From Volume 38, Issue 4, May 2011 | Pages 267-276
Causing damage to the inferior alveolar nerve (IAN) when extracting lower third molars is due to the intimate relationship between the nerve and the roots of the teeth. When the proximity radiologic markers between the IAN and the root of the third molars are present, the technique of coronectomy can be proposed as an alternative to extraction to minimize the risk of nerve injury, with minimal complications.
Operations on mandibular third molars are common and are complicated by temporary injury to the inferior alveolar nerve in up to 8% and permanent injury in under 1% of cases.1,2 Risk factors include advanced age1 and difficult operating conditions,3 but the most important one is the proximity of the third molar to the mandibular canal.4 Coronectomy (Figure 1) avoids the inferior alveolar nerve by ensuring retention of the roots when they are close to the canal.5 This has been illustrated in Cases 1 (Figures 2–5), 2 (Figures 6, 7) and 3 (Figures 8–11).
The relationship between the roots of mandibular teeth and the inferior alveolar nerve can often be assessed radiographically, particularly with a panoramic radiograph. Computed tomography scanning can be used to visualize the relationship in the third dimension,6,7 and with the combination of these techniques it can be ascertained which teeth may represent the greatest risk to the inferior alveolar nerve upon removal.8
When the radiologic markers – on panoramic radiography – of proximity of the IAN to the root of the third molars are present, the incidence of damage can be as high as 35%.3 Howe and Poyton, by comparing the radiographic appearance of the tooth root and the IAN as to whether or not the nerve was visible in the socket at operation, produced predictors for possible damage to the nerve. When these radiologic predictors were present, the incidence of inferior alveolar nerve injury was 35.64%.1
Rood and Shehab, by comparing the radiological signs to the actual incidence of damage to the IAN, found that three radiologic signs (darkening of the root, interruption of the lamina dura and diversion of the canal) were statistically significant as predictors of trauma to the IAN. They found that, when one of those signs was present, the nerve was affected in 30% of cases.9
The technique of coronectomy, or deliberate vital root retention, has been proposed as a means of removing the crown of a tooth but leaving the roots, which may be intimately related with the inferior alveolar nerve, untouched so that the possibility of nerve damage is reduced.8 O'Riordan, in a study of 100 patients, showed that the risk of subsequent infection was minimal and morbidity was less after coronectomy than after the traditional operation. Over a period of 2 years, some apices migrated and were removed uneventfully under local anaesthesia.10 On the premise that coronectomy reduces the risk of nerve injury, it has been recommended for those patients for whom there is great risk of nerve injury.11
| Paper | Study design | N (extraction) | N (coronectomy) | Mean follow-up | Infection | Root migration | Nerve injury design extraction/coronectomy |
|---|---|---|---|---|---|---|---|
| Knutsson et al(1989)8 | Retrospective study, post-op status at one year | – | 33 | 1 year | 3 | ||
| O'Riordan (2004)20 | Case series | – | 52 | 10 years | 5.7% | 3 cases of temporary sensory disturbance | |
| Renton et al (2005)5 | Randomized controlled trial : incidence of IANI, coronectomy versus removal | 102 | 94 | 25 months | 10% (extraction group), 12%(coronectomygroup) | 13–15 % | 19% (extraction group) 0% (coronectomy group) |
| Pogrel et al (2004)12 | Prospective cohort study | – | 50 | 22 months | 30% | None | |
| Hatano et al (2009)13 | Case-control study | 118 | 102 | 13 months | 5% (extraction group) 1% (coronectomy group) | ||
| Leung and Cheung (2009)14 | Randomized Controlled Trial RCT: safety of coronectomy versus extraction | 178 | 171 | 24 months | 6.7% (extraction group) 5.8% (coronectomygroup) | 62.2% (1 year after coronectomy) | 5% (extraction group) 0.65% (coronectomy group) |
Pogrel et al12 evaluated 41 patients who underwent coronectomy on 50 lower third molars, with follow-up of at least 6 months. This technique was used because there was radiographic evidence of a close relationship between the roots of the tooth and the inferior alveolar nerve. The authors reported that there were no cases of inferior alveolar nerve damage in this study.
Renton et al evaluated 128 patients who required operations on mandibular third molars and who had radiological evidence of proximity of the third molar to the canal of the inferior alveolar nerve. Patients were divided in two groups: extraction group (102 teeth) and coronectomy group (94 teeth). The mean follow-up was 25 months. The authors reported that 19 nerves were damaged (19%) after extraction and none after coronectomy.5
Hatano et al compared coronectomy with traditional extraction on 220 patients, 118 in the extraction group and 102 in the coronectomy group. The mean follow-up time was 13 months in the extraction group and 13.5 months in the coronectomy group. Six inferior alveolar nerve injuries (5%) were found in the extraction group. In the coronectomy group, 1 patient (1%) had symptoms of nerve injury.13
In the study of Leung and Cheung, nine patients in the extraction group (n = 178) presented inferior alveolar nerve deficit, compared with one in the coronectomy group (n = 171). The follow-up of the study was 24 months.14
Coronectomy involves transection of the tooth 2–3 mm below the enamel of the crown. The pulp is left in place after the crown has been levered off.5,12 The technique of leaving the retained root fragment at least 2 mm inferior to the crest of bone seems appropriate and does appear to encourage bone formation over the retained root fragment. This distance of 2 to 3 mm has been validated in animal studies.12
There is no need for a root canal treatment or any other therapy to the exposed vital pulp of the tooth. Following a periosteal release, a watertight primary closure of the socket is performed with one or more vertical mattress sutures.12
Pogrel considered that all patients must be placed on prophylactic antibiotics pre-operatively, because it is felt that antibiotics should be in the pulp chamber of the tooth at the time it is transected. However, some studies have indicated that antibiotics were unnecessary.12
One logical question is the outcome of roots left in place after the coronectomy. Poe et al15 showed in dogs that, in vital retention of roots, all pulps survived and had calcific spurs attempting to bridge the pulp canal. Johnson et al16 showed the same results in humans. Subsequent papers,17,18,19 some of which examined roots and adjacent bone ‘en bloc’, found that the pulp remained vital. Animal studies have shown that vital roots remain vital with minimal degenerative changes. Osteocementum usually extends to cover the roots.
When the crown is removed the pulp is no longer enclosed in a rigid compartment with a tiny apical outlet, so any hyperaemia or inflammatory oedema after the surgical ‘insult’ can expand without restriction. However, the cut pulp should be irrigated well and any manipulation avoided.20
Radiographs after 6 months can show bone formation having occurred superior to the retained root fragment. Migration of the root has been noted in every article published on the subject and appears to occur in between 14% and 81% of cases, depending on the length of follow-up.21
The study of Leung and Cheung revealed that more than half of roots migrated at a high rate for 3 months post-operatively and then gradually stopped at 12 and 24 months.14
In the study of Pogrel et al, root migration was noted in approximately 30% of patients over a 6-month period (Table 1).12 Case reports have suggested that it can take up to 10 years for the root fragments to erupt.22
Knutsson et al carried out a prospective trial on 33 patients. The surgeon resected the crown at an ‘adequate’ level without further grinding and the flap was closed with interrupted sutures. After one year, all but six root fragments had migrated, most between 1 and 4 mm.8 Dolanmaz et al recorded similar migration.23
All authors, however, point out that this migration means that the root fragment comes away from the IAN and, therefore, facilitates uncomplicated removal.19
In the study of Hatano et al four remaining roots had signs of post-operative infection, and the patients underwent extraction of the root. No nerve damage resulted in these patients after repeat extraction.13
Apical radiolucency without symptoms was reported. But, although the radiolucency was still evident, a lamina dura could be seen around the apex.19 This appearance may be due to the migration; the area from which the root moved may have been filled with immature, more radiolucent bone, or may have been there pre-operatively owing to large cancellous spaces around the apex.20
O'Riordan evaluated the rate of infection of retained lower third molar roots after coronectomy in a retrospective study of 52 patients who were operated on over a 10-year period. Only 3 of 52 patients had to have roots removed because of pain or infection.20
Freedman published a retrospective series of 33 cases. Only one root had to be removed because of infection.22
In the study of Hatano et al, four remaining roots had signs of post-operative infection, and the patients underwent extraction of the root. No nerve damage resulted in these patients after repeat extraction.13
In the study of Renton et al, the incidence of dry socket and infection was similar in the extraction group and in the coronectomy group.5
In the study of Leung and Cheung, pain and dry socket incidence was significantly lower in the coronectomy group. But there were no statistical difference in infection rate between the two groups.14
There are relatively few contra-indications to carrying out coronectomy under the appropriate circumstances, but these do include the following:
There are currently no standards regarding the timing and frequency of follow-up of patients having coronectomy. Most authors take radiographs immediately post-operatively and 6 months later. Later radiographs are taken if the patient becomes symptomatic.
For Renton, the follow-up after coronectomy of 25 months was considered sufficient to evaluate the incidence of nerve injury, dry socket and early eruption, but not of late eruption, which can occur up to 10 years after the initial operation. A longer review period may therefore show that a proportion of these retained roots do eventually erupt and may cause a late infection or require removal. One possible advantage, even if second operation is required, is that, unless the root is close to the IAN, it is likely to erupt away from the nerve, so reducing the potential of nerve injury during the second operation.5
Nerve injury after the extraction of the mandibular third molar is a serious complication. The technique of coronectomy can be proposed to minimize the risk.
