Extraction cases with aligners are considered one of the most complex cases to treat with aligners due to the mechanical properties of the plastic. When we treat an extraction case with brackets, we have the possibility to choose the stiffness and size of the archwire to achieve greater control of en-masse movements of the teeth during space closure. However, in treatments with clear aligners this approach is not possible.
To overcome these limitations, aligner companies have developed a number of different solutions. For example, Invisalign, despite using the same type of plastic, introduces additional forces to improve root control of the teeth. Others, such as Orthocaps or Angel Align, combine aligners of different stiffness throughout the treatment to achieve better movement control. The latter work in a very interesting way: each stage consists of two aligners, one more flexible and one more rigid. The latter serves to "consolidate" the movement, making the movement more predictable.
If we leave aside the type of plastic and focus on the aligner mechanics, we must understand that virtual planning is the orthodontist's way of creating a system of forces that will be applied later on the patient's teeth when the aligners are placed. The Clincheck does not always reflect the final position we want to reach. This discrepancy between the final situation on the ClinCheck and the actual situation of the patient's teeth increases as the complexity of the case increases.
When an extraction space is closed, the teeth tend to lean towards the extraction space. For example, in case of first premolar extraction, the canine will tilt distally towards the space and the molars and second premolar will tilt mesially toward the space. Even if we have planned on the ClinCheck an en-masse movement to close the space, it is difficult to produce it in this way.
How can we improve the predictability of these movements?
- Sequencing the space closure: closing a space simultaneously by moving all the anterior and posterior teeth at the same time, will create a posterior open bite due to bowing caused by the lack of control of the vertical plane. If we sequence the movements, the arch length will be reduced in a more controlled way, and there will not be so many undesired effects during space closure.
- Plan an anti-tipping movement: when closing a space with brackets, we sometimes make some kind of compensations to improve root control, such as tip-back bends. There are several studies that have looked at the possibility of doing something similar in virtual planning. If we tilt the molar and premolar crowns distally and tilt the canine crown mesially beyond their ideal final position, the aligner will apply additional force to counteract some of the tipping tendency of those teeth.
But...How much can we exaggerate these movements?
Generally speaking, it is difficult to determine a specific number of degrees. What is clear is that the greater the translational movement, the greater the overcorrection should be. You can see an example of the study carried out by Feng et al. in which they show the degrees of overcorrection recommended for cases of maximum anchorage in which space closure is achieved mainly by anterior retrusion.
If we look at these values, we will see how the canine, as it has a greater retraction movement, will need more degrees of overcorrection (mesial crown tip). The molars and the second premolar, which act as anchorage teeth, have about 8º-9° of distal crown tip, which increases the posterior anchorage and prevents unwanted mesialization.
We should not forget that, even if we make these overcorrections, en-masse movement is very difficult to achieve, and there are more factors that affect its predictability (the size of the clinical crowns, the inclination of the teeth, the presence/absence of crowding...) so we should be prepared to use auxiliary techniques to upright the teeth and close the extraction space correctly if we do not achieve it with the aligners.
Feng X et al. Comparison between the designed and achieved mesiodistal angulation of maxillary canines and posterior teeth and influencing factors: First premolar extraction treatment with clear aligners. Am J Orthod Dentofacial Orthop 2022;162:e63-e70.