Jung and Cho [6] reported that the mandibular canal was clearly visible in 50% of CBCT images in the first molar and in 58% in the second molar region. Similarly, Oliveira-Santos et al. [5] reported that it was visible in 63, 66, and 67% of second premolar, first molar, and second molar regions, respectively. As described above, these studies did not discriminate between the superior and the inferior walls. In our study, cases in which more than two thirds of the superior wall was identified on CBCT images (visibility ratio of 0.7 or more) in areas 1, 2, and 3 were 44, 62, and 66%, respectively (Table 3). It may be difficult to compare the results of our study with those of previous studies because of marked differences of evaluation methods. However, we consider that our results indicate nearly the maximum visibility of the mandibular canal when using CBCT, because we used a limited volume CBCT device with inherent small voxel size, as described above.

Because of poor visualization of the superior wall of the mandibular canal, some ingenuity may be necessary when using CBCT for treatment planning of dental implant surgery. The simplest method is to utilize the average diameter of the mandibular canal. Koivisto et al. [26] evaluated CBCT images and reported that the average diameter of the right and the left mandibular canal in the premolar/molar region was 2.91 and 3.03 mm, respectively. Utilizing these data, the approximate location of the superior wall can be estimated in cases in which the inferior wall was visible. As another method to localize the mandibular canal on CT, the use of panoramic views in addition to cross-sectional views has been recommended [27]. Probably, the imaging modality with the highest visibility of the mandibular canal is high-resolution MRI with small voxel size. Deepho et al. [28] recently reported that 3D-VIBE images at 3T MRI with voxel size of 0.8 mm clearly demonstrated the mandibular canal in 144 out of 147 areas of 62 mandibles. However, MRI has not become a routine imaging technique for dental implant treatment because of its low availability and high cost.