Lower Lip Numbness After Surgical Removal of an Impacted Third Molar
A 29-year-old woman undergoes surgical extraction of an impacted mandibular right third molar (tooth 38). The crown is at the cervical level of the adjacent second molar, and a CBCT shows the root apices overlapping the mandibular canal, with no visible cortical bone between the root apex and the canal. The surgeon removes buccal bone, sections the tooth, and elevates the roots; the procedure lasts 55 minutes and bleeding is controlled with local measures. The next day the patient reports numbness and tingling of the right lower lip and chin. She has no tongue numbness, no taste change, and no facial weakness. Two weeks later she reports slight improvement. She asks whether the nerve was cut and whether she should have another operation immediately. The surgeon must explain the anatomy, classify the injury, and decide on management.
1. Which nerve is most likely injured, and what anatomical findings explain numbness of the lower lip and chin with sparing of the tongue?
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The most likely injured nerve is the inferior alveolar nerve (IAN), a branch of the mandibular division of the trigeminal nerve (V3). It enters the mandibular foramen and runs in the mandibular canal, giving off the mental nerve at the mental foramen. The mental nerve supplies general sensation to the lower lip and chin, which matches the numbness. The lingual nerve is a separate branch that runs anterior and medial to the IAN and supplies general sensation to the anterior two-thirds of the tongue; it is spared, so the tongue is normal. No taste change also argues against chorda tympani involvement. The CBCT finding of root apices overlapping the canal with no cortical bone places the nerve at risk during elevation and sectioning. Sparing the tongue does not mean the IAN is undamaged; it localizes the injury to the IAN or mental nerve.
2. How would you classify the injury clinically, and what findings at follow-up would make you more concerned?
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The history is paresthesia, not anesthesia, and there is slight improvement at two weeks. That favors a mild injury such as neurapraxia (Sunderland first degree) or possibly axonotmesis (second degree) rather than a transection. Test light touch, pinprick, two-point discrimination, and thermal sensation, and map the affected area against the contralateral side. More concern would be caused by complete anesthesia, dysesthesia or pain, no improvement by six months, or a retained root or foreign body compressing the canal. Recovery is not guaranteed; neurapraxia may recover in weeks to months, while axonotmesis regenerates at about 1 mm/day and can take 6-18 months. Document the baseline because improvement is judged against it.
3. Should the surgeon re-explore immediately? Outline a management and documentation plan.
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Immediate re-exploration is not indicated for paresthesia alone without evidence of nerve transection or compression; surgery carries a risk of further injury. The reasonable plan is conservative review at 1-2 weeks, then 1, 3, and 6 months, with serial sensory testing and a CBCT to confirm no root fragment or material in the canal. Refer to an oral and maxillofacial surgeon or microneurosurgical service if there is no improvement, worsening, or dysesthesia. Advise the patient to avoid further local trauma and to report changes. Document the specific pre-operative consent for nerve injury, the post-operative findings, the sensory tests, and the referral. If imaging shows a retained root compressing the canal, removal may be considered, but that is a different decision from routine re-exploration.