Third mandibular molar coronectomy: a way to prevent iatrogenic inferior alveolar nerve injuries—an systematic review

This is an Open Access article d unrestricted use, distribution, Abstract Introduction: In the field of oral and maxillo-facial surgery, the avulsion of third mandibular molar is a very common procedure. However, and although the injury on the alveolar inferior nerve is very rare, the neurological risk must not be underestimated. Indeed, it may lower the patient’s quality of life in a significant way. The coronectomy is a technique that allows us to avoid this risk. It consists in remaining in place the third mandibular molar’s roots. Educational objectives: After a clinical introduction to this surgical technique, the main characteristics of this type of procedure will be presented with the help of an exhaustive literature review. Thus, we will refer to the following subjects: the obvious decrease of neurological risks, the potential pre and post-operating complications, the potential necessity of an endodontic treatment for the residual roots, the becoming of these same roots, and finally the bony and mucosal cicatrization of the operated area. Conclusion: Every oral surgeon should have in mind this technique of coronectomy and master it. Indeed, when the case justifies it, the benefits are numerous for the patient.


Introduction
The extraction of third molar, especially mandibular, is a very standard procedure in oromaxillofacial surgery. In cases where the inferior alveolar nerve (IAN) is in close proximity to the roots of the mandibular third molar (3MM), there is a minimal risk of nerve injury. Previous studies have reported that third mandibular molar extraction causes approximately 0.5-1% of chronic IAN injuries [1,2]. IAN injury must nevertheless be minimized to avoid adverse consequences impeding the patient's quality of life. These consequences may be caused by sensorineural disorders such as hypoesthesia, dysesthesia, anesthesia, etc., as well as the neuropsychological and functional disorders that may also result (chronic pain, long-term changes in the body schema, a functional deficit that may impact social function: drooling, biting, the altering of certain facial expressions etc.) [3,4].

Pedagogical objectives Preoperative assessment
A standard preoperative assessment is performed to identify the current and past medical history of the patient and to have a better grasp of the anesthesia modalities required for the intervention. Some X-rays are required for planning the intervention. First, a panoramic X-ray and initial examination of the mandible will make it easy to detect possible risks. Seven predictors of increased risk of IAN injury have been reported in previous studies [5,6]: -Disruption of the lamina dura* (Fig. 1a) -Dental root translucency* (Fig. 1b ) -Root canal deviation* (Fig. 1c) -Translucency around the roots (Fig. 1d) -Root curvature (Fig. 1e) -Narrowing of the roots (Fig. 1f) -Narrowing of nerve root canal (Fig. 1g) -*: Statistically significant according to Rood et al. (1990) [5].
Following this, a three-dimensional cone-beam computed tomography (CBCT) should ideally be performed, which will pinpoint nerve proximity. If a bone plate is interposed between the nerve tissue and the tooth root to be extracted, possible risks to the nerves are decreased. A coronectomy can only be considered when direct contact is observed between IAN and the root.
This technique, also known as a partial odontectomy, was first outlined by Ecuyer et al. in 1984 [7]. It is widely used in Anglo-Saxon countries but is rarely used in France.
The patient will receive prescriptions, including analgesics, adapted to the clinical situation (palliative 1 or 2), a local antisepsis (chlorhexidine 0.2%) to be taken 24 h after the intervention, and antibiotic prophylaxis (in accordance with current recommendations for dental avulsions of mandibular wisdom teeth: 2 g of amoxicillin 1 h before surgery or 600 mg of clindamycin in case of a penicillin allergy [8]).
Precise and reliable information is provided to the patient and informed consent is obtained.
At the end of this preoperative assessment: -The indications for a coronectomy should be decided in a rational manner by the surgeon; -Informed consent should be obtained from the patient; -Drug prescriptions appropriate to this intervention should be prescribed.

Operative technique
After intraoral disinfection (with 10% povidone iodine or 0.2% chlorhexidine), an intrasulcular incision is made from the first to second molar. This is followed by a distovestibular relieving incision of a few millimeters at a 45°along the external oblique line (Fig. 2a).
Next, a full-thickness gingival flap is elevated (Fig. 2b). The flap is supported on a retractor so that a clear view of the operating site is ensured. Some authors advocate using a lingual protection plate, which will create a deep coronoradicular separation without the risk of lingual nerve injury [9].
Alveolectomy is the performed using either round or fissure bur attached to a contra-angle handpiece with constant saline irrigation until the enamel-cement junction is exposed (Fig.  2c).
Coronoradicular separation is performed using a fissure or Lindhemann bur attached to a contra-angle handpiece with saline solution irrigation. Then, the crown is separated from the radicular complex (Fig. 2d). The separation axis should be perpendicular to the long axis of the tooth. The risk here is the creation of an incomplete segment, resulting in a residual amelodentinal substance which would require a repeat surgery. (Fig. 3). The immobilization of the root complex is required; otherwise, infection risk is increased. [9].
Amelodentinal substance 2-4 mm deep to the alveolar rim is delicately removed with a round bur, which is reported to enhance new bone growth (Fig. 4) [9].
The dental pulp tissue does not require any special treatment [10]. Bone smoothening, rinsing, and suturing are performed. A postinterventional retroalveolar or panoramic dental X-ray should be performed to confirm that there is no residual amelodentinal lamina and to serve as a reference   for any residual roots. Regular clinical and radiological followup is required every 6 months for a period of 1-2 years and annually thereafter. This will make it easier to intercept any complications, to assess the migration of the residual roots and, if necessary, to predict possible avulsions (Fig. 5).
The operative technique must allow: a clear view of the optimal operating site; a clear section of the crown without any mobility of the radicular complex.

Literature review
The aim of this literature review is to answer the following issues regarding the coronectomy: decreasing the risks of nerve injury; risk of infection; the clinical outcomes of roots retained in the bone; the need for the endodontic treatment of residual roots; periodontal healing of the operative area.
Consequently, a keyword search was conducted using such scientific databases as "PubMed," "Google Scholar," and "ScienceDirect." The French and English results for the keywords "coronectomy" and "partial odontectomy" were analyzed and 298 references were found.
Each article was assessed according to the study design, sample size, including how many 3MM, the presence of a control group, the radiological diagnosis which allowed for the inclusion of subjects, the average follow-duration up (in months), and the level of proof according to the French National Authority for Health (HAS) scale [28].

Decreasing nerve injury risk
The randomized controlled studies by Renton et al. (2005) and by Leung et al. (2009) found statistically significant outcomes, showing that coronectomy showed a lower incidence of nerve damage than conventional extraction. In radiologically at-risk patients, nerve damage was reported in 0-0.65% cases for coronectomies, whereas it was reported in 2.64-19% cases for standard avulsions [13,14]. In addition, two metaanalyses, one by Long et al. In summary, many studies (nonrandomized controlled, prospective, and retrospective) found similar results favoring coronectomy ( Table I).
The coronectomy minimizes the risk of IAN nerve damage compared to conventional surgical techniques.

Intraoperative complications
Apart from the complications inherent to any oral surgery (for example; bleeding, pain, fracture, etc.), two specific complications should be highlighted.
In the event of intraoperative mobilization of the radicular complex, it must be avulsed. Indeed, according to Patel et al. (2013), this increases the risk of postoperative infection [26,29]. This risk is higher in conical root cases and in female patients.
In the case of insufficient coronoradicular separation, amelodentinal residue may be observed at the end of the procedure. This will require reintervention to avoid the increased risk of infection.

Postoperative infections
For methodological reasons, controlled group studies were used to investigate short-term (<2 months) postoperative infections, such as dry and suppurative alveolitis. On the other hand, long-term infections (≥2 months) caused specifically by coronectomies were investigated by prospective studies, because they have long-term follow-up.   [6,13]. Other studies did not specifically address this parameter.
Long-term infections: The presence of residual roots in the socket forces the practitioner to consider the potential risk of a long-term infection. The study conducted by  which monitored a cohort of 612 coronectomies for 60 months, discovered only two (0.33%) infections (at 12 and 24 months) [17]. All the other studies had shorter postoperative monitoring and found no long-term infections.
It seems that the postoperative infection rates are similar for both techniques.

The fate of the residual roots
Different studies highlight a phenomenon of root migration toward the crown. Leung et al. (2009) note a migration of 3.06 mm (±1.67 mm) after 24 months with a migratory peak within the first 3 months, followed by a stabilization up to 36 months (13) (Fig. 7). These results have been confirmed by Kohara et al.'s study (2015) (111 teeth monitored for a period of 36 months), which found that 68% roots had migrated, with a peak at 3 months and stabilization at 3.5 mm after 36 months [18]. This study found increased migration in women and younger subjects. The study done by Goto et al. (2012) focusing on conical roots also confirms this [21]. Hatano et al. (2009) and Kouwenberg et al. (2016) found similar migration rates of 84-85% [6,22].
In certain situations, the roots re-erupt in the oral cavity after migration. A reintervention will therefore be guided by symptomology: pain, infection, or the patient's discomfort.      during the 60-month follow-up of 612 coronectomies, observed a re-eruption rate of 2.1% [30]. The coronectomies are conducted without nerve damage, thanks to the apparent distancing between the nerve structures and the roots as a result of migration. Cilasun et al. (2011) and Renton et al. (2005) indicated that no reintervention was needed after a follow-up of 17 and 25 months, respectively [14,16].
A mesiocrestal migration of residual roots (frequently observed) sometimes requires reintervention, but all safety protocols are observed with respect to IAN.

Endodontic treatment is required
Sencimen et al. (2010) established two groups with each group having eight patients. In one group (experimental group), the coronectomy was supported by endodontic treatment with mineral trioxide aggregate of the roots. In the other group (the control group), the tooth with the coronectomy received no treatment [18]. A significant complication rate is found in the group receiving the endodontic treatment (seven avulsions needed for the eight residual root complexes).
Some studies have histologically analyzed the pulp tissue of the extracted teeth after their re-eruptions [18,31]. The dental pulp of these teeth was the key determinant in most cases (some cases of partial necrosis were discovered with the necrotic tissue located coronally to the vital apical pulp tissue).
Finally, the long-term prospective studies in which clinical and radiological follow-up was performed reported no signs of pulpal necrosis [17].
Endodontic treatment of residual roots is not necessary.

Periodontal healing
According to Vignudelli et al. (2016), from the 9th month after the coronectomy, the pocket depth distal to the second molar and the distance between the bone crest and the bottom of the bone defect reach normal physiological levels [32]. Kohara et al. (2015) confirmed this using 3D CBCT imaging, which revealed that in >90% cases, there were pockets <4 mm as well as new bone growth above the roots [18].
Leaving residual roots in place does not affect the periodontal prognosis of the adjacent tooth.

Conclusion
The use of the coronectomy as a technique for extracting mandibular wisdom teeth seems to significantly decrease the risk of nerve damage in cases of proximity between IAN and the dental roots without increasing the intraoperative or postoperative complications. It is a simple technique that does not require endodontic treatment of the residual roots. Long-term reinterventions and infections are infrequent.
Coronectomy is therefore a surgical technique of interest which should be more widely taught and practiced. The use of this technique could significantly decrease the postoperative neurological complications that are often the reason for medicolegal cases.

Conflicts of interests:
The authors declare that they have no conflicts of interest in relation to this article.