Forces and moments generated by removable thermoplastic aligners: incisor torque, premolar derotation, and molar distalization

The widespread admiration for clear aligners is in part due to the development of Align Technology’s Invisalign system in the early 2000s. Its concept of removable thermoplastic appliances to cover all teeth coupled with the ever-evolving technology of CAD/CAM stereolithography and tooth movement software has ed it to satisfy the demands of today’s generation.

Clear aligners were originally developed to treat mild to moderate occlusion issues in a more aesthetic manner. However, due to rapid developments in clear aligner technologies, the number and complexity of cases that are treated with this method have skyrocketed just in the last few years.

Aesthetics, comfortable wear, and enhanced oral hygiene, among other benefits, have brought clear aligners to the limelight. But do we really know everything there is to know about these aligners? Can clear aligners successfully achieve complex tooth movements that have been deemed impossible with removable appliances, say, torque, derotation, or even distalization of the molars?

This study investigates the forces and moments generated by removable thermoplastic aligners (RTA) and if they are sufficient to bring about the above-listed tooth movements.

A team from Germany conducted this study. It was published in the American Journal of Orthodontics & Dentofacial Orthopedics.

Forces and moments generated by removable thermoplastic aligners: incisor torque, premolar derotation, and molar distalization.

Simon M, Keilig L, Schwarze J, Jung BA, Bourauel C.

Am J Orthod Dentofacial Orthop. 2014 Jun;145(6):728-36. doi: 10.1016/j.ajodo.2014.03.015.

What they asked

The aims of this experimental study were:

“(1) to quantify the exact initial force systems that are delivered by an individual aligner,

(2) to measure the force systems generated by a series of aligners, and

(3) to investigate the influence of auxiliaries (attachments, power ridges) on the force transfer.”

What they did

The authors investigated the initial force systems delivered by 970 aligners of the Invisalign® system (60 series of aligners) from 30 consecutive patients (11 males, 19 females, mean age: 32.9 years). The inclusion criteria for the patients were patients who were seeking orthodontic movements and the need for 1 of these 3 movements: incisor torque, premolar derotation, or molar distalization. The exclusion criteria included cleft lip and palate or any other syndrome-associated orofacial malformation.

The 30 patients were divided into 3 movement groups with 10 patients in each group. In each movement category, 20 tooth movements (2 per patient) were determined. Each of these 3 groups was further subdivided so that 10 movements were supported with an auxiliary (attachments/power ridges) and 10 were not.

• In group 1, maxillary central incisor torque was greater than 10, supported by a “horizontal ellipsoid attachment” or power ridges.
• In group 2, premolar derotation was greater than 10, supported by an “optimized rotation attachment” or with no auxiliary.
• In group 3, molar distalization was greater than 1.5 mm, supported by a 4-mm-long “horizontal bevelled gingival attachment” or with no auxiliary.

All the attachments we used were engineered by Align Technology to achieve predictable tooth movements.

The patients’ ClinCheck (Align Technology, Santa Clara, Calif) was planned so that the movements that were to be investigated were performed in isolation in the respective quadrant.

They also took resin replicas of the patients’ mouths before the start of the investigated movement were taken and mounted in a biomechanical measurement system. Then, an aligner was put on the model, the force systems were measured, and the calculated movements were experimentally performed until no further forces or moments were generated. Subsequently, the next aligners were installed, and the measurements were repeated.

What they found out

They found that each aligner created high initial forces, followed by an exponential decrease of forces/moments just above 0 N/N·mm. They were also able to conclude that the forces and moments between consecutive aligners differed even though a continuous movement was planned in ClinCheck.

Regarding the effect of auxiliaries like attachments and power ridges on the force transfer, they found that distalization and derotation supported by an attachment and incisor torque supported by a power ridge have higher forces or moments corresponding to the direction of movement. In nearly all cases, the differences were statistically significant.

The initial mean moments were about 7.3 N·mm for maxillary incisor torque and about 1.0 N for distalization. There were also significant differences found in the generated moments n the premolar derotation group, whether they were supported with an attachment (8.8 N·mm) or not (1.2 N·mm). All measurements showed an exponential force change.

What we can conclude

These were the conclusions offered by the authors of this study:

• The measured forces and moments created by aligners of the Invisalign system are consistent with the literature values.
• The force decay is exponential during RTA wear, independent of attachments.
• Force systems differ in a series of aligners, even if a constant movement in ClinCheck is planned.
• Premolar derotation should be supported with an attachment, especially if these teeth have short crowns and few undercuts. Otherwise, moment transfer from the RTA to the tooth is possible only to a limited degree.

Overall, we can conclude that the forces and moments generated by aligners of the Invisalign® system are within the range of orthodontic forces, although the force change is exponential when the patient is wearing an RTA.