Coronectomy procedures are widely carried out in secondary care, involving the removal of the dental crown, while retaining the roots in situ. This paper defines and explains the rationale behind coronectomy. It also seeks to review the indications for referral of wisdom teeth, and how to identify high-risk wisdom teeth radiographically using two- and three-dimensional imaging. Using this information, this article aims to provide the practitioner with information on short- and long-term management of high-risk wisdom teeth and discusses coronectomy versus extraction. It also discusses the complications of coronectomy and the importance of adequate consent.
Educates clinicians on the indications for coronectomy.
Discusses the consent procedure, as well as the indications for referral.
Discusses the clinical and radiographic justification for coronectomy procedures.
Removal of lower third molars is a commonly practiced procedure in oral and maxillofacial surgery departments in primary or secondary care.1 Extraction of lower third molars carries a risk to the inferior alveolar nerve (IAN), from 0.4% to 8.4%.2,3 This may be increased in 'high-risk' teeth. With the increasing use of cone beam computed tomography (CBCT), the relationship of the IAN, a mixed nerve, can now be accurately ascertained from its roots.4,5
A coronectomy procedure involves the removal of the dental crown of a tooth while leaving the roots in situ, classically practised with lower wisdom teeth. The main aim of the procedure is to reduce the risk of damage to the inferior alveolar nerve for teeth with roots in close proximity.2,4,6,7,8,9,10
Assessment of lower third molars
Not all wisdom teeth require removal. Those that are asymptomatic, and causing no damage to their neighbours, may be left well alone. If symptomatic or causing insidious damage to adjacent teeth they may be candidates for extraction or coronectomy (see Box 1 for NICE guidelines for extraction of third molars).11 Mandibular third molars must be reviewed on a case by case basis and assessed both clinically and radiographically and before deciding on a definitive treatment plan, a number of factors must be considered. Impacted third molar teeth are in close proximity to the lingual, inferior alveolar, mylohyoid, and buccal nerves, but usually paraesthesia relates to damage to the IAN or lingual nerves.12 Figure 1 demonstrates simplified anatomy of the mandible, and the third molar region. Before any procedures are carried out, dental and patient factors need to be considered (shown in Table 1).4
Proximity to the inferior alveolar nerve
Narrowing of the roots
Curving of roots
Overlapping of root
Darkening of the roots, or apical radiolucencies
This is shown diagrammatically in Figure 2.
Those teeth identified as being high risk on plain film (for example, and orthopantogram) may be further assessed using a cone beam CT (CBCT). A CBCT allows the clinician to visualise the high-risk lower molar tooth in all fields of view in high quality. However, although there has been evidence to show that cone beam images do not affect the sensitivity or specificity for predicting ID nerve damage, they can be useful in aiding surgical planning, and reducing the risk parameters for when the roots of lower third molars are in close proximity to the IAN.13 This may in turn affect the surgical approach – the use of CBCT is explored later in this paper.
Consent for coronectomy procedures: risks and complications
As for any dental or surgical procedure, informed consent is required before treatment can progress. The process of informed consent involves the communication of the potential risks and benefits to the patient, who may then voluntarily give permission to the clinician to carry out the defined procedure (where the patient has capacity). The consent process for coronectomy procedures is very similar to that of extraction of mandibular third molars, with some exceptions:14
There may be a need to extract the tooth if the roots are seen to be mobile during the procedure (9–38%)14
The residual roots may migrate and erupt through the gingivae over time, and need to be extracted eventually, the idea being that the secondary procedure is of lower risk, as it is further away from the nerve. Pogrel et al. noted approximately 30% of coronectomised roots migrating over a 6-month period,7 but some case reports have noted eruption up to ten years following the procedure15
Alveolar osteitis: Leung and Cheung discovered the incidence of pain and dry socket to be lower in coronectomised patients compared to extraction patients, but there were no statistical differences in both groups.2,3 Renton reported no difference in incidence, which was approximately 5%14
Pain: Renton et al.14 postulated that coronectomised patients may experience less pain compared to extraction patients, possibly due to less bone removal, but further research is required
A plain film radiograph is a two-dimensional representation of a three-dimensional object.17 However, if a plain film classically displays the following radiographic signs, it may indicate the need to consider a coronectomy procedure, as opposed to an extraction:5,12,14
Deviation of 'tram lines', or interruption of the white lines representing the inferior dental canal
Darkening of the roots
Deflection or sudden narrowing of the roots of the lower third molar
Narrowing of one or both of the white lines representing the ID canal
Periapical radiolucency of the lower wisdom tooth
Curving of the roots
Loss of ID canal cortex (>3 mm)
Loss or interruption of the lamina dura of the third molar.18
Rood and Shehab postulated that only three of the factors mentioned above were seen to be significantly related to IAN damage, these being diversion of the canal, darkening of the root, and interruption of the white line of the ID canal.5
How to assess a CBCT?
When any of these are visible on the two-dimensional image, and the tooth is deemed higher risk, a CBCT scan can be used to assess the 3D anatomy. If this shows that the IAN is in close proximity then coronectomy is a viable treatment option. However, this is something which is best carried in secondary care or with specialists in primary care. Again, clinicians can read the report of the scan to deduce this if there is any difficulty. Significant high-risk signs include:
Perforation of the tooth root by the ID canal – the classic 'polo mint' appearance
Narrowing of the ID canal
Direct contact or flattening of the nerve by the molar root
Decortication of the bone
A lingual course of the nerve with/without perforation of the cortical plate
Clinicians should refer to the SEDENTEXT Guidelines, when looking at criteria for CBCT.20 There have been multiple clinical studies about the use of CBCT for pre-surgical assessment of impacted third molars.21,22,23,24,25,26,27,28,29 The SEDENTEXT guidelines have postulated that CBCT can offer advantages for surgical planning, and the position of the IAN and the roots of the third molar but should not be used routinely for all pre-surgical third molar assessments.
Figure 3 an example of a high-risk wisdom tooth, the image taken using computer tomography.
A summary of the SEDENTEXT criteria is shown in Box 2.
Tips for the GDP
If there is any doubt about the lower third molar, the GDP can attempt a periapical radiograph of the area. However, in many cases, the patient may not tolerate a film holder posteriorly, especially if impacted, in which case, an OPG is advisable. If there are any radiographic signs or doubt about the lower third molar, the GDP should refer through to the specialist oral surgery service for further investigation. If taking an OPG in primary care, it is imperative that the OPG is of good quality – sending a digital copy which cannot be tampered with is preferable, which may avoid the need to re-expose the patient.
In addition, GDPs may follow up patients in practice who have had coronectomy or those who may have post-operative sequelae such as dry socket and inflammation, which the dentist can manage conservatively. In the long term, the GDP may notice migration of the root through the gingivae. A second procedure is necessary to remove the migrating roots.
There has been some evidence to suggest that bone grafting can improve the outcome of coronectomy. A study by Elo et al., followed 78 patients from five to nine years after coronectomy with bone grafting, and demonstrated marked improvement in post-operative probing depths and bone levels, with no migrations or secondary procedures. Elo postulated that bone grafting may be considered for patients with probing depths of >4 mm on the distal of adjacent molar teeth.30 Similarly, Leung et al. (2017) discussed the possibility of guided bone regeneration in reducing the risk of root exposure after coronectomy procedures.31
Coronectomy is a valid procedure as an alternative to surgical removal of wisdom teeth in high-risk wisdom cases. It is important for the general dental practitioner to clinically and radiographically assess this when referring for coronectomy, as well as gaining the confidence to deal with lower risk teeth in practice.
A special thank you to Mr Michael Millwaters, Consultant in Oral and Maxillofacial Surgery, Princess Alexandra Hospital, for use of the radiographs of the patients at the trust.