Lateral Incisor Root Resorption: A Consequence of Ectopic Maxillary Canine Teeth

Background

An ectopic impacted tooth is one that fails to erupt at its appropriate site in the dental arch within a normal period of growth, based on clinical and radiographic assessment.³ In the permanent dentition, impacted maxillary canines have an incidence rate of 1.5%, second only to mandibular third molar teeth.⁴ These teeth usually erupt at the age of approximately 10.5 years in girls and 11.5 years in boys, and should be palpable from age 8 years.⁵ Ectopic permanent canines can have many adverse effects including cyst formation, displacement of adjacent teeth, loss of vitality and external resorption of adjacent teeth, all of which can complicate and lengthen restorative and orthodontic treatment for the patient. Further investigation with plain radiographic films for localization and advanced imaging with cone beam computed tomography (CBCT) is often required if a maxillary permanent canine is not palpable by the age of 10, or if there is prolonged retention of the primary predecessor.

It is known that when the maxillary permanent canine develops it moves downwards into the dental arch and becomes more upright when it reaches the mesial aspect of the primary canine and distal aspect of the permanent lateral incisor. If this does not occur, the permanent canine will be considered ectopic.³ The aetiology of impacted permanent canines is, however, unclear.⁶ Two possible theories related to their impaction have been raised: the genetic theory and the guidance theory.^7,8 The genetic theory notes a dental anomaly of genetic origin as the cause of an impacted canine, such as hypodontia, microdontia or enamel hypoplasia.⁹ The guidance theory follows the belief that the adjacent permanent lateral incisor guides the canine during the course of eruption. If the permanent lateral incisor is missing, or the root malformed, the permanent canine will become ectopic.¹⁰ Neither theory is currently supported by compelling evidence.

Interceptive management of ectopic maxillary canines

Interceptive management of ectopic permanent canines is desirable to prevent long-term complications and lengthy remedial treatment for the patient. Possible interceptive treatment options include:

Interceptive extraction of primary canines was first suggested in 1936; however, since then, there has been varying evidence surrounding the efficacy and justification of these interceptive measures.⁶ Extraction of the primary canine is the most common treatment undertaken to encourage spontaneous correction in position of the permanent successor. In a systematic review by Almasoud,¹¹ mounting evidence to support interceptive extractions was found. Others, however, including the Cochrane review conducted by Parkin et al,¹² concluded that there was no convincing scientific evidence to support this intervention. The randomized control trials evaluated were found to have deficiencies in design, conduct, analysis and reporting, resulting in a high risk of bias. Interceptive extractions may often be the patient's first experience of dental treatment, therefore must be clinically justified considering the impact they may have on young patients. The current Royal College of Surgeons (RCS) Guidelines for management of the palatally ectopic maxillary canines support interceptive extraction of the primary canine in select cases. The patient should be aged 10–13 years, and the permanent canine should not be severely displaced. It notes improvement has been found in 68–72% of cases, although recognizes that this evidence is inconclusive.⁴

Root resorption as a consequence of ectopic maxillary canines

External root resorption is a pathological process in which the immune system dissolves the root of a tooth. It can occur as a result of orthodontic treatment, infection or adjacent to unerupted teeth.¹⁴ The mechanism of root resorption is uncertain; however, its cause is thought to be related to physical pressure. It was previously believed that pressure from an enlarged follicle could have been a potential explanation, but this theory has since been dismissed.¹ It is now believed that root resorption as a result of an ectopic permanent canine occurs solely when the unerupted tooth is in direct contact with the adjacent permanent lateral incisor. The alveolar bone surrounding the adjacent tooth is resorbed and the protective layer of cementoblasts and collagen fibres are lost, making way for dentinoclasts to resorb the incisor root.² Risk of resorption is greatest when the canine root is well developed and its cusp overlaps 50% of the lateral incisor root.³ An unerupted maxillary permanent canine also moves mesially as it develops, further increasing the risk of lateral incisor root resorption.² Lateral incisors are more commonly affected than central incisors because they have a conical root with an apex located deep in the palate, where the permanent canine develops.³ Average-sized permanent lateral incisors are more likely to be resorbed than peg-shaped teeth, or those with abnormal morphology because they are more likely to be in close proximity to, or obstruct, the eruption path of the unerupted permanent canine.¹

There have been many suggestions on how to grade apical resorption defects. The most common system was developed by Levander and Malmgren.¹³ Defects are assessed and grouped from 1 to 4, group 4 being the most severe and involving over one-third of the original root length. Resorption can, however, affect different areas of the root, with the middle third affected in 82% of cases. The apical and cervical sections of the tooth are less commonly affected.³

Permanent lateral incisor root resorption can be difficult to diagnose due to superimposition, overlap of adjacent teeth and distortion of conventional radiographs.³ For this reason resorption defects are often diagnosed late in relation to the extent of the defect and the patient's age, with approximately 60% of all resorption defects classified as severe at the time of diagnosis. Resorption usually occurs at an early age, mostly between 10 and 13 years. CBCT is the gold standard for the diagnosis of incisor root resorption because it more accurately depicts the position and extent of the resorption defect. CBCT studies have found permanent lateral incisor root resorption to be associated with 12.5% of ectopic canines.¹⁵

Incisor root resorption associated with impaction of permanent maxillary canines can threaten the long-term survival of the affected tooth.¹⁶ Management of root resorption can be complex and is often case dependent, aiming to address the cause and to aid stabilization and/or regeneration of the lesion. There is lack of evidence regarding the most effective treatment for external root resorption, with management often left to the clinician's discretion, taking into account their clinical experience and patient-related factors. In cases of slight resorption caused by pressure from an adjacent tooth with no signs of infection, removal of the source of pressure will often prevent further progression of the defect. In these cases, the tooth can be maintained, and even undergo orthodontic movement; however, it will always have a guarded prognosis, and be at risk of mobility or causing pain. In some cases of severe resorption with pulpal involvement, endodontic treatment can arrest the resorptive process. A biocompatible material, such as mineral trioxide aggregate (MTA) or Biodentine (Septodont, Saint Maur des Fosses, France), is often used to provide an apical seal. If the defect is very extensive, or there is cervical margin involvement, extraction is often the only viable treatment option.¹⁴

Case 1

A 15-year-old male undergoing orthodontic treatment was referred to the Department of Child Dental Health at the University Dental Hospital of Manchester by his general dental practitioner (GDP). His orthodontist had noticed a cervical external resorption lesion on the distal surface of the UR2. The resorption defect was quite extensive, prompting the orthodontist to request advice on the treatment options and prognosis for this tooth. The tooth was asymptomatic and there was no history of pain, swelling or infection.

The patient had previously been referred to a hospital orthodontist aged 14 who noted the presence of impacted UR3 and UL3 and retained deciduous canines (Figure 1). Following this, the deciduous canines were extracted and the UR3 and UL3 erupted spontaneously. Active orthodontic treatment with fixed appliances was commenced 12 months later. Pre-treatment peri-apical radiographs demonstrated the resorption defect on the root of UR2. Consequently, the UR2 was not engaged in the appliance to minimize any further resorption.

On examination, the UR2 was slightly instanding, but this was not an aesthetic concern for the patient. A labial periodontal defect related to the UR2 was noted with no increase in probing depth around the tooth. The UR2 was not mobile, was of a normal colour, and gave a positive response to Endo Frost (ROEKO, Langenau, Germany) on sensibility testing. The tooth elicited a normal sound and was non-tender to percussion. Peri-apical radiographs showed a moderate distal root surface defect of the UR2 that appeared to communicate with the supracrestal region (Figure 2a). The UL2 showed a slight distal defect on the middle third of the root, which was an incidental finding (Figure 2b).

A report from a maxillofacial and dental radiology consultant confirmed that the defects were likely to have been caused by the previously ectopic maxillary permanent canines. Considering the clarity of the peri-apical radiographs, it was decided that a CBCT was not justified.

Noting the minimal resorptive defect, the UL2 was deemed to have a good long-term prognosis. The UR2, however, had a more extensive resorptive defect that was extremely close to the pulp canal, complicated by an area of adjacent periodontal bone loss. The patient was made aware that this tooth had a guarded long-term prognosis with failure likely to be hastened through orthodontic movement of this tooth. No treatment was deemed necessary at this time because the patient was asymptomatic. Extensive oral hygiene education was delivered, stressing the importance of subgingival cleaning. The risks and benefits of orthodontic treatment were explained to the patient, referring dentist and orthodontist. Failure of the tooth during orthodontic treatment may result in extraction and replacement with a denture, bridge or implant, as any endodontic treatment would likely be unsuccessful due to the extent of the lesion and periodontal bone loss. The management plan was, therefore, to accept the position of the UR2 and maintain the tooth in the arch with close monitoring. The UR2 would, in the meantime, act as a space maintainer. This would allow any future prosthetic replacement to have more ideal mesiodistal and vertical dimensions, helping to preserve aesthetics and ensure that the upper dental centreline remained correct.

Case 2

A 14-year-old female was referred to the Department of Child Dental Health at the University Dental Hospital of Manchester by her GDP who had diagnosed a non-vital UL1 and UL2 following surgical removal of an ectopic UL3. The surgeon had noted that the extraction of the UL3 was difficult, resulting in slight mobility of the UL1 and UL2 (Figure 3). Recovery was complicated by an infection at the extraction site, which was treated with antibiotics.

On examination, the patient presented with a labial gingival abscess relating to the UL2, which had a grey discolouration. The UL2 was not mobile, but was tender to percussion and had a negative response to sensibility testing with Endo Frost. The UL1 was also not mobile and responded positively to sensibility testing. A plain film peri-apical radiograph showed evidence of apical root resorption in UL1 and UL2. Further detailed investigation in the form of a CBCT found that the UL1 root was short due to disto-labial resorption. The UL2 was resorbed on the mesial aspect of the apical third of the root, extending to the root canal. It also showed a round cortical defect of the labial cortex overlying the space between the UL1 and UL2 roots (Figure 4). The CBCT report confirmed that these defects were likely to have been due to pressure resorption and/or iatrogenic damage caused by the ectopic UL3 and its subsequent removal. The patient attended a multidisciplinary clinic with a consultant orthodontist, paediatric dentist and restorative dentist for comprehensive planning. A decision was taken to complete root canal treatment on the UL2 and monitor the UL1.

Root canal treatment was undertaken on the UL2 and the apical 3 mm of the root canal was found to be non-negotiable. An MTA apical plug was, therefor placed and the canal backfilled with thermoplastic gutta percha (Figure 5). Annual review following completion of root canal treatment of the UL2 noted mild tenderness to percussion. The patient was otherwise clinically asymptomatic with no other signs of pathology related to the UL2. The patient had commenced active orthodontic treatment with fixed appliances and the UL2 had shown signs of successful movement. Although treatment for this patient has been successful so far, the short roots of the UL1 and UL2 pose an increased risk of further resorption through orthodontic movement, a risk for which the patient was consented.

Conclusion

Root resorption secondary to displaced permanent maxillary canines can have severe consequences for affected teeth. Early recognition and interceptive treatment is key in the prevention of complex treatment or loss of severely resorbed teeth. Referral for further investigation and opinion should always be considered if the permanent maxillary canine is not palpable by 10 years of age.

Although further high quality randomized controlled trials are needed to provide a clear and evidence-based treatment plan for patients, the current guidelines do promote the extraction of retained deciduous canines. Extraction of the maxillary primary canine at ages 10–13 may result in spontaneous eruption, if the permanent successor is not severely displaced, reducing the risk of resorption to adjacent teeth.

Future updates in guidance and a pending Cochrane review focusing on intervention promoting eruption of displaced canines should provide more clarity in the management of these patients.