- A, Darkening of root;
- B, Deflection of root;
- C, Narrowing of root;
- D, Bifid root apex;
- D, Diversion of canal;
- F, Narrowing of canal;
- G, Interruption in white line of canal.
虽然在不同的报道中以及不同的统计方法得出的各种征象与神经损伤的风险大小不尽一致，但是一般来说deflection of the root, narrowing of the root, diversion of the canal, and narrowing of the canal这几种情况具有较高的损伤神经的风险。(Xu GZ, Yang C, Fan XD, Yu CQ, Cai XY, Wang Y, He D. Br J Oral Maxillofac Surg. 2013 Dec; 51(8):e215-9.)
另外有文献对牙根与神经管的相对位置与神经损伤的风险进行了分析得出如下结果，提示牙根位于神经管颊侧时损伤神经的风险似乎更高（Kim JW, Cha IH, Kim SJ, Kim MR. J Oral Maxillofac Surg. 2012 Nov; 70(11):2508-14.）：
Methods of treatment
Orthodontic extraction technique
具体方法如下(Pogrel, M. A., et al. (2004). “Coronectomy: a technique to protect the inferior alveolar nerve.” J Oral Maxillofac Surg 62(12): 1447-1452.)：
- Using a 701-type fissure bur, the crown of the tooth was transected at an angle of approximately 45°. The crown was totally transected so that it could be removed with tissue forceps alone and did not need to be fractured off the roots. This minimizes the possibility of mobilizing the roots. However, the lingual retractor is essential during this technique because the lingual plate of bone can be inadvertently perforated, and otherwise the lingual nerve would be at risk.
- Following removal of the crown of the tooth, the fissure bur is used to reduce the remaining root fragments so that the remaining roots are at least 3 mm below the crest of the lingual and buccal plates in all places (this involves removing the shaded portion).
- An alternative technique is to use a round bur from a superior aspect and remove the crown and superior part of the roots by drilling it away. In this case, only minimal lingual retraction may be required.
可能发生的并发症包括（Complications after coronectomy are rare but include
- Alveolar osteitis
Failed coronectomy i.e. mobilisation of the roots (9-38 per cent)
- Inferior alveolar nerve injury
- Root migration (30 per cent)
A novel surgical technique
这一方法是在第二磨牙远中制造出足够的间隙，以通过第三磨牙的萌出而使其远离神经管后，再次手术拔除牙齿。过程如下(Landi, L., et al. (2010). “A novel surgical approach to impacted mandibular third molars to reduce the risk of paresthesia: a case series.” J Oral Maxillofac Surg 68(5): 969-974.):
A periapical radiograph of the area is taken before surgery and stored for follow-up comparison. The surgery is approached as it would be for extraction of an impacted M3. Block anesthesia is administered with local infiltration of the buccal nerve. A hockey-stick incision is outlined, and a full thickness flap is raised. After ostectomy, using a carbide and diamond bur, is completed, access is gained to the impacted tooth; then, using a fissure bur, the mesial portion of the anatomic crown is sectioned and removed. Care should be taken to avoid pulp exposure at this stage (Fig 3). The distance between the distal aspect of M2 and the mesial aspect of the sectioned M3 is measured and recorded for migration evaluation. Postoperative management includes pain medications (ibuprofen 400 mg, 3 times a day) and mouth rinses with clorhexidine (0.2% twice a day for 10 days). After removal of the sutures, patients are instructed to clean the area. A monthly exam is scheduled for the first 3 months. At 3 months, a new periapical x-ray is taken to assess the degree of migration of the M3 (Fig 4). If indicated, a new panorex is also prescribed to assess the relationship between the roots and the IAN. After migration of the M3 is judged adequate for a risk-free extraction, the surgical removal of the impacted tooth may be scheduled.
- radiographic proximity of the M3 roots with the IAN confirmed on a computed tomography scan;
- horizontal or mesioinclined M3 impaction;
- contact of the M3 crown with the distal aspect of the M2;
- an established pathological process is detectable in the area of impaction (pericoronitis, caries, or deep periodontal defect), indicating the need of M3 removal;
- orthodontic-assisted extraction is judged complex to apply or is not accepted by the patient;
- preferable (but not exclusive) young patient age because higher residual eruption activity may be expected and age is considered a risk factor for M3 extraction complications.