Relationship between hypoesthesia of the lower lip after extraction of the mandibular third molar and preoperative imaging findings on panoramic X-ray films and multi-planer reconstructive CT scans

The mandibular third molars are usually located near the inferior alveolar nerve. Surgical removal of these molars may damage the nerve and cause hypoesthesia of the lower lip. Before surgery, it is important to examine the exact positional relationship between the mandibular third molars and the mandibular canal to prevent such complications. A total of 329 lower third molars from 198 patients were studied on panoramic radiographs and computed tomography (CT). We studied the association of hypoesthesia with the spacial relation between the tooth roots and the mandibular canal. Hypoesthesia occurred in 23 of 329 teeth (7.0 %). Almost all cases of hypoesthesia resolved within 6 months. However, hypoesthesia of 3 teeth (0.9 %) persisted approximately 7 to 12 months after surgery. In 206 of 329 teeth (62.6 %), the assumed location of the mandibular canal differed between panoramic radiography and CT. Furthermore, the prediction rate of the incidence of type 1 hypoesthesia on CT (35.5 %) was higher than that on panoramic radiography (15.9 %). Risk factors related to hypoesthesia included loss of the white line of the root, curve of the mandibular canal, and close proximity of the tooth to the mandibular canal (Type 1) on panoramic radiography. There were also teeth in contact with the canal (0 mm) in all patients in whom hypoesthesia developed. In particular, patients whose mandibular canals were located between the roots of the mandibular third molar or on the lingual side of the mandibular third molar had a high incidence of hypoesthesia (26.0 %). These results suggest that the positional relation between the mandibular third molars and the mandibular canal should be accurately evaluated by CT. Because of its high resolution and ability to examine patients in detail, CT was found to be useful for predicting the risk of inferior alveolar nerve damage before mandibular third molar surgery.