Interobserver agreement was substantial or almost perfect agreement (Table 1).

The mean values of the visibility ratio of the superior and inferior walls in each area are shown in Table 2 and Fig. 3. In all areas, the ratio of the superior wall was significantly lower than that of the inferior wall (p = 0.0000). As for variance among the three areas, the ratio was highest in the most posterior area (area 3) and tended to decrease gradually towards the mental foramen for both walls. For the superior wall, the ratio of area 1 was significantly lower than that of area 3 (p = 0.0006). In contrast, for the inferior wall, significant differences were found between area 1 and area 2 (p = 0.0001), area 1 and area 3 (p = 0.0000), and area 2 and area 3 (p = 0.0132). A representative case is shown in Fig. 4.

Table 3 shows the frequency of cases with visibility ratios of 0.7 or greater (i.e., more than two thirds of the wall was visible) in each area. Cases in which more than two thirds of the superior wall could be identified on CBCT images in areas 1, 2, and 3 were 44, 62 and 66%, respectively.