The exchange of position of two adjacent teeth constitutes a transposition. Maxillary canines are rarely affected; transpositions of upper canines occur in only 0.2% of the orthodontic population. They can be detected early in radiographic exam- inations and later, clinically, after they begin to erupt. Most writers affirm that transpositions, which are often associated with other dental anomalies, are caused primarily by some genetic defect but that local factors may contribute to their development.
Practitioners should consider their muco-gingiva environment as an impor- tant factor in their evaluation of the iatrogenic risks that might accompany treatment and consider improving soft tissue status as a preparatory soft tissue therapeutic pro- cedure. They must also differentiate between cases of incomplete transposition and partial complete transposition, which can almost always be successfully treated, from cases of total complete transposition that are relatively difficult if not impossible to treat.
INTRODUCTION
Transpositions of maxillary canines, which occur quite infrequently, can be detected before they erupt on rou- tine X-Ray examination, notably on panoramic films, or found clinically after they do erupt. (fig. 1).
Even though it is rare, the phenomenon is far from a recent development. Nelson45 in 1992 and Lukacs37 in 1998 showed transpositions on crania that were more than 5000 years old (fig. 2).
As early as 1817, a French dentist named Miel reported a case of transposition.
To begin with, we shall define what is meant by transposition and then we shall discuss how frequently it occurs, its association with other anomalies, and, then, the consequences it has for periodontal status before we describe and evaluate the orthodontic means that are used to deal with it.
DEFINITION
A transposition is usually defined as a rare dental phenomenon in which two adjacent teeth in the arch exchange places (fig. 3). S. and L. Peck, in 1995, proposed a “common” definition, asserting that a transposition was an exchange of posi- tion of two adjacent teeth, especially of their roots. It could also be the eruption of a tooth into a position usually occupied by a non-adjacent tooth.
But in extreme cases of ectopic eruption of permanent teeth that change the natural arch positioning, the term employed is transmigration.
• Peck’s classification, 1995 (fig. 4) The transposition most frequently encountered is between the canine and the first premolar, 71% of the time, and canine with lateral incisor, 20% of the time. Occasionally bilat- eral, these transpositions are usually unilateral and occur more frequently in females than in males.
Only Plunkett disagrees with this genetic hypothesis. Other possible causes of transposition that have been advanced include:
– The failure of the root of a temporary canine to resorb, causing the permanent tooth bud to move to another position.
– A mechanical deflection if the eruptive path is disturbed.
– The loss of adjacent teeth.
– The presence of an odontoma.
– The presence of a cystic lesion.
These widely varying conjectures reinforce the consensus that the origin of transpositions is primarily genetic but that local factors may con- tribute to their development.
TRANSPOSTIONS AND ANOMALIES ASSOCIATED WITH THEM
Transpositions of maxillary canines are frequently associated with other dental anomalies:
– congenitally absent teeth, particularly upper lateral incisors in 37 to 48% of cases (fig. 5);
– reductions in size of adjacent teeth, in the form of dwarfed or peg- shaped laterals in 10 to 25% of the cases (fig. 6);
– it would seem in addition, from clinical observation, that in cases of dental anomalies the risk of root resorption is great, but no statistical evidence to prove this has as yet been shown in the literature. Clinicians, however, should be aware of this pos- sibility and guard against it.
THE PERIODONTAL ENVIRONMENT OF TRANSPOSITIONS
The environment and nature of the muco-gingival tissues investing the teeth are factors upon which long- term periodontal health depends. Korbendau and Guyomard in 198030 showed clinical examples of kera- tinised gingiva following teeth in their movements. Using a multi-disciplinary approach to orthodontic treatment, surgical technique and reinforcement of periodontal health should be of concern in all stages of mechano- therapy.
During orthodontic treatment peri- odontal tissue is routinely subjected to stress that can be iatrogenically harmful especially if that tissue is fine in texture, type II or IV according to the Maynard and Wilson40 classifica- tion. Inattention to periodontal health can lead to gingival recession or even chronic inflammation. These harmful phenomena are often intensified by deposits of biofilm resulting from the difficulty in controlling plaque around the orthodontic appliance.
So orthodontic therapists should plan on muco-gingival care from the outset of therapy with the goal of restoring a thickened keratinised gin- giva over thin cortical plates or, even more important, of areas of dehis- cence to encourage osseous recon- struction throughout the course of orthodontic therapy.
During the initial phase
Depending on the quality of the soft tissue investing the canine, practitioners should select from a variety of possibilities, the most appropriate way to deal with it.
– if they determine that an orthodontic force vector is the iatro- genic cause of the difficulty they might be able to adjust that force and wait until the close of mechano- therapy and treat the soft tissue prob- lem under optimal conditons34;
Orthodontists can almost always treat these incomplete transposi- tions, which are usually between maxillary laterals and canine teeth, successfully because while their crowns are transposed to varying extents their roots usually remain in their proper sites.
Mechano-therapy, which always employs light forces, can be of two types, one leaving a relative liberty to the affected teeth to slide back into their correct positions and the other to exert more direct “controlling” force in order to guide the movement of the transposed tooth as perfectly as possible (fig. 7).
The incomplete transposition, fol- lowing trauma, of a maxillary canine whose crown erupted in the position of the central incisor but whose root remained effectively in the canine site is a special case58. After evaluating the problem with a set-up, the orthodon- tist was emboldened to make the unusual decision that the best course of action in this case was to replace the central incisor lost in a traumatic accident with the canine, and accept an end of treatment asymmetrical molar occlusion, class II on the left, Class I on the right, which would require careful equilibration. Of course, the canine would have to be modified cosmetically, during and after orthodontic treatment, to allow it to play its role as a central incisor successfully. (fig. 8).
Complete maxillary trans- positions
It is important to differentiate between cases of partial complete transposition where the transposed tooth remains outside the curvature of the arch that can be treated orthodontically and cases of total complete transposition where the tooth lies within the curvature of the arch and is virtually impossible to treat orthodontically.
Complete total transpositions
These complete total transposi- tions are almost always between canines and first premolars. Every writer dealing with this subject has proposed that a transposed canine sometimes be left in place for multiple reasons, the diffi- culty of moving it and the risks of root resorption, of tissue damage, and loss of pulpal vitality that such movement might entail.
A canine tooth left in its trans- posed position is aesthetically acceptable especially if it is modi- fied cosmetically to resemble a premolar if it is reshaped to adapt to the occlusion in that position. Most transposed teeth dealt with in this way pose no aesthetic or functional problems but do work best, after appropriate equilibra- tion, as participants in group func- tion. (fig. 9).
Partial complete transpositions
– In cases of complete transposition of a canine and a premolar if the treatment plan calls for the extrac- tion of the four first premolars, obvi- ously the treatment can conclude with all the canines in their proper position and the occlusion in a proper Class I relationship (fig. 10). 
– In cases of partial complete transposition of a canine and a premolar accompanied by either con- genitally absent laterals or peg-shaped laterals that cannot be conserved, it is possible to treat the problem orthodontically, closing spaces, and moving the transposed canine into the lateral position thus avoiding putting a premolar in contact with a central incisor, which would have been both unaesthetic and non functional.
This solution, even though it is dif- ficult and not without considerable inconveniences, would seem to be preferable. But the risks are considerable, including:
– root resorptionso frequent that some authors assert that there are relationships between trans- positions and dental anomalies as well as root resorption even if there is not yet any statistical confirmation of this opinion;
– loss of pulpal vitality;
– weakening of supporting soft tissues.
Despite all this, it is still possible to correct the transposition orthodontically by:
– intruding the canine;
– torqueing, or moving the premo- lar bodily toward the palate, using, Langlade suggests, a quad helix;
– then moving the canine mesially.
The patient depicted in figure 11 had a Class III malocclusion with con- genitally absent maxillary incisors and an upper left canine and first premolar that were transposed. She was treated with a surgical-orthodontic protocol that, orthodontically, was accompanied by the undesirable side effects of root resorption and peri- odontal and occlusal difficulties asso- ciated with the closure of the lateral spaces.
So early detection of such a part complete transposition when the canine is still in a high position is desirable because, with early treat- ment these problems can be averted, especially if the patient has no con- genitally absent teeth. The orthodon- tic technique for this early treatment would keep the canine in as high a position as possible (fig. 12).
CONCLUSION
The transposition of a maxillary canine is a rarely occurring phenomenon that can be accompanied by other dental anomalies such as con- genitally absent or dwarfed lateral incisors.
Transposition can be incomplete, totally complete, or partially complete. Complete and even partially complete transpositions are difficult, and risky, to treat. But when the anomaly is discov- ered early, treatment prospects improve, especially if the orthodontist can begin to move the unerupted transposed tooth when it is still in a high position. But when the tooth is already erupted into the transposed position the better course of action may be simply to leave it between the two premolars and not attempt any orthodontic therapy for it.
However, thanks to advances in periodontal techniques and in orthodontic mechano-therapy with the improved anchorage of mini-screws and mini-plates, orthodontists may be able to treat almost all transposed teeth when appropriate. But, treatment of these conditions is still complicated and accompanied by multiple risks. Orthodontists, accordingly, should always make a careful risk/benefit eval- uation before embarking on the treat- ment of a transposed tooth. Not infrequently this assessment will sug- gest that mechano-therapy be avoided.
Để truy cập toàn bộ bài viết, xin vui lòng xem thêm tại đây