PurposeThe positional relationship between the mandibular canal and corresponding third molars is a key anatomic factor of inferior alveolar nerve (IAN) injury. The aim of the present study is to classify the anatomic three-dimensional relationship between the mandibular third molar and the mandibular canal on cone-beam computed tomography (CBCT) images.MethodsThis study used CBCT images to classify the positional relationship between the mandibular canal and corresponding third molars. CBCT images of 749 patients (1296 mandibular third molars) were analyzed to draw up a classification.ResultsOn a total of 1296 third molars, the mandibular canal relative to the roots of the mandibular third molar was on the apical side (88.1%), followed by the buccal side (7.9%), the lingual side (3.5%), and then between the roots (0.5%). Ninety-five (7.1%) third molars had a close relation with the mandibular canal, while 1201 (92.7%) third molars had no direct contact. The percentage of the mandibular canal contacts with the mandibular third molar was higher when the mandibular canal was lingually positioned.ConclusionsThe anatomic structures of the mandibular third molar and the mandibular canal may be helpful to make adequate surgical planning to avoid or reduce nerve involvement.

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