In this study, we only used CBCT images of the mandible obtained with the smallest FOV, 40 × 40 mm. On those images, the range of 30 mm in length in the mandible just posterior to the mental foramen was divided into three equal areas, each of which was 10 mm in length. They were designated as areas 1, 2, and 3, from anterior to posterior. After that, the visibilities of the superior and inferior walls of the mandibular canal in each area were evaluated separately. Although the location of the mental foramen differs among individuals, it is mostly situated below the second premolar or between the apices of the first and second premolars [17, 18]. Thus, it is considered that areas 1, 2, and 3 in our study nearly corresponded to the second premolar to first molar region, the first molar region, and the second molar region, respectively. Visualization of the superior wall in our study was significantly poorer than that of the inferior wall in all areas. Further, concerning the variance among areas, the visibility ratio was highest in the most posterior area (area 3) and tended to decrease gradually towards the mental foramen for both walls.

Although there have been no detailed studies using CBCT, poorer visualization of the superior wall compared with the inferior wall has been reported by some studies using conventional radiographs or medical CT images [19,20,21]. Whether the wall of the mandibular canal is visible or invisible on images largely depends on the presence or absence of corticalization of the wall surrounding the neurovascular bundle. Bertl et al. [22] performed histomorphological observation of the mandibular canal wall using thin sections of the first molar region of the mandible from 50 cadavers. They identified corticalization of the cranial (superior) and caudal (inferior) wall in 65% and 81%, respectively. Although they only observed the first molar area, their results may be considered consistent with ours of poorer visibility of the superior wall on CBCT images. The presence of nerves and vessels rising to the lower teeth from the mandibular canal may partly explain the lower corticalization rate of the superior wall [23, 24]. Further, the presence or absence of corticalization of the canal wall may be correlated with the trabecular bone volume or density [22, 25]. However, quantitative evaluation of the trabecular bone was difficult in our study using CBCT images. On the other hand, concerning the variance in the visibility of the mandibular canal based on anteroposterior location, several studies using CBCT reported that the mandibular canal can be more easily identified in the posterior region compared with the anterior region [3, 5,6,7,8]. An anatomical study using cadavers has reported similar results [23]. Our study evaluated the superior and inferior walls separately, with similar results, although a significant difference was only found between area 3 and area 1 for the superior wall.