One of the most common procedures in oral surgery is the removal of third molars. Third molars may be erupted, partially impacted and completely impacted. The removal of these teeth is not without risks and complications.22 One of the more serious complications with removal of the lower third molars is injury to the inferior alveolar nerve (IAN). By virtue of its anatomic position in the mandible, a close relationship between the third molar roots and the mandibular canal places the IAN at risk for injury during the removal of these teeth. Damage to the nerve is often due to compression from the root during elevation, but may also be directly injured from surgical instruments.18 Nerve transection may occur from rotary instruments used deep in the socket or from removal of a root perforated by the nerve.18 Patients who have suffered an injury to this nerve may experience paresthesia, anesthesia and dysesthesia to the lower lip, chin and buccal gingivae. The incidence of IAN damage after third molar removal leading to temporary symptoms ranges from 0.26 percent to 8.4 percent, and is generally accepted as being around five percent.1,16 Symptoms lasting longer than six months are around one percent.19 There are several other factors that are associated with neurosensory deficits in third molar removal. Increasing age, unerupted tooth, deep impaction, intra-operative IAN exposure as well as specific radiographic signs have all been shown to be risk factors in IAN injury.1 In assessing lower third molar position relative to the mandibular canal, panorex radiography is the most commonly used and available modality. There are five radiographic signs that indicate proximity of the roots of the lower third molar to the mandibular canal.14 These signs correlate well with root proximity to the mandibular canal diagnosed with cone beam computed tomography.15 As the mandibular canal is seen approximating or crossing the root, there may be a loss of the white lines representing the cortex of the canal, deviation of the canal, root deflection along the canal, and darkening and narrowing of the roots.6 The incidence of IAN injury has been shown to be the highest in lower third molars showing radiographic signs of diversion of the mandibular canal (30 percent), followed by darkening of the roots (11.6 percent) and deflection of the root by the mandibular canal(4.6 percent).1
FIGURE 1. An impacted tooth #38 with an intimate relationship to the IAN.
A 48-year-old female presents with a symptomatic impacted tooth #48. (A) A ConeBeam CT scan was ordered to evaluate the tooth and proximity to the IAN. The roots of the vertically oriented tooth exhibit hypercementosis, and the periodontal ligament is clearly not visible. This suggests ankylosis may be present. The nerve courses lingually to the apex of the roots and the nerve is moderately compressed. (B)Pre-operative X-ray. (C) Immediate post-operative X-ray. (D) Six-month post-operative X-ray.
The coronectomy procedure was first reported by Knutsson et al., in 1989 as a procedure to decrease the incidence of IAN injury in third molar cases with root proximity to the mandibular canal as compared to conventional complete removal of the third molar.15 The crown of the impacted third molar is often the cause of various pathologies, such as cysts, pericoronitis, food impaction and caries. By removing the crown and leaving the roots, the clinical problems are solved and the IAN injury is significantly decreased.4 The coronectomy procedure is indicated when there are radiographic signs of root proximity to the mandibular canal. The teeth must be vital and non-infected, and there can be no evidence of periapical pathology, in order to reduce infection in the retained root.2 Mobile teeth should be excluded as the retained root may act as a foreign body and become infected.2 Horizontally impacted teeth along the course of the mandibular canal may be unsuitable for this technique as crown sectioning may injure the IAN.2 It has also been suggested that this procedure can also be used in patients concerned regarding a potential nerve injury.6
Coronectomy of the mandibular third molar is may be performed with IV sedation techniques or local anesthesia in select patients. It is a technique sensitive procedure, and proper sectioning of the tooth may require more dexterity and perceptual acuity than removal of the tooth in its entirety.5 Exposure of the tooth is achieved with raising a buccal mucoperiosteal flap with a disto-buccally angulated releasing incision. Bone is removed with rotatory instruments to expose the cemento-enamel junction. The crown is sectioned from the roots below this junction and may require multiple sections to minimize forces on the root portion. All residual enamel surfaces remaining on the roots must be removed. The root stump is reduced to a level of a minimum of 4mm below the alveolar crest using a round bur. No attempt is made to remove or protect the partially resected pulp tissue. The roots are then assessed for mobility, the wound irrigated and debrided with saline and closed with a resorbable suture.2,5,9 A radiograph should be taken at this time to assess for any residual enamel, which will require immediate removal to ensure proper healing.21
A 55-year-old female presented with a symptomatic impacted tooth #38. (A) Pre-operative X-ray. (B) One-month post-operative X-ray.
Numerous clinical trials, systematic reviews, retrospective and prospective studies have assessed the effectiveness of the coronectomy procedure. It can be concluded that this technique is significantly superior when compared to total removal for reducing IAN damage in high risk cases.2,3,4,5,7,8,9,10,11,12, 22 Even with this high success rate, complications may occur. Numerous studies have demonstrated that the residual root will continue to migrate and may erupt into the oral cavity.19,7,11,17,4,21,3,9,6,2 The range of migration may be between two to 4.8 mm.3,19 The greatest migration time is within the first year after the coronectomy procedure, and this tends to stabilize during the second and third year, due to the formation of overlying bone. A minimum of a three-year follow up is recommended in order to monitor this migration.17 If the roots become symptomatic or erupt into the oral cavity, they will need to be removed in a second procedure. Due to the migration away from the nerve, removal may be performed at a lower risk of IAN injury.3 Women, and a younger age group (<29 years), tended to have a higher mean root migration, most likely due to less dense bone.17 Re-operation rates range from zero to 12.1 percent.9 Care must be taken during coronal sectioning to limit forces applied to the roots. If root mobilization occurs during the procedure, it must be removed to prevent infection, and with the attendant risks of IAN injury.9 Most studies find that the incidence of usual post-operative sequellae of infection, pain, swelling and dry socket were either the same, or better than, for complete extraction cases.4,11,9 No specific management is considered for the remaining pulpal tissue in the retained roots, as pulpal necrosis in the retained root is rare. Sencimen et al. performed endodontics on retained roots after pulpectomy and found that rates of failure and infection were lower in the group that did not receive endodontic treatment.10 CT images one-year after coronectomy did not detect any periapical pathology of the retained roots in a study by Goto et al.20 It is known that fractured roots of vital teeth generally remain vital and heal without complications, an idea upon which the coronectomy procedure is based.12 This is also supported by animal studies showing that roots remain vital with minimal degenerative changes.13
A 50-year-old male presented with symptomatic impacted #48. (A) X-ray from 10 years earlier when he had symptomatic #18 and elected not to remove #48 at that time. (B) Pre-operative X-ray. Carious symptomatic #48. (C) Immediate post-operative X-ray. One week after surgery. (D) Eight months post-operative X-ray.
Coronectomy is a useful procedure to significantly minimize the risk of IAN injury in select mandibular third molar removal. It is technique sensitive in order to obtain good outcomes and it should be planned from the beginning of the procedure. It is not a technique to be used in the event the operator is unable to remove roots after a crown has fractured. The plan for coronectomy should be discussed pre-operatively with the patient and all risks fully explained. The patient needs to be aware of the prolonged follow-up necessary to ensure proper healing.
Immediate post-operative radiographic imaging is recommended, as well as a follow-up evaluation six to 12 months after surgery.OH
Robert A Green, DDS, MD, MSc, FRCD(C). Private practice, Stoney Creek, ON, Staff, Hamilton Health Sciences Center, Hamilton, ON, Canada. Corresponding author: firstname.lastname@example.org.
David J. Wilson, BSc, DDS, MD, FRCD(C).Private practice, Thunder Bay, ON; Staff, Thunder Bay Regional Health Sciences Centre, Thunder Bay , ON, Canada.
Bruce R. Pynn , MSc, DDS, FRCD(C), Private practice, Thunder Bay ON; Staff, Thunder Bay Regional Health Sciences Centre, Thunder Bay, ON, Canada.
Oral Health welcomes this original article.
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