Is sequential distalisation predictable?

Before the use of aligners, Class II correction by upper distalisation was a difficult strategy to apply. Most cases were solved with Class II elastics. This treatment option solves the sagittal problem at the expense of mesialising the lower teeth. Due to their greater complexity, in patients where we planned to distalise, auxiliary techniques were used to increase anchorage, such as micro-screws, and/or we made the movements sequentially with coils, moving the molars, premolars and the rest of the anterior teeth step by step. But one thing was clear to us: brackets alone were not capable of distalisation.

Since we started using clear aligners, sequential distalisation has become the main method for correcting Class II cases. Watching in the ClinCheck how the teeth moved progressively until Class I was reached, led us to believe that aligners were capable of anything. Interestingly, in a survey carried out a few years ago asking orthodontists and general dentists how they corrected Class II with aligners, only 37% of the doctors surveyed said they used elastics. Clearly, the ease with which movements occur in a virtual programme can be confusing.

During the first stages of a Class II correction, when the aligners push one or two teeth distally, the force that is transmitted to the teeth will generate an opposing force of the same magnitude on the rest of the arch. The greater the number of teeth we push, the greater this effect will be.

In early studies on molar distalisation, researchers claimed that it was one of the most predictable movements with aligners, reaching figures above 80%. I ask you the following question: does the predictability of molar distalisation guarantee the correction of Class II? The answer can be found in the article published by Taffarel et al.

Distalising molars requires anchorage to avoid the opposing force that is produced when the aligner pushes the teeth distally. This anchorage can be either elastics or mini-screws. Relying solely on aligners to correct Class II will result in unwanted movements. Let's take the best-case scenario: We distalise the molars achieving 88% of the planned movement, but... what will happen when premolars and the rest of the anterior teeth are distalised?

Molars are not going to provide sufficient anchorage to retract all those teeth, even if the movements are done sequentially. Moreover, if proper anchorage is not used, some of the molar distalisation that we have achieved may be lost when all those teeth are retruded. That's why most of us have seen patients who were in molar Class I end up in Class II by the final stage of the phase. Whether it was because of this loss of posterior anchorage, lack of collaboration or unwanted occlusal contacts, distalisation did not occur as planned.

For this reason, when distalisation is performed, it must be understood that ClinCheck does not always represent the final position of the teeth. This programme is used to plan the forces we are going to apply to the teeth. Like brackets, aligners are not designed to make all the movements on their own. Therefore, when we detect that the force system we have planned is not optimal to achieve our objectives, we must assess which auxiliary elements we need to complete this system effectively and achieve the desired final position.

Ta ffarel IA et al. Distalization of maxillary molars with Invisalign aligners in nonextraction patients with Class II malocclusion. Am J Orthod Dentofacial Orthop 2022;162:e176-e182.