When we try to correct the torque or inclination of the incisorsHow many of us have seen aligners misaligned when trying to correct the negative torque of the anterior teeth? With power ridges, with vestibular attachments, with palatal attachments... Different tools, but the same result: having to re-scan in the middle of the treatment to order new aligners.
Fortunately, we can learn of the mistakes of the past. The way we move teeth is the same, regardless of the appliance we use to do so. For this reason, the optimal system of forces for each type of movement is what will give us the key to achieving a satisfactory result.
As of today, the protocol P.I.R. (proinclination, intrusion and retrusion) is the most widespread method of achieving a change in incisor inclination with aligner treatment. If we turn to 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 how the movements have been planned, something that has been taken into account in the article by Jiang et al.
In this study, the authors have differentiated between 4 different movements and analysed the effectiveness of each of these movements with the Invisalign aligners. The predictability of each of them, in order from highest to lowest, is as follows:
- Pure tilt: 72,48%
- Controlled inclination: 65,24%
- Translation: 49,50%
- Torque (root movement): 35,21%
As can be seen in the images, each of these movements has used a axis of rotation different. The drawing on the left, where the axis of rotation is closer to the centre of resistance of the tooth, represents the case of pure tilt. This is the most predictable movement of all, followed by controlled tilt. Both movements are more efficient than translation and torque. Coincidence? Probably not. The results of the study confirm the importance of the way in which movements are planned in their achievement.
If we translate this into practice, we can deduce that, when planning a change of inclination of the incisors by radiculolingual movement, using the incisal edge of the teeth as the axis of rotation, the predictability of the movement will be very low, which will increase the risk of misalignment of the aligners during treatment. If, on the other hand, we plan a pure coronal tilt or controlled tilt movementIf the incisor inclination is more predictable, we will be able to achieve a more predictable inclination of the incisors.
In conclusionIt is clear that the type of appliances we use in our orthodontic treatments can affect their evolution and final result, but it is in our hands to understand the functioning, limitations and virtues of these appliances in order to get the most out of them and achieve an excellent finish.
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