Orthodontic Conundrums Part 1: Transposition – an Update on Presentation and Treatment Modalities

Background and presentation

This is the most common form of transposition and the most reported form in the literature.^1,5 In these cases, the transposed maxillary canine is most often excluded from the arch and positioned buccal to the first and second premolars (Figures 2 and 3). The canine is also often rotated mesiobuccally, and the first premolar rotated mesiopalatally and tipped distally. Transient crowding is a common finding as the primary canine is frequently retained (Figures 1 and 2).¹

Unlike other types of maxillary transposition, local factors, such as a history of dental trauma, appear to play less of a role in Mx.C.P1 transpositions.^1,10,11

Treatment

In cases of Mx.C.P1 transposition, interceptive extraction of a retained primary canine may allow spontaneous correction, particularly if the roots of the first premolar are distally angulated.⁹ Although retention of primary teeth may result in transient crowding, the simultaneous presence of true crowding and transposition is uncommon as transposition is usually associated with microdontia and hypodontia (Figure 4).⁵ The presence of crowding or the need to correct an increased overjet, however, may allow for the correction of transposition through extraction of the first premolars.

Individuals with maxillary transpositions are more likely to exhibit dental abnormalities, both in general and on the same side (Figure 5).^5,6

Where Mx.C.P1 transposition is accompanied with absent lateral incisors, there are two possible approaches, depending on the space available in the arch. In a spaced arch, the transposition may be accepted with a view to redistribute spaces and plan for long-term prosthetic replacement in the lateral incisor position. Attempting closure of the space requires mesializing the premolar into the lateral incisor space. This potentially risks placing the buccal root of the maxillary first premolar outside of the envelope of bone and may result in an alveolar dehiscence. In addition, the first premolar provides a poor aesthetic substitute for a lateral incisor, owing to its bulbosity and emergence profile. Given these issues, extraction of the premolar and opening of the lateral incisor space for a prosthesis may be a more suitable approach.¹²

In cases of Mx.C.P1 transposition, where extraction is not indicated, the transposed positions may be accepted. Figure 6 illustrates the end result of treatment of the case in Figure 2 where transposition was not corrected. In such scenarios, the appearance can be improved with careful grinding of teeth or the addition of composite material. The upper first premolar makes an acceptable substitute for the canine when considering a canine guidance, with mesiopalatal rotation of the premolar and reduction of its palatal cusp facilitating smooth guidance on lateral excursion.⁹

Compared to accepting Mx.C.P1 transposition, full correction can require complex mechanics. Some authors advocate initial palatal movement of the displaced premolar, to facilitate correction of the canine from its ectopic starting point to a normal position.¹³ (Figures 7 and 8).

Once the canine is repositioned, the premolar may then be brought into alignment. Orthodontic correction can prolong overall treatment time, but the potential for an optimal aesthetic and functional outcome may be greater.^13,14 In the case presented, there has been some compromise in the gingival health, although this gives rise to little appreciable aesthetic concern as the patient does not have a high smile line (Figure 9).

Background and presentation

This is the second most common form of transposition, comprising 20% of the total number of maxillary transpositions.¹ Incomplete transposition is more common than complete transposition in these cases, and the characteristic features of Mx.C.I2 transposition are as follows:⁶

Local factors, such as a history of dental trauma, appear to play more of a role in Mx.C.I2 than in other maxillary transpositions. These cases frequently demonstrate the sequelae of early dental trauma, such as dilacerated or shortened roots.^1,10,11

Treatment

Treatment should consider the prognosis of the affected teeth and the nature of the malocclusion as a whole. If the central incisor has significant root resorption, from previous trauma or the eruptive path of the ectopic canine, it may be extracted and the canine moved into the central incisor position.¹⁵ This approach would obviously necessitate restorative camouflage of the canine to deliver acceptable aesthetics.

Where the central incisor is unaffected, the principle problems in Mx.C.I2 transposition arise from the ability of the lateral incisor to function as a canine, and the capacity for the canine and lateral incisor to be camouflaged as one another⁹

The relatively small root of the lateral incisor makes it a poor substitute for a canine in canine-guided occlusion. In such instances, to aim for group function may be wise. Should the appearance of the canine be unsuitable for camouflage, or if the lateral incisor demonstrates significant root resorption, consideration can be given to extraction of the lateral incisor and movement of the canine into its normal position, creating space for prosthetic replacement of the lateral incisor.

Where the transposition is accepted and the canine is camouflaged in the lateral incisor position, enameloplasty and the addition of composite can be used to optimize the aesthetic result.¹⁶

As with Mx.C.P1 transposition, orthodontic correction and full resolution of the transposition often results in prolonged treatment, with the additional increased risk of root resorption should root interferences arise.^11,17 Attempts at orthodontic correction have the potential to move the root of the canine out of alveolar bone, as it is distalized labial to the lateral incisor, resulting in the appearance of a long clinical crown.¹⁸

Background and presentation

This is an extremely rare condition with very few cases being reported in the literature.¹ In these instances, the maxillary canine occupies the position of the maxillary first permanent molar, after the molar's earlier loss. The literature on Mx.C.M1 transposition is scant, and a search found no reported cases in the last 50 years. As this form of transposition appears to require premature loss of first permanent molars, it may be reasonable to conclude that a global reduction in caries experience, through effective fluoride exposure, has led to a reduction in incidence of Mx.C.M1 transpositions.¹

Treatment

It would seem prudent to accept the transposition in such cases as moving the canine to its normal position would likely be extremely difficult and fraught with risk. The one case report concerning the treatment of Mx.C.M1 described extraction of the transposed canine.¹⁹

Background and presentation

This form of transposition is unique in that it is the only type, reported on multiple occasions, where there is no canine involvement. These cases are typically preceded by dento-alveolar trauma, resulting in changes to tooth form or orientation, such as dilacerations or rotations.¹

Treatment

The lower and narrower gingival contour of the lateral incisor presents the main challenge when managing this type of transposition. The transposed positions may be accepted and aesthetics improved with restorative treatment, but an interdental ‘black triangle’ may persist at the gingival aspect. An alternative approach would be to extract the lateral incisor, move the transposed central incisor into its correct position and provide prosthetic replacement of the lateral incisor.⁹

Background and presentation

This is another rare form of transposition. In a similar manner to Mx.C.M1 transposition, it appears to be the result of aberrant canine migration following early loss of the central incisor. There has been little reported on the treatment of this variant.¹

Background and presentation

In rare instances, an unerupted mandibular canine may migrate intra-osseously and cross the symphyseal midline (Figure 10). This phenomenon appears to be unique to the mandible.²⁰ Authors disagree on what constitutes transmigration, with some accepting it to have occurred when only the cusp tip has crossed the midline, and others requiring the canine to have crossed over by at least half its length.^20,21,22,23

Treatment

Treatment options for transmigrated mandibular canines include: surgical removal, transplantation, exposure and orthodontic alignment, and observation.²² Reports in the literature find surgical removal to be the most commonly performed treatment for transmigrated canines. The teeth are often in unfavourable positions, frequently orientated horizontally, and consequently orthodontic movement is difficult, complex, and in some instances mechanically impossible.^20,21,22,23

Background and presentation

Mandibular transpositions occur less frequently, and with fewer permutations, than those in the maxilla.^2,7 In Mn.I2.C transpositions, the canine usually erupts in a relatively normal position, but distal migration of the lateral incisor is principally responsible for the positional interchange.

Mn.I2.C transposition frequently presents with distal tipping, rotation and displacement of the lateral incisor as it erupts into the position normally occupied by the ipsilateral canine and premolar (Figure 11). When the mandibular canine eventually erupts, it is typically positioned mesial to the ectopic lateral incisor, resulting in a transposition.⁷

The early mixed dentition phase, between ages of 6 to 8, is thought to be the optimal time to assess the development and path of eruption of the mandibular lateral incisors. Radiographic investigation is likely to prove useful if an aberrant eruptive pathway or ectopic eruption are suspected.²⁴ At the early stage of this form of transposition, approximately age 9, the mandibular lateral incisor begins to tip distally, and its crown rotates mesiolingually. The canine can be seen to be developing mesial to the lateral incisor radiographically. The lateral incisor and canine crowns appear transposed, but the roots occupy their normal positions. The ectopic lateral incisor also frequently undermines the primary first molar, precipitating its premature exfoliation.⁷ The later stage of Mn.I2.C transposition, approximately age 12, can result in the roots of the lateral and canine fully transposed and upright, to give a mature complete transposition.⁷

Treatment

As before, treatment options centre on either aligning the teeth in their transposed positions, extraction or attempted orthodontic correction. The choice of which approach to take is typically based on when the anomaly is detected.²⁵

Should the transposition be detected early, the primary canine and lateral incisor should be removed. Thereafter, fixed appliances may be used to correct the rotation of the lateral incisors, upright them and move them mesially to occupy the space adjacent to the central incisors (Figure 12). However, one should be mindful of the position of the developing canine, as attempting mesial movement of the transposed lateral when the canine lies in its path may precipitate root resorption.

Once the lower incisors occupy their normal positions, space may be maintained to allow the mandibular canine and first premolar to erupt into the appropriate sequence.

Rarely is Mn.I2.C transposition a true transposition (Figure 13) but in such a case management would involve accepting the tooth sequence.