Coronectomy: a case series demonstrating its value in younger patients

The role of Cone Beam Computed Tomography (CBCT) in coronectomy

The risk of damaging an adjacent vital structure, such as a neurovascular bundle, during the extraction of teeth is recognized by all dentists. Those practitioners who undertake surgery at sites where this is predictably problematic will be familiar with the radiographic signs that are predictive of increased risk of IAN injury during third molar removal. These have been evaluated¹² and loss of continuity of the radio-opaque lines corresponding to the cortication of the inferior dental canal (IDC) and diversion of this canal have been identified as predictors of heightened risk due to close nerve-root contact. These radiographic signs can also be observed, less frequently, in relation to the roots of erupted or unerupted mandibular molars or premolars and must be interpreted appropriately by those responsible for the treatment planning of younger patients in order to avoid preventable post-extraction neuropathy.

When plain radiographs are suggestive of an increased risk of a close relationship between the roots of a tooth and the IDC, the attending clinicians should consider the indication for CBCT. This will allow the anatomical relationship between these two structures to be examined in detail in three dimensions, providing an appreciation of the feasibility of tooth removal without nerve injury. When ‘intimate’ tooth-nerve contact is suggested by the appearance of the loss of cortication or a change in the morphology of the IDC on CBCT, the case for coronectomy of a healthy tooth becomes stronger. Where associated pathology precludes this, the additional information obtained from the scan allows the surgeon to plan and modify their surgery to render iatrogenic damage to the nerve as unlikely as possible. CBCT also provides valuable information about the relationship of an unerupted tooth to its neighbours and allows the extent of resorption, if any, to be accurately assessed.

In keeping with best practice, IR(ME)R and European Commission radiation protection guidance, the use of CBCT in children should be restricted to complex cases in which the benefit of additional diagnostic information obtained outweighs the additional radiation risk. The dose received by the patient should be further minimized by scanning the smallest possible volume size.¹³

We present three cases of patients treated at King's College Hospital to demonstrate the use of coronectomy in the developing dentition. These patients had CBCT scanning as part of their multidisciplinary assessment in a 3DX Accutomo scanner (J Morita Corporation, Tokyo, Japan).

Case 1

A 10-year-old girl was referred from a specialist orthodontic practice regarding her unerupted LL6. The patient presented with a Class I incisal relationship on a Class II skeletal base complicated by an unerupted LL6, increased overjet of 5.5 mm, and a 2 mm shift to the left of the mandibular centre-line. On examination, the LL7 appeared to be partially erupted whilst the LL6 was neither visible nor palpable. There was no buccal or lingual expansion of the alveolus in this region.

A dental panoramic tomograph (DPT) was provided by the referring orthodontist (Figure 1). This showed that the LL6 was unerupted and unfavourably positioned with its apices at the lower border of the mandible. A well-defined radiolucency associated with the cemento-enamel junction of this tooth was suggestive of a dentigerous cyst which appeared to communicate with the oral cavity distally. A CBCT of the left mandibular molar region was obtained to clarify the anatomy of the LL6 and to assess its relationship to the IDC. The scan revealed that this tooth had four rootlets which appeared to have developed as a result of the deflection of the developing mesial and distal roots around the cortical margins of the mandibular canal. The apices of all four rootlets were noted to be lying within the cortical plate at the lower border of the mandible and appeared to demonstrate significant curvature (Figure 2). The IDC was seen to be intimately related to the tooth, running between the two buccal and two lingual rootlets (Figure 3). From the imaging available it was appreciated that the complete surgical removal of the unerupted LL6 would be technically challenging because of its anatomy and associated with heightened risk of IAN injury due to the relationship between the roots of the tooth and the nerve.

After comprehensive discussion with the patient and her parents it was agreed that, in order to facilitate the orthodontic treatment she was keen to have, and to prevent acute symptoms arising from the LL6, a coronectomy of this tooth would be undertaken. They were fully aware that some residual spacing in the left molar region was to be expected on completion of orthodontic treatment due to the presence of the retained roots of the LL6. The family were prepared to accept this in order to avoid the risk of neuropathy which could have complicated the complete removal of this tooth. It was agreed with the consultant orthodontist involved in the patient's care that the coronectomy would be performed at a low level to minimize the interference with the orthodontic alignment of the adjacent teeth.

Coronectomy of the LL6 was performed under general anaesthetic (Figures 4 and 5) and primary closure was achieved. The soft tissue associated with the crown of the tooth was curetted and sent for histopathological analysis. The specimen was reported to be an enlarged fibrous dental follicle.

At review two weeks after surgery, the patient reported no pain or altered sensation in the distribution of her left IAN and soft tissue healing at the coronectomy site was noted to be progressing well. A sectional DPT taken at this time confirmed the complete removal of the crown of the tooth LL6 (Figure 6).

Orthodontic treatment included the use of head gear and removable and fixed appliances to distalize the molar teeth, reduce the overbite and align the remaining teeth. This treatment was started 18 months after coronectomy following the full eruption of the second molars and took 30 months to complete. The DPT and photographs taken just prior to de-bond (Figure 7) demonstrate an excellent result in closing the space overlying the retained roots of LL6, albeit accepting a Class II buccal relationship on the left-hand side.

Case 2

A 6-year-old boy was referred by his dentist regarding his missing LRE. There was no history of pain or swelling. Clinical examination confirmed that this primary molar was absent; otherwise the dentition was developing as expected, although the eruption of the LR6 was less advanced than that on the left.

Radiographic assessment showed the LRE to be present with its apices at the lower border of the mandible. The mesial and distal roots appeared divergent. A well-defined unilocular radiolucency was noted to be associated with the crown of the tooth and there was a suggestion of some small radio-opacities within this. The LR5 was not seen (Figure 8).

A CBCT was carried out utilizing a small (4 x 4 cm) field of view to examine the anatomy of the unerupted primary molar and its relationship to the IDC. This scan confirmed the inferior position of the tooth within the mandible and that the LR5 was absent. The unerupted primary molar was identified as having two mesial and one distal root which split into two apices, so there were four apices in all. The roots and the apices were flattened, curved and divergent (Figure 9). The IDC was observed to run lingual to the roots of the unerupted tooth, compressed against the lingual plate (Figure 10). The crown of the unerupted primary molar was seen to be surrounded by an enlarged follicle which contained several small, round opacities with a radiodensity similar to that of enamel. The margins of this lesion were noted to be well defined and corticated; differential diagnoses included: dentigerous cyst, adenomatoid odontogenic tumour and ameloblastic fibro-odontoma.

After a six month period of monitoring, the clinical and radiographic pictures remained unchanged, however, it was felt that the lesion overlying the unerupted tooth should be removed in order to establish its nature, prevent acute symptoms arising as a result of the development of a communication with the oral cavity, and to minimize the adverse effect on the rest of the developing dentition. The complete removal of the lesion was believed to necessitate the removal of at least the crown of the tooth. The treatment options discussed with the child's parents included; continued monitoring with no active treatment, surgical removal of the LRE and coronectomy of this tooth. Given the technical challenge of attempting to remove the whole tooth and the risk of nerve injury associated with this, the decision was made to proceed with coronectomy. The family were warned of the risk of damage to the IAN and that a second procedure may be required to retrieve the roots if healing was incomplete or symptoms persisted.

The coronectomy of the LRE was carried out under general anaesthetic. The associated soft tissue lesion and mineralized material (which resembled small denticles) within it were removed completely and sent for histopathological analysis. The histopathological appearance of the specimen was essentially that of a microscopic odontome within disorganized odontogenic tissue and not entirely consistent with any of the defined odontogenic tumour entities, so a diagnosis of a follicular hamartoma was suggested. At post-operative review the patient reported no altered sensation or pain and the soft tissues in the region of the coronectomy had healed well. A radiograph taken one year after surgery demonstrated good bony infill and that the roots of the coronected primary molar had not migrated (Figure 11). Intriguingly, the radiograph also appeared to show a tooth-like structure developing superior to the retained roots. Time (and continued monitoring) will tell whether this will take the form of a premolar of normal morphology or a simpler supernumerary tooth but, given the extent of the bony defect at the time of coronectomy, this was an unexpected development!

Case 3

A 15-year-old girl was referred by her dentist regarding the presence of multiple supernumerary ($) and impacted teeth. She was known to suffer from neurofibromatosis type II.

On examination the LRE, LRD and LLE were noted to be retained whilst the LR4, LR5, LL4, LL5 and the UR5 were absent. A DPT (Figure 12) demonstrated the presence of one supernumerary tooth in the premolar region of both maxillary quadrants and two supernumerary teeth in the premolar region of each mandibular quadrant. The UR5 and all four mandibular premolar teeth were seen to be unerupted and impacted. A CBCT was obtained to assess the position, orientation and relationship of the unerupted teeth to adjacent structures and to ascertain whether the teeth adjacent to the supernumeraries had been resorbed. The scan revealed that the UR$ had caused only minimal resorption of the distal aspect of the UR4, but that it had prevented the eruption of the UR5 which had a curved root and had itself caused a degree of resorption of the UR6 roots (Figure 13). The UL$ was noted to have caused a degree of resorption of the UL4 (Figure 14). In the mandibular arch the supernumerary teeth were seen to lie inferior to the primary molar teeth bilaterally. The LRE was noted to have two divergent roots between which the incisive branch of the IDC passed. Bilaterally, the crowns of the LR5 and LL5 were closely associated with the mesial roots of the first molars and a degree of resorption was suspected (Figure 15). The IDC was seen to run on the buccal aspect of the impacted LR5 and LL5 before reaching the mental foramina (Figure 16).

Extraction of the supernumerary and retained primary teeth in addition to the UR5 was carried out under general anaesthetic. Post-operative recovery and healing was uncomplicated. At review a year later the maxillary arch appeared well aligned and the position of the LR4 and LL4 had improved markedly, although these teeth remained unerupted. The position of the LR5 and LL5 appeared more unfavourable than before (Figure 17).

A further CBCT scan confirmed that the LR5 and LL5 had moved in a disto-angular direction and had caused resorption of the mesial roots of the LR6 and LL6, respectively. The IDCs were again noted to run in contact with the buccal aspect of the roots of the second premolars.

Although continued monitoring of the mandibular arch was an attractive option to avoid further surgery and the risk of bilateral nerve injury in this young patient, intervention was necessary to prevent further resorption and possible loss of the mandibular first molars. Given the intimate relationship observed between the LR5 and LL5 and the canals of both the mental and inferior alveolar nerves on CBCT, coronectomy of both teeth was considered the most appropriate treatment modality and this was subsequently carried out under general anaesthetic. At review six months later the patient reported neither transient nor persistent neuropathy. A DPT confirmed that there were no radiographic signs of pathology associated with the retained roots (Figure 18).

Discussion

The three cases presented demonstrate how coronectomy can be used as an effective alternative to the removal of teeth in situations where extraction would be associated with an unacceptably high risk of post-operative neuropathy. The possibility of this rare but highly undesirable and disabling outcome should be avoided in the treatment planning of all patients but does, however, seem particularly important when considering the management of young patients who often present with an asymptomatic dentition.

It is important to appreciate that, although coronectomy is a useful technique, it can be complicated by post-operative neuropathy or the iatrogenic damage of adjacent teeth. There is also a small risk that, either intra- or post-operatively, this procedure will fail and the retrieval of the roots with attendant risks will be unavoidable. It should therefore be considered a risk reducing but not risk free procedure and the pre-operative counselling of patients and their carers regarding this is essential. Chalmers et al⁸ suggested that, following a coronectomy, the presence of the retained roots would necessitate the acceptance of residual spacing in the overlying dental arch at the completion of orthodontic treatment. Our experience is that close liaison with orthodontic colleagues from the time of initial treatment planning allows the outcome of any appliance therapy to be optimized. Discussion regarding the level at which the coronectomy should be performed allows the surgeon to understand what he/she can do to minimize the disturbance to the alignment of the dentition attributable to the deliberate retention of roots.