Recently, several aligner manufacturers (ClearCorrect, Spark, Angel Aligner) using ZENDURA FLX plastic have proposed alternating "soft" aligners with "rigid" aligners, arguing that rigid aligners help promote root movement.
This is proposed with an analogy to what is done with fixed orthodontics: flexible nitinol archwires and steel completion archwires.
However, although it may "sound good" and that the parallelism wants to make us imagine a better result of our treatments without doing anything other than changing the material of our aligners, the truth is that this is a falsehood, since:
- The dental movement of an aligner is caused by its flexibility, not by its rigidity, which only contributes to increase its retention (which can be improved, for example, by using attachments).
- Increased stiffness also leads to increased microfractures in the plastic of the aligner and deformation due to mechanical stress (remember that flexibility is more important for movement).
- The aligner is removable: it cannot be compared to the final root torsion of a steel archwire that is constantly placed on the patient, something that can only be suggested by someone with total ignorance of tooth movement... or with an interest in making sense of it.
In fact, as a final comment, Zendura FLX is a multilayer polyurethane that has two versions, the rigid one and the even more rigid one, due to its tri-layer structure (the outer ones are "waterproof") of PU, so that in reality there is no "soft" plastic as such, since the structure itself.
In conclusion: it has not been demonstrated in any way that alternating the hardness of the aligner plastics improves the result in any way, and the inefficiencies of using a material with excessive stiffness have been demonstrated, which we should avoid in our planning, always opting for flexible materials with low relaxation stress.