Abstract: PURPOSE : The aim of the present study was to measure the alveolar bone thickness of the deeply impacted mandibular third molar, and classify the molar based on the proximity to the external cortical bone. METHODS : Cone-beam CT(CBCT) data from 110 deeply impacted mandibular third molars were measured. At the center of crown, cenmeto-enamel junction and root, the distance between the tooth and the external bone surface was measured for both buccal and lingual side. The samples were classified into 3 groups (buccal, central and lingual position) by the following 3 methods: the first method was distance comparison: if the average difference of buccal-lingual alveolar bone thickness was less than 1 mm, the tooth was classified as central position,otherwise, the tooth was classified as buccal or lingual position. The second method was visual observation on CBCT images. The third method was to compare the relationship of third molar and dentition. Take distance comparison as gold standard to evaluate the sensitivity and specificity of the latter two methods. RESULTS : The samples were classified by distance comparison as: 1 buccal position (0.9%), 13 central position (11.8%), and 96 lingual position (87.3%). The samples were classified by visual observation as: 1 buccal position (0.9%), 16 central position (14.5%), and 93 lingual position (84.5%). The samples were classified by dentition relationship as: 59 buccal position (53.6%), 48 central position (43.6%), and 3 lingual position (2.7%). The sensitivity of visual observation for lingual, central and buccal position was 91.67%, 61.54% and 100%, respectively,the specificity was 64.29%, 91.75% and 100%, respectively,the youden index was 0.56, 0.53 and 1, respectively. The sensitivity of dentition relationship classification for lingual, central and buccal position was 3.13%, 0% and 100%, respectively,the specificity was 100%, 50.52% and 46.79%, respectively,the youden index was 0.03, 0.49 and 0.53, respectively. CONCLUSIONS : Among deeply impacted mandibular third molars, lingual position ranks the first, central position ranks the second and buccal position ranks the third. Visual observation is reliable for lingually or buccally displaced molar. Dentition relationship classification is not related to alveolar bone thickness.

Key words: Cone-beam CT, Deeply impacted mandibular third molar, Bone thickness