## 4 Discussion
Since the subject of this study was patients who underwent CBCT, this might have affected the outcome as patients with a mandibular third molar impacted by a vertical or a mesial impaction, which is relatively far from the IAN, were not included. According to the Pell and Gregory classification, the most prevalent was Class IIA (30.0%), followed by Class IIB (19.3%). This is consistent with previous results [1,14,44]. [PERSON] [45] reported that Class A (56.2%) and Class II (63%) were the most common among Italians, and [PERSON] [46] and [PERSON] [43] reported that Class IIB was the most common. It can be seen that Koreans do not differ in the degree and distribution of impaction from different races. In addition, it was found that the depth of the impaction increased as age increased. This suggests that patients with low impacted mandibular third molars remove their teeth at a relatively young age, so as the age increases, deep impacted mandibular third molars, which are difficult to extract, are present at a higher rate. Panoramic radiographic signs were reported to occur at a low frequency of 0.1%-3.3% in the study by [PERSON] and [PERSON] [20]. [PERSON] et al. [11] reported the occurrence at a rate of 11.8-35.9%, similar to this study. This difference is thought to be due to the difference between observers in determining the symptoms, as the evaluation of the signs in the panoramic radiograph is not standardized. In addition, the subjects of this study had panoramic radiographs of the mandibular third molar and inferior alveolar nerves that were in contact with each other, so it is thought that the incidence of signs was higher than that of a completely randomized study. In CBCT, the contact between the mandibular third molar and the inferior alveolar nerve was observed in 61%, which was less than the cases with contact on the panoramic radiograph (88.67%). This means that it was not possible to accurately determine whether the mandibular third molar and the inferior alveolar nerve were actually in contact with the panoramic radiograph. There was a significant correlation between the Pell and Gregory classification, the contact between the mandibular third molar and the IAN in CBCT, and the narrowing of the IAN in CBCT. This means that the deeper the degree of impaction of the mandibular third molar, the closer it was to the IAN. As for the signs on the panoramic radiograph, there was a correlation between the actual contact between the mandibular third molar and the IAN in CBCT, except when the root was narrowed and there was a dark line at the root apex [47, 48, 49]. There was a correlation between the disappearance of the continuity of the lingual cortical bone at the root of the mandibular third molar and the dark line at the root apex. Dark lines were observed in 55.7% of cases with lingual cortical bone perforation, and were absent in 44.3% of cases with lingual cortical bone perforation. In the literature examining the dark area of the root and lingual cortical bone perforation, it was suggested that the dark area showed proximity to the IAN rather than the lingual cortical bone perforation [50]. It is believed that this is caused by not distinguishing from dark lines when irradiating the dark area. If the root is in the lingual cortical bone, it is thought that the presence of the periodontal ligament in the dense cortical bone and the perforated area of the cortical bone appear as dark lines. The possibility of root fracture occurring during extraction increases, and the fractured root may fall into the submandibular space, so care should be taken during extraction. It would be useful if we could predict the existence of roots in the lingual cortical bone only with panoramic radiographs, not CBCT. In the case of proximal dental caries 46.9% had mesial impaction and 36.4% had horizontal impaction. This is consistent with other studies that stated that the mesial and horizontal impaction of the third molar accounts for most of the proximal caries of the second molar [51,52].