OBJECTIVETo investigate the actual relationship between the impacted mandibular third molars (IMTM) and the mandibular canals on panoramic radiographs and cone beam computed tomography (CBCT).

METHODSA cross-sectional study design was used and panoramic radiographs of 390 patients (645 sides developed IMTM) were included in the present study. The relationship between the IMTM and the mandibular canal on panoramic radiographs was defined in three types: 'untouched', 'touching in point', and 'superimposition or line touching'. The IMTM superimposing and line touching with mandibular canals on panoramic radiographs were examined by CBCT.

RESULTSThe relationship types on panoramic radiographs were as follows: 'untouched' type 441 IMTM (68.4%), the type of 'touching in point' 149 IMTM (23.1%) and the type of 'superimposition and line touching' 55 IMTM (8.5%). CBCT showed that 25 (50%) of 50 IMTM of superimposition and line touching type impinged on the mandibular canals.

CONCLUSIONSMore than 90% of IMTM which do not have intimate relationship with the mandibular canals can be screened out by panoramic radiographs. The IMTM which belong to the line touching and superposition type on panoramic radiographs should be examined by CBCT.