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The aim of this study was to objectively quantify the topographic relation of the mandibular canals, impacted third molars, and cortical plates using cone beam computed tomography (CBCT).A retrospective chart review was conducted using the database of a university-based imaging service. Two calibrated reviewers examined the CBCT images of 100 cases scheduled for mandibular third molar removal. They characterized the position and condition of the mandibular canal and measured its dimensions at 3 different points relative to the third molars.The mandibular canal is more often located buccal to the third molars but is more likely to be compressed when in a lingual position. The vertical (cephalocaudal) dimensions of the mandibular canal change significantly as the canal progresses toward the ramus. The horizontal (buccolingual) dimensions of the mandibular canal fluctuate very little but significantly narrow in proximity to the third molars. Thinning of the lingual cortical plate is common, whereas grooving of molar roots is uncommon.Minor variations in the horizontal dimensions of the mandibular canal close to the third molars signify an effect on the canal. This effect may indicate an increased risk of neurovascular injury. The mandibular canal can have a direct or indirect effect on the cortical plates.