et al.’s study are of very small diameter [5].
Regarding linear measurements, the present study found the distance between the RMF and the distal surface of the second molar to be 14.70 ± 5.07, which was comparable to the results of Han et al. who found the distance to be 14.08 ± 3.85, von Arx et al. who found the distance to be 15.16 ± 2.39 mm, and Park et al. who found the distance to be 12.1 ± 3.3 mm [14, 15, 21]. Park et al. and Ogawa et al. found the distance between the RMF and the third molar to be 5.8 ± 3.6 mm and 5.5 ± 2.1, respectively, which are not much different from our findings of 4.26 ± 4.21 mm [15, 42].
This study has the merits of being the first study done to investigate the RMC and RMF in the Egyptian population and highlight the incidence rate and the importance of knowing such abnormality; also, these measurements may aid the clinician to better localize the RMF in various clinical procedures. Thus, the 2nd and 3rd molars can be used as anatomical landmarks, hence saving the surgeon from the complication of damaging the retromolar nerve and its associated morbidity.
This study has the limitation of using a relatively large voxel size 0.25 mm which did not allow us to observe and investigate canals narrower than 0.5 mm; also, all data was collected from the Faculty of Dentistry of Ain Shams University located in Cairo governate and so the DICOM data investigated may not represent the whole Egyptian population.