When we try to solve the incisor torque or inclination, it is common to see how the planned movement does not match the clinical reality. How many of us have seen aligners mismatch when trying to correct the negative torque of the anterior teeth? Using power ridges, vestibular attachments, palatal attachments... Different tools, but the same result: mid-treatment rescanning to order new aligners.
Fortunately, we can learn from past mistakes. The way we move teeth is the same regardless of the appliance we use for it. That's the reason why planning the optimal force system for each type of movement is what will give us the key to achieve a satisfactory result.
Nowadays, the P.I.R. protocol (proinclination, intrusion and retrusion) is the most extended method to achieve a change of inclination in the incisors with aligners treatment. If we review the literature, we can see that disparate results are obtained in terms of the predictability of the inclination movement of the incisors. Many of these studies do not specify the way in which the movements have been planned, something that has been taken into account in the article by Jiang et al.
In this study, the authors differentiated between 4 different movements and analyzed the efficacy of each movement with Invisalign aligners. The predictability of each movement, ordered from highest to lowest, is as follows:
- Pure inclination: 72.48%.
- Controlled inclination: 65.24%.
- Translation: 49.50%.
- Torque (root movement): 35.21%.
As shown in the images, a different axis of rotation has been used for each of these movements. The picture on the left, where the axis of rotation is closer to the center of resistance of the tooth, represents the case of pure inclination. This is the most predictable movement of all, followed by controlled inclination. Both movements are more efficient than translation and torque. Coincidence? Surely not. The results of the study confirm the importance of the way in which movements are planned in their performance.
If we transfer this to daily practice, we can conclude that, when planning a change of inclination of the incisors by means of lingual root movement, using the incisal edge of the tooth as the axis of rotation, the predictability of the movement will be very low, which will increase the risk of aligner mismatch during the treatment. Otherwise, if we plan a pure crown inclination or controlled inclination movement, we will achieve a more predictable inclination of the incisors.
Finally , it is obvious that the type of appliance we use in our orthodontic treatments can affect their evolution and final result, but it is up to us to understand the characteristics, limitations and virtues of these appliances in order to get the most out of them and achieve excellent results.
Jiang T et al. A cone-beam computed tomographic study evaluating the efficacy of incisor movement with clear aligners: Assessment of incisor pure tipping, controlled tipping, translation, and torque. Am J Orthod Dentofacial Orthop 2021;159:635-43