Which is the best protocol for rotated canines?

Who hasn't had to scan a patient with rotated canines due to an aligner mismatch? Rotation of round/conical teeth is a movement that can be challenging to perform with aligners. The poorly retentive anatomy of the teeth means that the force delivered by the aligners may not be as effective as we would like.

To help achieve this movement, attachments are an essential tool, but they are not the only factor that affects the predictability of the rotational movement of the canines. The type of rotation that is planned, whether it is a rotation made on the long axis of the tooth or a hinge rotation, the degrees of canine rotation or the wear protocol (1-2 weeks) can affect the final result and the predictability of the rotation. The differences between the bone of the upper and lower jaw should also be taken into account. The lower density of the upper bone compared to the lower jaw bone makes lower canine rotations more difficult to achieve.

There are several studies about the capability of aligners to rotate these teeth. It is interesting to see how from the early studies by Kravitz et al. in 2008-2009 to more recent studies, such as that of Stephens et al, the predictability of canine rotation with Invisalign has improved dramatically. While in 2008-2009 we had a predictability percentage of around 30% in lower canines, in the study by Stephens et al. they observed an average predictability of 75%. Despite this, there are also recent studies that have found a lower predictability than that found in this study.

The introduction of the SmartTrack material together with the appearance of optimized attachments for each type of movement according to the anatomy of each tooth has been an important factor in the improvement of rotation predictability. Even so, these innovations have not achieved 100% predictability, something that is practically impossible to achieve with aligners, regardless of the movement performed.

As expected, since we have a certain percentage of error, we know that the greater the rotation, the greater the risk of misalignment. What can we do to improve predictability?

• Proper planning of the rotation sequence . When there is a simple rotation, less than 15-20°, it will generally not be necessary to ask for sequences and we can perform the rotation in a conventional way, on its long axis. If there is a rotation greater than 15-20°, we must consider sequencing the rotation with hinge movements. The hinged movements allow us to increase the moment of force applied, improving the efficacy of the aligners.
• Ensure that there is space to accomplish the movements. Any heavy contact that appears between the canine and the adjacent teeth or with the teeth of the upper arch can affect the correction of the rotation. Therefore, it is recommended to relieve heavy interproximal contact points with IPR strips. This will reduce the frictional force between the teeth and facilitate movement of the canine.
• Use appropriate attachments for rotation. If you use Invisalign, try to use an optimized attachment whenever possible, because it has been proven to work better than conventional ones. If you use any other brand of aligners, you should use a beveled vertical attachment so that the aligner generates the appropriate forces to correct the rotation of the canine.
• Add an extra 5-6° of rotation as an overcorrection. This extra 5-6° can be added during or at the end of treatment. Studies recommend adding them at the end. By adding them during treatment, the misalignment may appear at earlier stages. If we leave them for the end of treatment, we will decide whether they are necessary or not depending on how the canine rotation has been corrected. In some patients, canine rotations are satisfactorily corrected without the need for additional aligners.

To conclude, it should be noted that these authors have not found significant differences in the predictability of rotation by changing weekly or every 14 days. One possible reason for this finding may be the fact that in case of rotation there is no significant root displacement through the bone (as there can be in a case of translation movement), which is what most slows down the movement. As there is less bone remodeling, the change every seven days is sufficient to correct the rotation.

One movement, multiple factors to analyze. And, in turn, these factors can be extrapolated to many other movements, which in one way or another will help us to better understand how aligners work. One thing is clear: although they have their limitations, over the years these limitations are being reduced and the possibilities of invisible orthodontics are becoming greater and greater.

Stephens C. Et al. Clinical expression of programmed mandibular canine rotation using various attachment protocols and 1- vs 2-week wear protocols with Invisalign SmartTrack aligners: A retrospective cohort study. Am J Orthod Dentofacial Orthop 2022;162:e103-e115.

Kravitz ND et al. How well does Invisalign work? A prospective clinical study evaluating the efficacy of tooth movement with Invisalign. Am J Orthod Dentofacial Orthop 2009;135:27-35.

Kravitz ND et al. Influence of attachments and interproximal reduction on the accuracy of canine rotation with Invisalign. A prospective clinical study. Angle Orthod 2008; 78:682-7.