Risk assessment of M3Ms and decisions on ordering a CBCT and prescribing a coronectomy

Patient age

It is well understood that patients over the age of 25 years are more likely to be affected by persistent IANI when the nerve is damaged during oral surgery.²⁶ The reasons for this might be that the healing ability decreases with increasing age and that more bone is usually removed owing to completely formed roots or increased bone mineralization.²⁷ Blondeau and Daniel recommended that prophylactic M3M extraction should be avoided in patients aged 24 years or older because of a high possibility of complications, such as permanent neurosensory deficits, infection, and alveolitis.¹⁶

Gender

Females have been reported to be more at risk of persistent IANI.²²

Surgeon experience

The prevalence of IANI is also dependent on the surgeon experience and the methods used.^27,28,29

Eruption status and depth of impaction

A literature review of 32 prospective articles²⁹ highlighted that unerupted M3M status was the strongest indicator for IANI (erupted 0.32% deficit; partially erupted 0.67% deficit; unerupted 3% deficit). Interestingly, the depth of impaction and the pattern of impaction illustrated no statistical differences. However, this is likely due to the different systems used to describe impaction pattern and depth.

Type of impaction

In 2013, Smith presented a clinical study on 1,000 patients, removing 1,589 impacted M3Ms.²¹ The incidence of IAN neurosensory deficit was highest with horizontal impaction (4.7%) and lowest when the M3Ms were vertically impacted (0.9%). However, the difference between each type of impaction was not statistically significant (P > 0.01).

Intra-operative nerve exposure and bleeding during surgery

Clinical exposure of the IAN neurovascular bundle during surgery may notify the surgeon about an increased risk of post-operative IANI, resulting in a 20% risk of post-operative paraesthesia, with a 70% chance of recovery within one year.¹⁴ IAN bleed during surgery is also reported to be associated with increased IANI.²¹

Orthopantomography

The radiographic position of the M3M in relationship to the Inferior dental canal (IDC) has been shown to be useful in assessing the risk of damage to the IAN following extraction. Plane film radiographic signs on panorals indicative of possible IAN risk include:

Most studies (Table 2)^{29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49} report a combination of panoral risk factors predisposing to IANI. A study performed²¹ on a total of 12,842 patients who underwent extraction of the M3M, using univariate logistic regression analysis, reported that the significant risk panoral factors for panoral IANI were:

In contrast, another study, using multivariate logistic regression analysis, revealed that the significant risk factors were:

The narrowing of the root was the most significant type of superimposition, followed by dark and bifid apexes of the root.^30,31

A recent study⁴ reported the radiographic risk factors significantly (P < 0.001) related to IANI to be:

In a later study,³⁰ the same authors evaluated 178 M3Ms in 118 patients, where radiographic risk factors were identified including:

Of M3M radiographs examined, 65.7% illustrated a minimal of one radiographic signs, 27.5% two signs and 5.1% demonstrated three signs. Both darkening of root (P < 0.001) and deviation of IDC (P < 0.019) predicted the highest risk for IANI exposure during surgery. Darkening of root ONLY was a radiographic sign of significantly increased IANI risk (P = 0.016). If there was more than one radiographic risk factor, IANI was a significantly increased risk (P = 0.005). Intra-operative IAN exposure (P < 0.001) was also significantly related to exposure, 16.2% IANI deficit versus non-exposed 1.1% IANI deficit. Intra-operatively, 13.5% of M3Ms resulted in IAN exposure; of those with IAN exposure, 20.6% had post-operative IANI.

However, the presence or absence of these radiographic signs does not always determine the possibility of IANI, indicating that the panoral does not have high diagnostic accuracy in the assessment of risk in surgical extractions of lower M3Ms.^50/51 When the radiological marker on the panoral indicates that there is a close relationship between the M3M and the IDC, additional investigation using computed tomography (CT) may be recommended to verify the relationship in a three-dimensional view.^{38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53} This 3D imaging allows the surgeon to further scrutinize the need for modified surgery or intentional coronectomy,¹⁰ potentially resulting in a change of practice in up to 90–95% of cases. M3M removal was at ‘high risk’ based on panoral radiography, which would otherwise be recommending a coronectomy. The use of cone beam computed tomography (CBCT) for high-risk M3M may also be associated with reduced morbidity to the IAN.^41,42 CBCT has been introduced to improve conventional CT because it reduces the radiation dose, offers high spatial resolution, and decreases costs.⁵⁴ The software associated with cone-beam CT provides better imaging quality of M3Ms and their surrounding structures compared with conventional hospital-grade CT scans.^55,56

CBCT has higher radiation dose and added expense compared with panoral, however, the new generation of CBCT scans have reduced radiation dose (less than twice as much as a panoral radiograph) and lowered costs, but these may not be available internationally.

There are several important CBCT radiographic signs that may indicate a higher risk to the IAN and assist the clinician in case selection for coronectomy⁵⁷

IDC perforation or loss of LD

Shahidi et al reported that the loss or interruption of the cortical line (LD) on the panoral, having a close correlation with proximity of the IAN to the M3M, was confirmed on a CBCT scan panoral.⁵⁹ This is ‘highly suggestive of the risk of nerve injury’. Also, an IDC's LD perforation, as seen on a CBCT scan (Figure 5), can be closely correlated to darkening of the root seen on a panoral.⁵³

Deformation of the IDC at the point of contact with M3M roots (Figure 6)

This may indicate proximity and risk of IANI. The intimate proximity of a M3M can modify the common oval configuration of the IDC toward a more ‘dumbbell’ or ‘tear-drop’ shape or a concave configuration.⁶⁰ Invagination of the IDC or ‘compression’ (concave deformation) of the IDC can result from the proximity of root(s) of a M3M. In a second study, among 169 LM3s (115 patients), IAN injury was observed in 13 of 169 M3Ms (7.7%), and all 13 cases with IANI exhibited absence of cortication. A dumbbell-shaped IDC was considered a useful predictor for IANI (sensitivity, 69.2%; specificity, 84.6% [P = 0.005]).²⁵

Bifid IDCs are common in the M3M region⁴²

Care must be taken to scrutinize the presence of bifid IDCs as, often, a smaller branch may be passing close to the M3M roots and on extraction will cause partial permanent IANI (Figure 7).⁴⁶

M3M roots positioned between missing lingual cortex and IDC

Of high risk M3M cases reported, 30% have a defect of the lingual cortex.⁴² If the M3M root is sandwiched between the IDC and lost lingual cortex, this may be a secondary decision factor to undertake a coronectomy (Figure 8).¹⁰

Buccal position of roots

Buccal position of roots in contact with a lingually placed IDC may be associated with increased IANI rate (Figure 9).^43,61

Dental factors

Medical history

Immunocompromised, with likely poor healing, is a contra-indication for coronectomy and removal is advised whereby the patient may be more at risk of infection with retention of roots.

Social and psychological factors

A systematic review of four studies (two RCTs and two CCTs) involving 699 patients and 940 M3Ms report that coronectomy was converted to extraction during surgery due to root loosening or mobilization in 2.3% to 38.3% of cases. Re-operation was required in 0% to 4.9% of the coronectomy group due to persistent pain, root exposure or persistent apical infections. Root migration was only reported in three studies and ranged from 13.2% to 85.9%. Furthermore, it was reported that coronectomy can protect IANs in the extraction of M3Ms with high risk of nerve injury, as compared with total removal, and that the risk ratios of post-operative infections were similar between the two surgical modalities.⁶⁵

A recent systematic review⁶⁶ evaluated the clinical effectiveness of the surgical technique of coronectomy for third molar extraction in close proximity with the IAN. The authors assessed the following variables: IANI, lingual nerve injury, post-operative adverse effects, pulp disease, root migration and rate of re-operation. Ten articles qualified for the final analysis. The successful coronectomies varied from a minimum of 61.7% to a maximum of 100%. Coronectomy was associated with a low incidence of complications in terms of IANIs (0–9.5%), lingual nerve injury (0–2%), post-operative pain (1.1–41.9%) and swelling (4.6%), dry socket symptoms (2–12%), infection rate (1–9.5%) and pulp disease (0.9%). Migration of the retained roots seems to be a frequent occurrence (2–85.3%). Their conclusion was that coronectomy appears to be a safe procedure, at least in the short term, with a reduced incidence of post-operative complications.

A further systematic review⁶⁷ of four studies indicated that the pooled risk ratio (coronectomy vs total removal) was 0.11 (95% CI = 0.03–0.36), 1.03 (95% CI = 0.54–1.98), 0.55 (95% CI = 0.28–1.05), and 1.14 (95% CI = 0.57–2.30) for IANIs, post-operative infection, dry socket, and pain at one week after surgery, respectively. One of these studies indicated a relatively high rate of failed coronectomy (38.3%, compared with 2.3–9.4% in others). This may have been due to a higher proportion of narrowing roots and vertical impactions leading to easier mobilization of the M3M roots. Although the rate of root migration was high (13.2–85.29%), the distances of migration were short (3.06 ± 1.67 mm), and in directions of moving away from the nerves.

Furthermore, low rates of re-operation and root exposure were reported. The authors therefore concluded that coronectomy appears superior to total removal for reducing IAN damage and could be used in clinical practice for third molar extractions with high risk of nerve injury.

The role of the oral maxillofacial radiologist

Liability issues include who should be responsible for IANI when a bifid nerve is overlooked and partial permanent IANI results after M3M removal. The surgeon must be primarily responsible for the surgery and, as a result, must be competent in reading the CBCT and reporting the risk assessment. There are additional issues regarding CBCT, including radiographic reporting of a large anatomical area and the legal responsibilities of both clinician and radiologist.⁶⁴

Training

Should those not trained to remove M3Ms undertake a coronectomy? It is an adverse event knowingly to leave non high risk roots behind without informing the patient. Root retention may be indicated for ridge maintenance with prior root canal treatment (RCT) OR if the root is displaced. Therefore, surgeons undertaking coronectomy procedures must be suitably trained to remove and/or retrieve the tooth, should the roots be mobilized during coronectomy.

Patient consent

Joint decision-making involves both patient and clinician. The patient must understand the potential risk of his/her chosen intervention. Explaining the potential gain, avoiding life-long IANI is relatively simple for the patient to understand. However, explaining the complex potential complications of coronectomy (possible intra-operative root mobilization³⁴ with necessary extraction, early⁷⁷ and late post-operative infections⁶⁸) is difficult and an information sheet is recommended (Trigeminalnerve.org.uk pdf Coronectomy patient information leaflet). The patient must have a good grasp of potential risks, thus coronectomy should be undertaken when all possible information is available in order to prevent unnecessary patient distress and complaints.

Poor technique

The aim of the coronectomy technique is to leave the high-risk roots with a vital pulp to allow dentine and bone integration and healing,⁵⁸ and to remove all enamel. The technique is explained and illustrated in detail.⁹⁰ Most failures of coronectomy have been reported to be either related to root mobilization during the procedure or inadequate removal of enamel.

Bone rim

In an early study, Renton et al³⁴ recommended achieving an alveolus bony rim around the sectioned M3M root surface of 2–3 mm. This is often impossible due to the lack of a lingual plate adjacent to the M3M. As long as the enamel is removed and wound toilet is good, no further reduction of the root surface is indicated.

Primary closure

This is recommended by most studies and optimizes healing. Several studies mention early post-operative dehiscence persisting, leading to necessary root removal.⁸⁰ To date there are no periodontal health and osseous regeneration outcome studies that have compared coronectomy to extraction.

Antimicrobial (AM) cover for surgery

In early studies, antimicrobials were prescribed routinely for coronectomy,⁸⁹ however, recent studies avoid the prescription of AMs.^80,89

Bone grafting

A case report illustrates that grafting over the exposed root surface, may prevent root migration.⁹³ Leung (in a verbal presentation at ICOMS Melbourne 2015) reported a detailed series of patients where grafting had delayed root migration. However, as the coronectomy procedure is relatively successful without the additional expense of site grafting, further research is required to justify grafting coronectomy sites.

Pulpotomy, pulp dressing or root canal

As the pulpal tissue remains vital in the coronectomied root system there is no indication for pulpectomy, pulpotomy or root canal treatment.⁷⁹

Related cysts

The author routinely undertakes excision of benign odontogenic cysts whilst coronecting the involved M3M or M2M. However, the pathology must be confirmed and patient reviewed.^94,95

Ankylosed M3Ms or other high risk teeth

There are two reports of coronecting high risk ankylosed first and second molars in younger children.⁹⁶

Post-operative radiography

Some authors have recommended routine post-surgical panoral radiography, in order to determine whether or not migration has taken place or to check for the complete removal of enamel, Other studies recommended panoral follow-up at 1 month and at 6–12 months.⁷⁵ However, the author will only arrange post-operative radiography based upon the development of clinic symptoms. Post-surgical panorals or CBCTs are not recommended as part of routine coronectomy practice.