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Bulletin #7 January 2012

Opening space for the canine – it’s not as simple as it seems!

When a patient has a missing maxillary lateral incisor, we may decide to create space for the placement of an implant for the absent tooth. We will probably align and level the teeth in general and then use an expanded coil spring on the archwire to create the space between the central incisor and the canine, by sliding mechanics. But how many of us have done this and removed the orthodontic appliances, only to have the patient sent back to us by the prosthodontist or implantologist because the roots have not been adequately paralleled? While there is room for a good sized incisor crown, there is inadequate space between the roots of the adjacent teeth in the apical area to accommodate the implant.

With any of the brackets in common use today and regardless of their slot width, there is always a degree of tip that goes with the use of a compressed coil spring because we use an under-sized archwire to reduce friction and minimize binding. For this reason, many of us will complete the movement with a full thickness wire and/or will place the bracket at an “over-corrected” angle to compensate for this. Once the optimum outcome has been achieved, the active orthodontic treatment has been completed. The teeth adjacent to the space are all in ideal alignment, with uprighting, rotating, torque and crown height all achieved. Brackets are removed, retention instituted and the patient returned to the prosthodontist to complete the implant borne restoration. Quod erat demonstrandum.

This scenario is analogous to the treatment outcome that is required in relation to the resolution of a palatally impacted maxillary canine – although with a few subtle but important differences.

In the case of the impacted canine, there is rarely enough space in the immediate target location within the arch, either because the smaller deciduous canine is in interproximal contact with its immediate neighbours or because the lateral incisor and the first premolar have drifted into the space, following the extraction of the deciduous canine. Accordingly, our modus operandi dictates that orthodontic treatment should be planned on an overall and comprehensive basis, to treat the whole malocclusion, with alignment of the canine just one of the many items that need to be checked off in the list of treatment aims, in order to achieve the desired result¹. However, in this scenario alignment of the canine has high priority and needs to be addressed before most other items. To do this, an orthodontic appliance is placed, leveling and alignment achieved and space re-opened. There are few exceptions to this protocol, but the reader is referred to the September 2011 bulletin in this series to identify those cases where the protocol should not be followed and to understand the reasons for this.²

Surgical exposure of the canine and traction to its place may be started just as soon as space becomes available and all the other teeth in the dental arch can be used as the anchor base from which forces will be applied to the ectopic tooth – i.e. a heavy and rigid archwire is in place. This means that most other tasks and, particularly, finishing procedures must be subordinated to canine alignment.

Moving the lateral incisor mesially to close off any anterior spacing, including uprighting its root, is simple to perform and easy to recognize and control clinically. Not so the first premolar! A distally-directed force is applied to the bracket of the premolar, which is relatively free to move along the under-sized archwire. In so doing, the tooth acquires a degree of distal tip, the by-product of which is to provide a reverse tip of the two roots of the premolars mesially ……… directly into the path supposedly being prepared for the canine.

Fig. 1a. A panoramic view shows an impacted maxillary right canine displacing the lateral incisor root mesially. The canine is on the palatal side close to the line of the arch.

Fig. 1b. Leveling and alignment and space opening was achieved after an unusually long period of treatment. No account was taken of the adverse canine/lateral incisor relationship!

Fig. 1c. The relationship of the canine cusp tip to the incisor root has caused severe root resorption of the incisor, as seen on this periapical view.

Fig. 1d. The transaxial view from the CBCT shows the extent of the resorption.

Fig. 1e. Poor bracket placement and the compressed coil spring have generated a distal tip and mesio-buccal or bucco-distal rotation of the premolar, causing its roots to tip mesially and the palatal root to be rotated into direct contact with the neck of the unerupted canine.

Furthermore, since the force is applied to a bracket on the buccal side of the tooth and at a distance from the tooth’s centre of resistance, the force has a second vector which serves to rotate the buccal aspect of the tooth towards the distal interproximal area (a so-called mesio-buccal or bucco-distal rotation). This then causes the palatal root of the premolar to rotate more mesially, further impinging on the space intended for the aberrant canine (Fig. 1a-e).

Fig. 1f. Bracket position has been changed. The locating red guide jigs are used to re-site the bracket in a position which will effect distal uprighting and mesio-palatal rotation of the tooth, when engaged by a labial archwire. The lateral incisor was freed from its bracket.

Fig. 1g. The final alignment of the teeth. Labial root torque of the resorbed lateral incisor was considered inappropriate due to its severe resorption.

Fig. 1h. The final panoramic view taken at completion of the treatment shows the short lateral incisor root. Note also the blunting of the roots of the central incisors.

Over-correction of the tip and the rotation, together with excessive space opening are probably the best answers in these cases, in order to give the canine as wide a berth as possible and, with it, as much freedom of movement as may be needed to facilitate its resolution (Fig. 1f-h). Perhaps the most efficient manner in which to do this is to place an 0.022” Tip-Edge bracket on the premolar.³ Because of the unique design of this bracket, the slot widens to 0.028” when the tooth is in the tipped position, encouraging a freer and more rapid movement. Once the tooth is in its distal position and ligated there, a rigid and heavy archwire (0.022” round or 0.022” x 0.028” rectangular) may be substituted. Using this as a base arch, an uprighting spring, followed by a rotating spring may be used in quick succession to complete the over-correction. Uprighting the premolar with any edgewise bracket will inevitably cause the lateral incisor to extrude initially, before it self-corrects after the uprighting has been accomplished. Appropriate use of a Tip-Edge bracket avoids this unnecessary “round tripping”.³

So, at the end of this careful preparation, there will be a beautiful vertically-oriented space between the lateral incisor and the first premolar, whose width is about 9mms all the way up from the crowns to the apices of these teeth. Unfortunately, that is not all we have to look for before we can assure a clear passage for the resolution of the palatal canine. There are still 2 aspects that have not been considered.

Fig. 2a. the original panoramic view shows a high palatal canine, with an enlarged follicle surrounding the crown. No other anomaly is noticeable.

Fig. 2b, c. 3-D views from the CBCT images taken from the labial and palatal aspects show the palatal root to be curved mesially and, apparently, “tickling” the underside of the canine.

Fig. 2d. a transaxial cut crosses the canine cervical area (superiorly) and the apex of the premolar (inferiorly). The premolar apex is certainly a complicating factor in the resolution of the canine impaction and possibly its cause.

The first is in relation to the anatomy of the roots of the premolar. In the December 2011 bulletin in this series, a case was presented in which the palatal roots of the two first premolars were each shaped with a mesially-facing apical third which was certainly an impediment to canine correction, but probably also intimately involved in the aetiology of the impactions themselves¹. There were very many other complicating features attributed to that particular case, which made it highly exceptional. Nevertheless, a mesially curved root is not as rare as might be assumed. Unfortunately, it is a difficult diagnosis to make, since the root may not be easy to distinguish on a routine panoramic or periapical radiograph and, as such, it is often missed (Fig. 2). This is an anomaly that would be quickly recognized on a cone beam CT (Fig. 3). ⁴

Fig. 3a. a 3-D CBCT view of a more obvious case of abnormal root development that clearly interferes with the normal eruptive development of the canine. It is almost certainly a contributory factor to the impaction.

Fig. 3b. a transaxial cut of the same case to show the relationship between the roots of the canine and the premolar.

While we may have produced an adequate vertically-oriented space in the target location, the orientation of the long axis of the palatal canine is not always in the same plane. This is the second aspect that needs to be addressed. Most frequently, the apex of the canine is high in its appropriate site in the maxilla, with a mesial and palatal inclination and the crown palatal to the lateral or central incisor. If there is a direct and uninterrupted line between the crown and the canine location – a group 1 canine¹ - then all that is needed is for a wide arc swing of the crown to be described, with the fulcrum of the movement in the apical area, for successful alignment to be achieved. However, with a group 2 canine, the roots of the incisors impede this direct line, which means that a 2-stage indirect approach is necessary. The swing of the crown must initially be directed vertically downwards to bring about eruption on the palatal side of the alveolar ridges. From this vantage point, the tooth is offered a clear and unimpeded view of the canine location for direct traction to be applied.¹

There should be serious concern for those unusual cases in which the apex is found to be more distally placed, above the apices of the second premolar or first molar and, sometimes, displaced a little medially (palatally)(Fig. 1e). In these instances, the roots of the premolars will not permit the canine to be drawn to its place directly and only with difficulty by the 2-stage indirect method described above. Because apex position is notoriously difficult to diagnose from plane film radiographs alone, these cases will lack the necessary accurate positional diagnosis. Treatment failure² is likely unless the orthodontist can avail him or herself of the benefits of computerized tomography.⁴ On the other hand, case reports have been published in which the canine was drawn into place along a path above and between the buccal and palatal roots of the first premolar – an exercise that would need careful planning with the aid of a cone beam CT or an outsized helping of good luck!^{5, 6}

It should be clear from the above description that the policy of opening the space in the dental arch for resolution of the canine impaction needs to redefined with several factors in mind:-

1. Mesio-distal width of the space between the crowns of the adjacent teeth, in relation to the mesiodistal width of the impacted tooth
2. Mesio-distal width of the space along the entire length of the roots of the adjacent teeth and in relation to mesiodistal width of the impacted tooth
3. Elimination of adverse secondary rotation and tipping of the adjacent teeth
4. Anatomy of the premolar roots
5. 3-D orientation of the of long axis of the impacted tooth in relation to 3-D orientation of the prepared space

At the end of the treatment and as an integral item within the finishing procedures, any over-corrections that were made earlier should then be resolved either by re-siting the over-corrected edgewise/straight wire bracket or by placing a reverse-direction uprighting spring in the vertical slot of the same Tip-Edge bracket.

Exceptions to the rule

As was presented and discussed in the September 2011 issue of this series of bulletins, there are situations in which early space opening will compromise the position of the impacted tooth or generate/exacerbate existing resorption of the root of a neighbouring tooth (Fig. 1e). In those exceptional cases, the affected tooth should be exposed first and moved away from its initial location to resolve the impaction and to distance it from the immediate area of root resorption – upgrading it to item #1 on the treatment priority list. It is imperative to do this before any other treatment aims are embarked upon – and that includes deliberately delaying tooth alignment, leveling and space opening.⁷

References:

1. Becker A. The orthodontic treatment of impacted teeth. 3^rd edition. Oxford: Wiley Blackwell Publishers, to be published February 2012.
2. Becker A, Chaushu G, Chaushu A. An analysis of failure in the treatment of impacted maxillary canines. Am J Orthod Dentofacial Orthop. 2010;137:743-54.
3. Kesling PC. Dynamics of the Tip-Edge bracket. Am J Orthod Dentofacial Orthop 1989,96:16-25
4. Chaushu S, Chaushu G, Becker A. The role of digital volume tomography in the imaging of impacted teeth. World Journal of Orthodontics, 2004;5:120-132
5. Odegaard J. The treatment of a Class I malocclusion with two horizontally impacted maxillary canines. Am J Orthod Dentofacial Orthop. 1997,111:357-65.
6. Becker A. Comment about making outcome of treatment more predictable. Am J Orthod Dentofacial Orthop. 1997,112:17A-19A.
7. Becker A, Chaushu S. Long-term follow-up of severely resorbed maxillary incisors following resolution of etiologically-associated canine impaction. Am J Orthod Dentofacial Orthop 2005,127:650-654,