14.1. FACTS AND FIGURES
Development of the upper and lower canines commences between 4 and 5 months of age. The upper canines erupt, on average, at 11–12 years of age. The lower canines erupt, on average, at 10–11 years of age.
In a Caucasian population (Gorlin et al. 1990): congenital absence of upper canines, 0.3 per cent; congenital absence of lower canines, 0.1 per cent; impaction of upper canines, 1–2 per cent, of which 8 per cent are bilateral; impaction of lower canines, 0.35 per cent; resorption of upper incisors due to impacted canine, 0.7 per cent of 10–13 year olds; transposition, exact prevalence not known (rare).
14.2. NORMAL DEVELOPMENT
The development of the maxillary canine commences around 4 to 5 months of age, high in the maxilla. Crown calcifiation is complete around 6 to 7 years of age. The permanent canine then migrates forwards and downwards to lie buccal and mesial to the apex of the deciduous canine before erupting down the distal aspect of the root of the upper lateral incisor. Pressure from the unerupted canine on the root of the lateral incisor leads to flaring of the incisor crowns, which resolves as the canine erupts.
14.3. AETIOLOGY OF MAXILLARY CANINE DISPLACEMENT
Canine displacement is generally classified into buccal or palatal displacement. More rarely, canines can be found lying horizontally above the apices of the teeth of the upper arch (Fig. 14.1) or displaced high adjacent to the nose (Fig. 14.2).
The following have been suggested as possible causative factors. However, the aetiology of canine displacement is still not fully understood.
Displacement of the crypt. This is the probable aetiology behind the more marked displacements such as those shown in Figs 14.1 and 14.2.
Long path of eruption.
Short-rooted or absent upper lateral incisor. A 2.4-fold increase in the incidence of palatally displaced canines in patients with absent or short-rooted lateral incisors has been reported (Becker et al. 1981) (Fig. 14.3). It has been suggested that a lack of guidance during eruption is the reason behind this association. Because of the association of palatal displacement of an upper canine with missing or peg-shaped lateral incisors it is important to be particularly observant in patients with this anomaly.P.140
Crowding. Jacoby (1983) found that 85 per cent of buccally displaced canines were associated with crowding, whereas 83 per cent of palatal displacements had sufficient space for eruption. If the upper arch is crowded, this often manifests as insufficient space for the canine, which is the last tooth anterior to the molar to erupt. In normal development the canine comes to lie buccal to the arch and in the presence of crowding will be deflected buccally.
Retention of the primary deciduous canine. This usually results in mild displacement of the permanent tooth buccally. However, if the permanent canine itself is displaced, normal resorption of the deciduous canine will not occur. In this situation the retained deciduous tooth is an indicator, rather than the cause, of displacement.
Genetic factors. It has been suggested that palatal displacement of the maxillary canine is an inherited trait with a pattern that suggests polygenic inheritance. The evidence cited for this includes:
14.4. INTERCEPTION OF DISPLACED CANINESBecause of their high propensity for ectopic eruption, it is essential to palpate for unerupted canines when examining any child aged 9 years and older, as early detection of an abnormal eruption path gives the opportunity, if appropriate, for interceptive measures. It is also important to locate the position of the canines before undertaking the extraction of other permanent teeth. Canines, which are palpable in the normal developmental postion, buccal and slightly distal to the upper lateral incisor root, have a good prognosis for eruption.Clinically, if a definite hollow and/or asymmetry is found on palpation, further investigation is warranted. On occasion, routine panoramic radiographic examination may demonstrate asymmetry in the position and development of the canines.
the prevalence varies in different populations with a greater prevalence in Europeans than other racial groups;
affects females more commonly than males;
occurs bilaterally with a greater than expected frequency;
occurs in association with other dental anomalies (e.g. hypodontia, microdontia).
It has been shown that extraction of a deciduous canine may result in improvement of the position of a displaced permanent canine, sufficient to allow normal eruption to occur (Fig. 14.4). As the success of this approach reduces with the degree of displacement it is advisable to seek the advice of a specialist before this step is undertaken in those cases where the canine is markedly displaced. The likelihood of the displaced canine position improving is also reduced in cases with crowding. It is prudent to warn the patient and their guardian that it may be necessary to expose the unerupted tooth and apply traction via an orthodontic appliance. This interceptive approach has also been used successfully for displaced mandibular canines.
14.5. ASSESSING MAXILLARY CANINE POSITION
The position of an unerupted canine should initially be assessed clinically, followed by radiographic examination if displacement is suspected.
It is usually possible to obtain a good estimate of the likely location of an unerupted maxillary canine by palpation (in the buccal sulcus and palatally) and by the inclination of the lateral incisor (Fig. 14.5).
The radiographic assessment of a displaced canine should include the following:
location of the position of both the canine crown and the root apex relative to adjacent teeth and the arch;
the prognosis of adjacent teeth and the deciduous canine, if present;P.142
the presence of resorption, particularly of the adjacent central and/or lateral incisors.The views commonly used for assessing ectopic canines include the following.
Dental panoramic tomogram (DPT), also known as an OPG or OPT. This film gives a good overall assessment of the development of the dentition and canine position. However, this view suggests that the canine is further away from the midline and at a slightly less acute angle to the occlusal plane, i.e. more favourably positioned for alignment, than is actually the case (Fig. 14.6(a)). This view should be supplemented with a periapical view.
Periapical. This view is useful for assessing the prognosis of a retained deciduous canine and for detecting resorption (Fig. 14.6(b)).
Lateral cephalometric. For accurate localization this view should be combined with an anteroposterior view (e.g. a DPT) (Fig. 14.6(c)).
Vertex occlusal. This view is popular with oral surgeons, but involves a relatively high X-ray dose and irradiation of the orbit.
The principle of parallax can be used to determine the position of an unerupted tooth relative to its neighbours. To use parallax two radiographs are required with a change in the position of the X-ray tube between them. The object furthest away from the X-ray beam will appear to move in the same direction as the tube shift. Therefore, if the canine is more palatally positioned than the incisor roots it will move with the tube shift (Fig. 14.6 (b)). Conversely, if it is buccal it will move in the opposite direction to the tube shift. Examples of combinations of radiographs which can be used for parallax include two periapical radiographs (horizontal parallax) and a DPT and an upper anterior occlusal (vertical parallax).
14.6. MANAGEMENT OF BUCCAL DISPLACEMENT
The width of the maxillary canine is greater than the first premolar which in turn is greater than the deciduous canine.
Buccal displacement is usually associated with crowding, and therefore relief of crowding prior to eruption of the canine will usually effect some spontaneous improvement (Fig. 14.7). Buccal displacements are more likely to erupt than palatal displacements because of the thinner buccal mucosa and bone. Buccally displaced erupted canines are managed by relief of crowding, if indicated, and alignment. An upper removable appliance with a buccal canine retractor can be used where the canine tooth is mildly displaced, mesially inclined and tilting movements will suffice. Fixed appliances are indicated if the canine is upright or distally inclined and/or rotated. In such a case a sectional fixed appliance on the buccal segment teeth in that quadrant and the affected canine only may be useful to prevent ‘round-tripping’ the upper lateral incisor.
In severely crowded cases where the upper lateral incisor and first premolar are in contact and no additional space exists to accommodate the wider canine tooth, extraction of the canine itself may be indicated. In some patients the canine is so severely displaced that a good result is unlikely, necessitating removal of the canine tooth and the use of fixed appliances to close any residual spacing.
More rarely a buccally displaced canine tooth does not erupt or its eruption is so delayed that treatment for other aspects of the malocclusion is compromised. In these situations exposure of the impacted tooth may be indicated. To ensure an adequate width of attached gingiva either an apically repositioned or, preferably, a replaced flap should be used. In order to be able to apply traction to align the canine, either an attachment can be bonded or a band cemented to the tooth at the time of surgery. A gold chain or a stainless steel ligature can be attached to the bond or band and used to apply traction.
14.7 MANAGEMENT OF PALATAL DISPLACEMENT
14.7.1. Factors affecting treatment decision
Patient’s opinion of appearance and motivation towards orthodontic treatment.
Presence of spacing/crowding.
Position of displaced canine: is it within range of orthodontic alignment?
Condition of retained deciduous canine, if present.
Condition of adjacent teeth.
14.7.2. Treatment options
Surgical removal of canine
This option can be considered under the following conditions:
The retained deciduous canine has an acceptable appearance and the patient is happy with the aesthetics and/or reluctant to embark on more complicated treatment (Fig. 14.8). The clinician must ensure that the patient understands that the primary canine will be lost eventually and a prosthetic replacement required. However, if the occlusion is unfavourable, for example a deep and increased overbite is present, this may affect the feasibility of bridgework later, necessitating the exploration of alternative options.
The upper arch is very crowded and the upper first premolar is adjacent to the upper lateral incisor. Provided that the first premolar is not mesiopalatally rotated, the aesthetic result can be acceptable (Fig. 14.9).
The canine is severely displaced. Depending upon the presence of crowding and the patient’s wishes, either any residual spacing can be closed by forward movement of the upper buccal segments with fixed appliances, or a prosthetic replacement can be considered.
If space closure is not planned, it may be preferable to keep the unerupted canine under biannual radiographic observation until the fate of the third molars is decided. However, if any pathology, for example resorption of adjacent teeth or cyst formation, intervenes, removal should be arranged as soon as possible.
Surgical exposure and orthodontic alignment
Indications are as follows:
favourable canine position
space available (or can be created).
Whether orthodontic alignment is feasible or not depends upon the three-dimensional position of the unerupted canine:
Height. The higher a canine is positioned relative to the occlusal plane the poorer is the prognosis. In addition, the access for surgical exposure will be more restricted. If the crown tip is at or above the apical third of the incisor roots, orthodontic alignment will be very difficult.P.145
Anteroposterior position. The nearer the canine crown is to the midline, the more difficult alignment will be. Most operators regard canines, which are more than halfway across the upper central incisor to be outside the limits of orthodontics.
Position of the apex. The further away the canine apex is from normal, the poorer is the prognosis for successful alignment. If it is distal to the second premolar, other options should be considered.
Inclination. The smaller the angle with the occlusal plane the greater is the need for traction.
If these factors are favourable, the usual sequence of treatment is as follows:
Make space available (although some operators are reluctant to embark on permanent extractions until after the tooth has been exposed and traction successfully started).
Allow the tooth to erupt for 2 to 3 months.
With deeply buried canines there is a danger that the gingivae may cover the tooth again. If this is likely to be a problem, either an attachment plus the means of traction (for example a wire ligature or gold chain) can be bonded to the tooth at the time of exposure or about 2 days after pack removal.
Traction can be applied using either a removable appliance (Fig. 14.10) or a fixed appliance (Fig. 14.11). To complete alignment a fixed appliance is necessary, as movement of the root apex buccally is required to complete positioning of the canine into a functional relationship with the lower arch.
Most orthodontists would agree that this option is best confined to those cases where there is no other alternative. If transplantation is attempted, it must be possible to remove the canine intact and there must be space available to accommodate the canine within the arch and occlusion. In some cases this will mean that some orthodontic treatment will be required prior to transplantation.
The main causes of failure of transplanted canines are replacement resorption and inflammatory resorption. Replacement resorption, or ankylosis, occurs when the root surface is damaged during the surgical procedure, and is promoted by rigid splinting of the transplanted tooth, which encourages healing by bony rather than fibrous union. Careful handling of the root surface and prevention of desiccation during surgery, followed by a method of splinting which allows functional movement of the canine during the immediate post-surgical phase, is now recommended. This can be achieved by use of an acid-etch composite splint for 1 to 2 weeks. Alternatively, a fixed appliance with a bracket on the canine can be employed, and is most suitable if space has to be created prior to transplantation.
Inflammatory resorption follows death of the pulpal tissues, and for this reason early pulp extirpation has been advocated by some authors.
Despite a better understanding of the factors leading to failure with transplantation, the long-term survival rates are not good in practice. The prognosis is improved if transplantation can be accomplished before root is 75 per cent formed. However, as this stage is reached around 12 years of age, early detection and planning is required to accomplish this.
Unerupted and impacted canines can cause resorption of adjacent lateral incisor roots and may sometimes progress to cause resorption of the central incisor. Studies have indicated that incisor resorption is more common in females than males. Also, if the angulation of an ectopic canine to the midline on a DPT is greater than 25° then the risk increases by 50 per cent.
Swift intervention is essential, as resorption often proceeds at a rapid rate. If it is discovered on radiographic examination, specialist advice should be sought quickly. Extraction of the canine may be necessary to halt the resorption. However, if the resorption is severe it may be wiser to extract the affected incisor(s), thus allowing the canine to erupt (Fig. 14.12).
Transposition is the term used to describe interchange in the position of two teeth. This anomaly is comparatively rare, but almost always affects the canine tooth. It affects the sexes equally and is more common in the maxilla. In the upper arch the canine and the first premolar are most commonly involved; however, transposition of the canine and lateral incisor is also seen (Fig. 14.13). In the mandible the canine and lateral incisor appear to be almost exclusively affected. The aetiology of this condition is not understood.
Management depends upon whether the transposition is complete (i.e. apical transposition is evident) or partial, the malocclusion, and the presence or absence of crowding. Possible treatment options include acceptance (particularly if transposition is complete), extraction of the most displaced tooth if the arch is crowded, or orthodontic alignment. In the last case, the relative positions of the root apices will be a major factor in deciding whether the affected teeth are corrected or aligned in their transposed arrangement.
PRINCIPAL SOURCES AND FURTHER READING
Becker, A., Smith, P., and Behar, R. (1981). The incidence of anomalous maxillary lateral incisors in relation to palatally-displaced cuspids. Angle Orthodontist, 51, 24–9.
The aetiology and management of displaced maxillary canines are considered in this very thorough paper.
Edmunds, D. H. and Beck, C. (1989). Root resorption in autotransplanted maxillary canine teeth. International Endodontic Journal, 22, 29–38.
The factors that lead to root resorption, and methods of reducing this sequela, are discussed.
Ericson, S. and Kurol, J. (1986). Longitudinal study and analysis of clinical supervision of maxillary canine eruption. Community Dentistry and Oral Epidemiology, 14, 172–6.
Ericson, S. and Kurol, J. (1988). Early treatment of palatally erupting maxillary canines by extraction of the primary canines. European Journal of Orthodontics, 10, 283–95.
The first scientific evaluation of the widely held belief that extraction of a deciduous canine could improve the position of a displaced successor was given in this important paper.
Gorlin, R. J., Cohen, M. M., and Levin, L. S. (1990). Syndromes of the head and neck (3rd edn). Oxford University Press., Oxford.
This excellent reference book includes, amongst a wealth of other information, data on the development and incidence of canine anomalies.
Jacoby, H. (1983). The etiology of maxillary canine impactions. American Journal of Orthodontics, 84, 125–32.
Evidence that leads the authors to conclude that palatal and buccal displacements have differing aetiologies is presented in this paper.
McSherry, P. F. (1998). The ectopic maxillary canine: A review. British Journal of Orthodontics, 25, 209–16.
Good review article in which the options for management of displaced canines are discussed.
Peck, S. M., Peck, L. and Kataja, M. (1994). The palatally displaced canine as a dental anomaly of genetic origin. Angle Orthodontist, 64, 249–56.