## 4 Discussion  
In recent decades, the use of CBCT has increased dramatically in dentistry. CBCT images have less distortion than conventional two-dimensional radiographic diagnostic images and are not affected by overlapping structures. Although CBCT images provide more accurate information about dentoskeletal structures, clinicians should be aware that the effective dose of radiation with CBCT is much higher than that of panoramic radiographs or cephalograms. Thus, CBCT is only recommended for limited purposes, including assessment of tooth impaction, maxillofacial deformities, and facial asymmetry. The use of CBCT as a routine orthodontic diagnostic tool is still debated [19; 20; 21]. Most CBCT images used in this study were obtained for extraction of the mandibular third molar instead of orthodontic diagnosis. Those who wore braces or other orthodontic retainers were excluded from the study to exclude the effects of orthodontic treatment. It cannot be definitively stated that all the patients included in the study had never had orthodontic treatment. So, to completely eliminate any possibility of any effect due to prior orthodontic treatment, a study including information on the patients' complete dental history would be necessary. This study was initiated with a question of whether there is any difference in the root-cortical bone distance based on facial divergency and whether it is possible to predict patients at high risk of root-cortical bone contact based on the 2D radiographs commonly used in daily practice. In this study, the relationship between measurement values related to facial divergency (FHR, SNMP, and PPMP) and skeletal transverse discrepancy (J-J/Ag-Ag ratio), and root-lingual cortical bone contact was confirmed. Based on the findings of [PERSON] et al. [2], which showed a decrease in the distance between mandibular molar roots and the posterior lingual cortical bone as it approaches the root apex, we hypothesized that the risk of contact between the mandibular molar roots and the lingual cortical bone is greater in the lower root area. To test this hypothesis, we included the most posteriorier point of mandibular dentition (D/MP) in our measurement list. We considered that the area 1, 3, or 5 mm from the root apex would be the best spots to determine the relationship between the lower roots and the lingual cortical bone, assuming a root length of 12 mm for the mandibular molar. We observed that the distance between the mandibular molar roots and the outer surface of the mandibular lingual cortical bone decreased from mesial to distal. This finding was consistent with the observations of [PERSON] et al. [3]. The distance between the mandibular first molar roots and the lingual cortical bone increased from the cervical to the apex. In contrast, there was no significant difference in the distance between the mandibular second molar roots and the lingual cortical bone relative to the root level. In addition, the lingual cortical bone of the distal part of the second molars shows an S-shaped curve, which is in contrast to the lingual cortical bone near the first molar that is parallel to the dental arch. (Figure 5). The morphology of the lingual cortical bone is thought to be related to the location of the mylohyoid line. [PERSON] et al. reported that the mylohyoid line is 16 mm away from the mandible lower edge in the distal root area of the mandibular