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The Bracket On The Adjacent Lateral Incisor

Published: July 2017

Bulletin #68 July 2017

The Bracket on the Adjacent Lateral Incisor

Why do many maxillary canines remain unerupted, well after their normal eruption time? For the most part, this is due to an abnormal eruption path, influenced or not by adjacent anatomy or pathology. It has been widely quoted that the normal eruption path of the canine begins at a location close to the inferior border of the orbit and the tooth travels 22mm before it is able to take up its erupted position in the dental arch.1, 2 As the result, these teeth come into close proximity with the lingual, labial, distal or mesial aspects of the root of the adjacent lateral incisor. When there is a bucco-lingual superimposition of the canine crown and the incisor root, a situation is created whereby the two teeth are occupying the full width of the narrow rim of alveolar bone which is broad enough to accommodate just one of them. Notwithstanding the fact that the presence of the two teeth generates more bone in the immediate area, there is still a bony deficit to accommodate the two teeth, side by side. One or both of these teeth become displaced from their normal position, often with an altered orientation of their long axes and will most likely be in direct physical contact with one another. 68.__Fig._1

Fig. 1. An occlusal view of a patient with 2 unerupted canines on the labial side. Note the labial crown inclination of the two lateral incisors. The canines in this case are located labial to the lateral incisors and lingual to the central incisors.

One of the first clues that this scenario is present can be seen at the initial clinical examination, in which we may suspect canine impaction because the orientation of the crown-to-root long axis of the adjacent lateral incisor indicates a markedly displaced root apex (Fig. 1). As orthodontists, we tend to recognize this in terms of the need to mesio-distally upright or bucco-lingually torque the tooth, to achieve ideal alignment. Since the reason for the abnormal orientation of the long axis of the lateral incisor is related to the presence of the impacted tooth in the same narrow portion of the alveolar rim, one should always seek to palpate the crown of the tooth under the mucosa.

Before attempting to achieve full alignment of the adjacent teeth, including root uprighting and torqueing, two considerations are crucial to the assessment of 3D location and orientation of the impacted canine:-

1. if the presence and location of the impacted canine will impede the attempt to fully align the adjacent incisor teeth. It should be remembered that we generally align and level the remaining dentition, including the lateral incisors, in order to use the entire arch as a composite anchor base in preparation for the disimpaction task. Many of us will approach this with a series of archwire changes, culminating in the placement of a fairly heavy rectangular archwire. If, during its corrective root movement (uprighting and torqueing) the incisor root becomes snagged up against the crown of a displaced canine, this could become a very potent recipe for incisor root resorption!

2. conversely, one must determine whether this pre-surgical orthodontic alignment of the roots of the incisors will compromise the already displaced canine still further, possibly rendering it untreatable.

THE BUCCO-LINGUAL PLANE:

There are several ways to circumvent this potential hazard. “Customization” is the operative word to be used in this context and it requires a little understanding and common sense on the part of the orthodontist. 68.__Fig._2a__b

Fig. 2a, b. The anterior section of the lateral skull radiograph showing the superimposition of the 6 anterior teeth. The yellow broken line indicates the long axis of the central incisors, the blue broken line is drawn through the long axis of the lateral incisors and the red broken line shows the superimposed unerupted canines. The divergent V-shaped long axes of the central and lateral incisors form the “window of opportunity” through which the canine should find a way to resolve its impaction.


When an unerupted canine is located labial to the root of the lateral incisor, that root will almost certainly be displaced palatally and an orthodontist surveying the scene will conclude that the tooth requires to be labially torqued (Fig. 2a, b). This would obviously cause an intra-bony clash between the root and the impacted canine which would, in turn, nullify the effect of the mechanics. But, in the longer term, it would cause resorption of the incisor root. Therefore, the first step must be to avoid the use of rectangular archwires in the initial leveling and aligning stages, since these will automatically produce root torque. Employing only round archwires will eliminate the possibility of torqueing the roots in the bucco-lingual plane.

Alternatively, it is sometimes advantageous to leave the lateral incisor without a bracket until the canine has been safely distanced from the area. However, this has the disadvantage of leaving a long span of unsupported archwire between the central incisor and the first premolar. This is particularly relevant when the orientation of the long axis of the central incisor is also displaced, which renders the method inefficient. A better option is to pass a fine diamond disk through the horizontal slot of an edgewise bracket placed on this tooth, in order to eliminate its edgewise slot configuration. In this way, most of the bucco-lingual torqueing movement of the other teeth may still be realized and the heavy slot-filling archwire may still be used to resist unwanted movement of the anchor teeth.

Obviously, the deciduous canine will be scheduled for extraction, as part of the impaction resolution plan. However, for as long as the permanent canine is not imminently in need of the space, because of its physical distance from its deciduous predecessor, placement of a bracket on the deciduous canine in the interim can positively assist in stabilizing the archform and permitting effective activation.

THE MESIO-DISTAL PLANE:

68.__Fig._3

Fig. 3. A panoramic view of another patient whose unerupted right maxillary canine is mesially impacted against the apical area of th e lateral incisor, accounting for the mesial movement of the apex and the distal flaring of the crown of the incisor.

If the unerupted canine is located high on the distal side of the root of the incisor, it generally displaces the apex mesially, in close proximity with the central incisor root (Fig. 3). The danger here is in relation to the effect of an initial aligning wire tied into a “ideally” orientated bracket. The very first orthodontic aligning wire will attempt to upright the root apex and redirect the long axis of the tooth distally, as the first stage of the leveling and alignment process. This will drive the root into the mesially angulated canine crown, which will obviously resist. It is possible that this strategy may work in some cases, if the canine crown is not “square” on the incisor root but, if it works, it will generally occur quite rapidly, in as little as 2 or 3 months. If, therefore, the uprighting movement is slow or ineffective, there must be something preventing the movement – namely the canine crown. Persistent, stubborn and blind dedication to the task will bring about severe resorption of the root (Fig. 4).

68.__Fig.__4

Fig. 4. A periapical view of the same case after an unusually (!) long period of leveling and alignment has caused severe resorption of the root of the lateral incisor.

Once again, one may think in terms of leaving the lateral incisor without a bracket, which carries with it the same limitations described in relation to the bucco-lingual plane and root torque. Alternatively, it may be worth considering the use of an attachment that does not correct mesiodistal root tipping, such as the simple vertically-oriented eyelet, a Begg bracket or a Tip-Edge bracket. None of these attachments will upright the root - the Begg bracket requires an auxiliary uprighting spring and the Tip-Edge bracket require a NiTi wire in its deep slot, but each can generate a rotational moment, which is usually advantageous in these circumstances.

Another alternative is partial ligation with a steel ligature, which provides support without activity. However, partial ligation is not possible with most of the self-ligating bracket systems, which is a considerable drawback in cases where one or two teeth are impacted. Yet another approach is to initially align the Edgewise bracket on the incisor with its horizontal slot bonded parallel to the occlusal plane, rather than perpendicular to the long axis of the tooth.

Once the canine has been relocated out of harm’s way, the “customized bracket/eyelet” must be removed and substituted by the specific lateral incisor bracket of the type used on the other teeth and in its appropriate location, in order to achieve its optimal alignment and a harmonious relationship with the other teeth.

As has been described above, an unerupted canine that lies labial to the root of the erupted lateral incisor will displace that root palatally. It may be seen radiographically and clinically where it may be palpable under the palatal mucosa. In these cases, a side view (as seen on a lateral cephalogram) of the long axes of the central and lateral incisors will often reveal a widely diverging V-shaped orientation of their long axes, as they proceed apically. If the canine has a fairly active eruption potential, its further mesial migration may bring it on the lingual side of the central incisor, while its root remains labial to the lateral incisor. In this divergent V-shaped incisor root arrangement, resolving the canine impaction requires surgical access from the labial side and orthodontic traction also applied from the labial side, between the incisor roots. Thus, any pre-surgical orthodontic alignment which includes the use of rectangular archwires to labially root torque the lateral incisor will likely ring the death knell for the canine, by closing off its “window of opportunity”. A successfully treated case of this type is described and illustrated in bulletin #16, November 2012, in the present series.

References

1. Coulter J, Richardson A. Normal eruption of the maxillary canine quantified in three dimensions. Eur J Orthod. 1997;19:171-83.

2. McSherry P, Richardson A. Ectopic eruption of the maxillary canine quantified in three dimensions on cephalometric radiographs between the ages of 5 and 15 years. Eur J Orthod. 1999;21:41-8.