The Use of Coronectomy to Manage Symptomatic Mandibular Third Molars: Techniques, Pitfalls and Suggested Guidelines

Technique

The coronectomy technique was initially described by Ecuyer and Debien in 1984,⁷ although more informal use of ‘partial odontectomy’ predates this description.⁸ The procedure has evolved from initial recommendations of a relatively aggressive reduction of tooth tissue to within 2 mm of the IDN, to more conservative tooth reduction that ensures complete removal of enamel.⁹ A number of key steps has been proposed as the technique has evolved, and failure to adopt these steps may underlie the variable successes experienced between surgeons. Pogrel, one of the pioneers of coronectomy, recommended the use of antibiotics prior to surgery to avoid root infection with a continued post-operative course; the role of prophylactic antibiotics particularly in the pre-operative stage, has been challenged although available evidence to derive a suitable conclusion is limited.^10,11 A three-sided mucoperiosteal flap is raised in order to access the crown of the third molar. Initial descriptions of the procedure further recommended the use of a lingual flap in order to protect the lingual nerve from damage during decoronation with fissure bur.¹⁰ Conventional UK practice is to avoid retraction of lingual flaps during third molar surgery owing to the increased risk of lingual nerve injury; this has on occasion led to practitioners adopting inappropriate decoronation techniques in UK-based practice. While lingual retraction may allow direct protection of the nerve from fissure bur trauma risked by perforating the lingual cortex (Figure 1), penetration of the fissure bur beyond the lingual cortex without retraction risks lingual nerve injury. Indeed, the authors have received a number of tertiary referrals of lingual nerve injury sustained from fissure bur trauma in this manner during coronectomy. A commendable practice is, therefore, to limit fissure bur sectioning to just beyond the pulp chamber, with final crown split using a small sized Coupland's elevator or straight Warwick-James (Figure 2). This variation in technique may explain the increased incidence of root mobilization noted in UK studies,¹ because, should the fissure bur cut not extend to a sufficient depth, the operator may risk mobilization of the roots during the final crown split. Root mobilization during the process of crown sectioning should prompt a change of treatment to full surgical extraction, and it is, therefore, important that the patient is consented pre-operatively for potential full surgical removal should mobilization occur.

Pogrel¹⁰ reported that an important aspect of successful coronectomy is the complete reduction of the retained root to 3 mm below the alveolar crest in order to ensure post-operative healing with full mucosal coverage and to avoid subsequent root exposure to the oral environment, although some clinicians have also reported success with paracrestal coronectomy, as long as complete enamel removal has been achieved. In order to decoronate using Pogrel's recommendations, a fissure bur cut should be undertaken at the level of the amelo-cemental junction and, by virtue, initial buccal reduction will be at best paracrestal (Figure 2d). Buccal cortex may be removed in order to access the full contour of the crown, although bone removal should be kept to a minimum (Figure 2c). An angular burring path will ensure that adequate lingual tooth tissue is removed, although further buccal reduction will be necessary in order to achieve complete submergence of the root. Buccal root reduction is performed after decoronation, and can be completed with a combination of round and fissure burs held in a vertical orientation in order to erode the buccal aspect of the root to 3 mm below crestal level (Figure 2e). Although conventional wisdom has led us to believe that retention of roots with exposed pulp chambers may lead to pulpal necrosis and subsequent peri-apical infection, attempts at coronectomy in combination with MTA root treatment have led to increased failures.¹² More recent reports have also suggested pulp vitality is commonly maintained after coronectomy.¹³

There has been recent interest in undertaking guided bone regeneration (GBR) to assist with achieving full hard-tissue coverage of the retained root portion using bone substitute and a resorbable collagen membrane.¹⁴ Although preliminary data suggest that root migration is significantly reduced with GBR, long-term data on success are not available and post-operative pain appears to be increased. The reduction of root migration may however prove to avoid root re-exposure and, therefore, the need for second surgery.

Peri-osteal relief and final water-tight, primary closure of the three-sided flap using vertical mattress sutures is recommended in order to seal the surgical site from the oral environment, although standard interrupted sutures may be more appropriate in cases of difficult access or cases judged to be at risk of lingual nerve injury. In the recovery period, any early post-operative complications should be managed as ‘dry socket’, and the presence of the persistent root ignored. Resorbable medicaments such as Alveogyl (Septodont) have been recommended,¹⁵ although repeated use may lead to pulpal necrosis.¹³ Ongoing symptoms may prompt an open discussion with the patient as to whether repeat surgery for root removal is warranted, although re-coronectomy may be feasible if retained enamel is noted on post-operative radiograph.¹⁶

Indications for coronectomy

Role of cone beam CT

It is the view of the authors that undertaking coronectomy for all symptomatic mandibular third molars exhibiting plain film features of a close relationship with the IDN is both unnecessary and inappropriate. Foremost, evidence of an intimate relationship on a plain radiograph may not translate to an intimate relationship in 3D, and sectional cone beam CT assessment of any suspicious relationship is a prerequisite to the final decision-making process.⁸ In fact, as many as 98% of cases demonstrating a high-risk relationship on plain film were noted as low-risk on subsequent CT review;¹⁷ for these cases, CT imaging still acts as a valuable tool in prompting the clinician to recommend full surgical removal rather than coronectomy. CT assessment may, therefore, either confirm or refute a close relationship and, furthermore, may greatly assist in quantifying individual risk. In two recent studies of those cases judged as ‘high risk’ based on OPG features, full extraction was associated with 0% permanent IDN injury if CT assessment demonstrated presence of intact cortical bone between the roots and IDN.^18,19 Full extraction in such cases should, therefore, be regarded as treatment of choice unless an overriding patient factor or anatomical configuration warrants coronectomy.

Current evidence suggests that CT assessment does not significantly influence nerve injury rates if used solely to modify the surgical approach to undertaking full tooth removal,^19,20,21 and, therefore, it is important to apply relatively strict criteria for altering the treatment plan from full removal to coronectomy in order for CT planning to be of benefit to outcome. It is not unusual for a patient to desire full removal irrespective of IDN risk so as to gain emotional closure of the ongoing pain relating to a symptomatic third molar and avoid future root infection; a frank discussion as to whether a patient's choice of treatment will be altered in the light of CT findings should take place before the test is requested. Indeed, Cilsun et al found that approximately 6.5% of patients carrying high-risk CT features declined coronectomy in preference to full surgical extraction.¹⁸ The value of CT scanning in such cases is more difficult to justify; although, on an individual basis, CT planning may be of marked benefit to outcome even if full surgical removal is planned (Figure 3). In addition to CT data helping to quantify risk of full surgical removal, the exact location of the IDN may also influence the decision as to whether coronectomy itself presents an unacceptable risk of nerve injury, owing to the nerve lying in the direct path of the decoronation cut.

A comprehensive radiographic assessment of nerve injury risk for any one patient is multifactorial, taking into account not only nerve position and canal cortication, but also factors such as root morphology, anticipated regions exposed to burring, and the anticipated path of root extraction. These factors contribute to high inter-operator variability in the interpretation of both OPT and CT imaging.

Removal versus coronectomy: a multifactorial approach to decision making

The final decision-making process should take into account all patient, clinical and radiographic variables in order to determine the most effective individualized treatment strategy. Although much literature quotes a greater significance of IDN injury to patients whose livelihood is reliant on intact trigeminal afferents, such as public speakers and wind musicians,^1,9 IDN injury is a disastrous consequence of third molar surgery for anyone, and therefore, treatment strategy should probably not be heavily driven by occupation. Although the final decision process should be patient-led, we have provided Table 1 to help summarize those factors that are indicative, permissive, unfavourable and contraindicative of coronectomy and have summarized a general decision-making algorithm for coronectomy of symptomatic mandibular third molars (Figure 5).

Table 1. Factors influencing the decision-making process

Indicative
CT features	Nerve lingual to third molar or interradicular plus CBCT confirms absence of cortex between nerve and root
	Other anatomical high-risk relationship, eg nerve completely enclaved by roots, marked nerve/canal compression, particularly when adopting ‘dumbbell’ morphology
Path of extraction	Likely to traumatize IDN
Mandible	Root fused to lower cortical plate, full surgical removal presents unacceptable risk of pathological fracture
Permissive
Orientation	Mesioangular, distoangular or vertical
Crest	3-mm subcrestal reduction feasible both lingually and buccally
Roots	Low risk of mobilization
Patient	Agrees to root retention
	Older age group (>25)
Unfavourable
Orientation	Horizontal
Crest	3-mm subcrestal reduction not feasible
Roots	High risk of mobilization
Patient	Limited access to dental follow-up (eg military)
	Younger age group (<25)
CT features	Cortical bone between IDN and roots
Contraindicated
Patient	Declines root retention
Medical history	High risk of systemic sequelae
Tooth	Caries, periodontal disease or apical pathology
CT features	Nerve very high and judged to be at risk of bur trauma during coronectomy

Hazards

As eluded to in the preceding text, the major hazards of coronectomy relate to direct bur trauma of either the IDN or lingual nerve. Although both IDN and lingual nerve injury are reported as rare following coronectomy, we have received a disproportionate number of referrals in recent years. While disconcerting, this experience cannot be translated to accurately representing direct risk of the technique because those clinicians undertaking coronectomy are likely to be more mindful of nerve injury overall and, therefore, more likely to refer to tertiary care following injury. Most referrals have related to lingual nerve injury as a consequence of bur penetration through the lingual cortex without protection (Figures 1b, c), reinforcing the need for careful technique involving either direct protection or a crown-fracture approach. Longer-term sequelae of coronectomy remain uncertain owing to the relatively nascent technique, although much concern regarding apical root infection leading to IDN injury appears unfounded.

Conclusion

The vast majority of symptomatic mandibular third molars are most appropriately managed through full surgical removal, even when displaying evidence of an intimate relationship on plain film. Coronectomy is, however, a valuable technique for those cases carrying high risk of sequelae such as IDN injury or mandibular fracture, as confirmed through CT scanning. It is likely that recent studies evaluating the success of coronectomy through randomized controlled trial have been weakened by the inclusion of those cases not at significant risk of nerve injury, and the value of adopting a coronectomy approach for more specific relationships, such as direct contact between a lingual or interradicular IDN and the mandibular third molar, may therefore be underestimated. The available evidence suggests that in such high-risk cases, the coronectomy technique has a superior outcome compared to full removal, and adoption of this strategy should be regarded as an appropriate balance of risk and benefit rather than being dismissed as an ‘easy option’.