Guidelines for the assessment of the impacted maxillary canine

Development

Canine development commences at 4–5 months of age, high in the maxilla, lateral to the piriform fossa, and has the longest path of eruption at 22 mm.6 Crown calcification starts at 1 year, between the roots of the deciduous first molar, and is complete at 5–6 years of age. It then migrates forward and downwards to lie buccal and mesial to the apex of the deciduous canine, then continues to move down the distal aspect of the root of the upper lateral incisor. As the canine erupts into position, the physiological median diastema may close.7 Maxillary canines erupt on average at 11–12 years of age, erupting earlier in females than in males.8 Canine displacement is classified as buccal, palatal, or in the line of the arch. Occasionally, canines can be found lying horizontally above the apices of the maxillary incisors, or displaced near the nose.

Aetiology

Buccally and palatally impacted canines have different aetiologies. Jacoby's study found that 85% of palatally displaced canines had sufficient space to erupt, whereas 83% of buccally impacted canines had insufficient space to erupt. Therefore crowding was determined as the main aetiological factor in buccal impactions.9 Two theories have been proposed to explain the aetiology of palatally impacted canines. The guidance theory described by Becker et al,10 in 1981, suggests that the distal aspect of the lateral incisor is the guide for canine eruption. In his study, Becker found that palatally impacted canines were very closely associated with spaced dentitions and lateral incisors that are peg-shaped, of small mesiodistal width, or congenitally absent. Approximately half of the cases of palatal impactions that were examined were associated with anomalous lateral incisors. The genetic theory described by Peck et al,11 in 1994, considers the dental anomaly of impacted canines to be a product of polygenetic multifactorial inheritance. They based their theory on the fact that palatally displaced canines are concomitant with other dental anomalies, such as lateral-premolar hypodontia and peg laterals, that they occur bilaterally and that there is a gender, familial and population difference in occurrence. Other factors documented in the multifactorial aetiology for impacted canines include:

Investigations

Investigations for unerupted canines should be carried out clinically and radiographically. Clinical assessment will involve visual inspection and palpation. Radiographic assessment will involve the use of parallax or Cone Beam Computerized Technology (CBCT).

Sequelae

Possible sequelae of impacted canines include cyst formation, internal resorption of the impacted tooth, external resorption of impacted or neighbouring teeth, ankylosis, infection and migration of neighbouring teeth with loss of arch length.27 Resorption and pathology are more likely in females, in age groups greater than 14 years, and in cases where the angulation of the canine to the midline is more than 25°.28 Ericson and Kurol showed, in a study of 107 children, that resorption on the roots of incisors adjacent to ectopically positioned canines occurred in 38% of lateral and 9% of central incisors.25 Walker et al showed that 66.7% of lateral and 11.1% of central incisors were resorbed following ectopic eruption of adjacent canine.29 The reported incidence of resorption depends upon the imaging technique used. Superimposition of the incisor roots and the crown of the impacted canine on intra-oral radiographs obscures root morphology in 45% of cases.29 Computed tomography overcomes this problem. Ericson and Kurol25 concluded that resorption on maxillary incisors after ectopic eruption of canines was more common than previously thought. CT scanning increased detection of root resorption by 50%. The sensitivity of intra-oral films was low when diagnosing resorption, and was reported in 0.71% of children in earlier studies.13 The most likely cause of root resorption is inherent pressure due to migration of the displaced erupting canine, combined with physical contact between the root of the incisor and prominences on the canine crown (Figure 5).30 However, the response of incisors with resorbed roots following treatment of impacted canines was assessed by Becker and Chaushu31 and they suggested that, once the impacted canine had been moved away, there is no risk for further resorption. Falahat et al,32 in their follow-up of root resorption, also showed that root resorption associated with ectopic canines, in most cases, did not threaten the long-term viability of the affected incisor, even when resorption was severe. Results from their study showed good clinical and radiographic evidence of healing in all cases, except for one ankylosed tooth and one with pulpal obliteration.

Management

The management of impacted canines usually involves five treatment options:

The general dental practitioner is well placed for early detection of ectopic canines. When canine impaction is identified, extraction of the maxillary deciduous canine may, in some cases, allow the impacted canine to erupt in the correct position. In Class I non-crowded cases, where the permanent maxillary canine is impacted or erupting buccally or palatally, the preventive treatment of choice has been shown to be extraction of the deciduous canine from the ages of 10–13 years.33 It is important that no crowding is present. Power and Short34 showed that interceptive extraction of the deciduous canine completely resolved permanent canine impaction in 62% of cases, and a further 17% showed some improvement in terms of more favourable canine positioning. Ericson and Kurol35 analysed the effect of extraction of the deciduous canine on 46 palatally erupting ectopic maxillary canines in 35 individuals aged 10–13. They found that, in 78% of palatally erupting ectopic canines, the eruption paths normalized within 12 months after extraction of the deciduous canine. In 64% of these cases, improved positions were noted after only 6 months, and in 36% the position improved after 12 months. Inclusion criteria specified that normal space conditions were present and no incisor root resorptions were found. Baccetti et al looked at interceptive treatment of palatally impacted canines by extracting the deciduous canine alone or in association with the use of cervical pull headgear.36 They showed that removal of the deciduous canine alone showed correction of palatal displacement in 65.2% of cases, compared with 87.5% where cervical pull headgear was used and 36% in the untreated control group. In a later study, Baccetti et al37 showed that the use of a rapid maxillary expander as an early interceptive approach is effective for increasing the rate of eruption of impacted canines. In the treatment group, 65.7% erupted successfully compared to only 13.6% in the control group. The success of interceptive treatment depends on the degree of impaction, age at diagnosis and canine position. Generally speaking, when the degree of overlap between the impacted canine and adjacent lateral incisor exceeds half the width of the incisor root, the possibility of total recovery using interceptive treatment alone decreases.34 Ericson and Kurol35 showed that, if the canine overlapped the lateral incisor by more than half of the lateral root, only 64% normalized, compared to 91% when the overlap was less than half the lateral root. The chance of successful eruption following extraction of the deciduous canine also decreases as the angle from the vertical increases.35 The degree of horizontal overlap with the lateral incisor has been found to have more impact on prognosis than angulation.34 A Cochrane review published in 2009 concluded that ‘there is currently no evidence to support the extraction of the deciduous maxillary canine to facilitate the eruption of the palatally ectopic maxillary permanent canine’.38 This review stated that randomized controlled trials were identified but, owing to deficiencies in reporting, they could not be included in the review. The authors recommended that further clinical trials should be conducted, and greater attention to the design and reporting of studies should be given to improve the quality of clinical trials. For these reasons, an orthodontic opinion should always be sought before embarking upon any interceptive treatment. Not all canines normalize, and an orthodontist would be equipped to manage those that fail to show improvement. Furthermore, greater success for re-alignment was shown by Baccetti et al37 with the use of space creation by cervical pull headgear and rapid maxillary expansion, where a specialist opinion again would be ideal.

Assessment for interceptive management

Four aspects of canine position should be assessed, as well as the age of the patient carefully being taken into account. The prognostic factors have been investigated by McSherry39 and Pitt et al,40 who suggested the use of these factors in an index to estimate treatment difficulty. These factors are discussed below:

The evidence basis for these prognostic indicators has been documented by McSherry,39 Stivaros and Mandall14 and Pitt et al.40 It is recommended in the RCS Eng Guidelines on The Management of the Palatally Ectopic Maxillary Canine that less experienced practitioners seek the opinion of an orthodontic specialist prior to initiating treatment.41 The above criteria may aid decision-making regarding management of cases. According to the criteria, if canine prognosis is good in all four categories, then a decision may be made by the orthodontist to extract the deciduous canine (Table 1). This may allow spontaneous eruption of the impacted canine. Many clinicians choose to remove the deciduous canine interceptively and monitor the situation. If the canine fails to erupt or improve within 12 months, the orthodontic treatment will most likely be exposure and alignment.35

In cases where prognosis is average, ie two categories suggest a good prognosis and two suggest an average prognosis (Table 1), definitive treatment could be the extraction of the canine, depending on the overall malocclusion and the associated factors, such as patient age, crowding and condition of the dentition. Therefore maintaining the canine could be important for the bone level or to avoid the need for restoration in an adolescent. The patient should of course be made aware of the eventual loss of the deciduous canine and the need for permanent restoration.

If one or more of the criteria suggest a poor prognosis, or there is evidence of pathology, then orthodontic treatment is essential and the deciduous canine should not be removed (Table 1). In these cases, all factors must be carefully considered before a decision on definitive treatment can be made.

Conclusion

Incidence of maxillary canine impaction is 1.7%, therefore a common encounter for general dental professionals. Practitioners should be aware of normal canine development, relevant investigations, and of dental anomalies such as peg-shaped lateral incisors that occur concurrently, so that early recognition and interceptive treatment can be carried out. Interceptive treatment alone is sometimes sufficient, depending on the degree of impaction. Surgical exposure and orthodontic alignment of the impacted tooth is often required. In such cases, early recognition and appropriate referral for orthodontic treatment are essential.