Impacted Third Molar

An impacted third molar is a wisdom tooth that fails to erupt into its normal functional position, because of a lack of space or an obstruction by bone, soft tissue or an adjacent tooth. About a quarter of adults have at least one impacted third molar.

Impactions are classified by angulation (mesioangular, distoangular, vertical, horizontal), by depth relative to the second molar, and by the relation to the ramus. The commonest problem is pericoronitis, and the most feared complication of removal is nerve injury.

Pathophysiology. Insufficient retromolar space, late development and a mesially angled path of eruption cause impaction. A partly erupted tooth has a gingival flap that traps plaque and food, which leads to pericoronal infection.

Other complications are caries of the third or second molar, distal cervical resorption of the second molar, periodontal pocketing distal to the second molar, dentigerous cyst, and rarely tumour.

Many have no symptoms. Pericoronitis causes pain, swelling and redness of the gum over the tooth, a bad taste, trismus, difficulty swallowing and fever, and may spread to the neck. Other features are pain from caries or pressure, food packing, and a swelling over the angle.

Ask about previous episodes of infection, antibiotics and the effect on function. Examine the opening, the tooth, the flap, second molar and lymph nodes.

Facial swelling that spreads, trismus under 20 mm, difficulty swallowing or breathing, and fever with systemic upset need urgent assessment for deep space infection.

• Clinical examination of the tooth, the operculum, second molar and periodontal probing.
• Panoramic radiograph to show the position and the relation to the inferior alveolar canal. Signs of a close relation: darkening of the root, interruption of the white line of the canal, and diversion or narrowing of the canal.
• Cone beam CT when the canal is close or the lingual plate is in doubt, to plan surgery and consent.
• Periapical radiograph for caries and for the second molar.
• Examine for cysts or tumour on radiographs (a radiolucency around the crown over 3 mm).

Differential diagnosis.

• Pericoronitis
• Dentigerous cyst or odontogenic keratocyst
• Caries of the second or third molar
• Periodontal abscess
• Temporomandibular disorder with pain at the angle
• Odontogenic tumour
• Tonsillitis or other causes of sore throat and trismus

• Acute pericoronitis: irrigate under the flap with warm saline or chlorhexidine 0.2%, debride, and give analgesia. Chlorhexidine 0.12% to 0.2% mouthwash 10 mL twice daily for 1 week. Antibiotics only for spreading infection, trismus or systemic upset: amoxicillin 500 mg orally three times daily with metronidazole 400 mg three times daily for 5 days, or clindamycin 300 mg four times daily in penicillin allergy. Reduce amoxicillin to 500 mg every 12 hours at eGFR 10 to 30.
• Removal is advised for recurrent pericoronitis (two or more episodes), unrestorable caries, cyst or tumour, resorption of the second molar, periodontal disease, and before orthognathic surgery, radiotherapy or immunosuppression. Routine removal of disease free, asymptomatic third molars is not advised, and these are kept under surveillance.
• Surgery under local anaesthesia, sedation or general anaesthesia: for a lower tooth, inferior alveolar nerve block with lidocaine 2% with adrenaline 1 in 80 000 (1.8 mL), with buccal infiltration of articaine 4% with adrenaline 1 in 100 000 (maximum articaine 7 mg/kg, and adrenaline 0.04 mg in significant cardiovascular disease). Raise a flap, remove bone, section the tooth and extract, irrigate, and suture.
• Coronectomy (removal of the crown and leaving the roots) for a tooth with a high risk of nerve injury on imaging, with consent for a second operation if roots migrate.
• Perioperative: a single dose of dexamethasone 4 to 8 mg orally or intravenously before surgery reduces swelling and trismus. Chlorhexidine 0.2% gel in the socket or 0.12% mouthwash reduces dry socket. A single dose of amoxicillin 2 g orally 1 hour before surgery, or 1 g intravenously, reduces infection and dry socket but is not routine in healthy patients.
• Analgesia: ibuprofen 400 mg every 6 to 8 hours with food for 3 to 5 days (maximum 2400 mg daily, avoided in renal impairment, peptic ulcer, anticoagulation and late pregnancy) with paracetamol 1 g every 6 hours (maximum 4 g daily). Ice packs for 24 hours.
• Complications: alveolar osteitis (about 1 to 4%, higher with mandibular impactions and in smokers), infection (about 1 to 5%), bleeding, trismus, temporary inferior alveolar nerve injury (about 1 to 5%), permanent injury (under 1%), lingual nerve injury (temporary 1 to 5%, permanent under 1%), mandibular fracture, and for upper teeth oroantral communication.
• Anticoagulated patients: continue warfarin when the INR is under 4.0 within 72 hours, with local haemostasis and tranexamic acid 5% mouthwash 10 mL four times daily for 2 days.

Special groups. Pregnancy: delay elective removal until after delivery, or use the second trimester for urgent treatment, with paracetamol and avoidance of ibuprofen after 28 weeks.

Monitoring. Review at 1 week for wound, infection and sensation, with a sensory review at 2 weeks and 3 months for any nerve injury. Asymptomatic retained teeth need clinical and radiographic review every 1 to 2 years.

Referral and escalation. Refer to oral and maxillofacial surgery for tooth close to the canal, medically complex patients, cysts or tumours, or nerve injury. Urgent referral for deep space infection or airway concern.