Extractions with aligners. Part I

Extraction cases with aligners are considered some of the most complex cases to treat with aligners. due to the properties of the plastic. When treating extractions with brackets, we have the possibility to choose the stiffness and thickness of the archwire to achieve greater control of the mass movement of the teeth during space closure. However, in treatments with aligners we do not have this possibility.

To make up for these shortcomings, aligner companies have devised different systems. For example, InvisalignThe new system, despite using the same type of plastic, introduces additional forces to improve the root control of the teeth. Others, such as Orthocaps o Angel AlignThe aligners combine aligners of different stiffness throughout the treatment to achieve better control of movement. The latter work in a very interesting way: each stage consists of two aligners, one more flexible and the other more rigid. The latter serves to "consolidate" the movement, achieving a priori a more predictable movement.

If we leave aside the type of plastic we are going to use and focus on the mechanics of the alignersIn order to be able to plan virtually, it must be understood that virtual planning is a way to
The Clincheck is a way for orthodontists to create a system of forces that we will later apply to our patients' teeth when the aligners are fitted. The ClinCheck does not always reflect the final position we want to achieve. This discrepancy between the final situation on the ClinCheck and the actual situation of the patient's teeth increases as the difficulty of the case increases.

When we close an extraction space, the teeth tend to tilt towards the extraction space. If, for example, we have extracted a first premolar, the canine will tilt distally into the space and the molars and second premolar will tilt mesially into the space. Even if we have planned a mass movement in the ClinCheck to close the gap, it is difficult to do it this way.

How can we improve the predictability of these movements?

• Sequencing the closing of spacesclosing a space simultaneously, 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 as many unwanted effects during space closure.
• Planning a anti-tipping movement in ClinCheck: when closing a space with brackets, we sometimes make some kind of offsets 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 crowns mesially beyond their ideal final position, we will get the aligner to apply additional force that will partly counteract the tipping tendency of those teeth.

But how much overcorrection can we do?

As is often the case, it is difficult to determine a specific number of degrees. What is clear is that, the greater the translational movement, the greater must be the
overcorrection. You can see an example of the study carried out by Feng et al. in which they show the recommended degrees of overcorrection for anchor cases
maximum in which space closure is achieved mainly by anterior retrusion.

If we look at these values, we can see that the canine, as it has a greater retraction movement, will need more degrees of overcorrection (coronodistal tip). The molars and the second premolar, which act as anchorage teeth, have about 8º-9° of coronodistal tip, which increases the posterior anchorage and prevents their mesialisation.

We must not forget that, even if we make these overcorrections, mass movement is very difficult to achieve, and there are many more factors that influence it.
The use of a dental crown is a very important factor in the achievement of these goals (the size of the clinical crowns, the inclination of the teeth, the presence or absence of crowding...) so we will have to be prepared to use
auxiliary tools with which to finish straightening the teeth and close the extraction space correctly if the aligners fail to do so.

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.