Forces and moments generated by aligner-type appliances for orthodontic tooth movement: a systematic review and meta-analysis.

The orthodontic market today is booming with clear aligner-type appliances that have quickly established themselves as the mainstay of orthodontic treatment. This form of treatment involves a series of aligners that are fabricated to cover all of the teeth and the gingiva without causing overt damage to the periodontal tissues. The orthodontist instructs the patient to wear these removable aligners for at least 20 to 22 hours a day and change them every 7 to 14 days for optimal performance.

The rate at which the tooth moves to its desired position is dependent on the rate of the physiologic process of bone remodelling and the coordinated force produced by the appliance on the tooth. Experts refer to tooth movement in aligner therapy as the predetermined “mismatch” between the aligner and the tooth. The force that is generated by the deformation of the appliance is transmitted to the tooth structure as well as the surrounding periodontal tissues, thus bringing on tooth movement and tissue remodelling.

And as is the case, the rate at which the aligners need to be changed is directly proportional to the rate of bone remodelling. This study evaluated the mechanics of aligner systems and the forces and moments generated across different types of these aligners.

This study was conducted by a team from Greece and Switzerland and was published in the Journal of Orthodontics & Craniofacial Research.

 

Forces and moments generated by aligner-type appliances for orthodontic tooth

movement: A systematic review and meta-analysis.

Iliadi A, Koletsi D, Eliades T

 Orthod Craniofac Res. 2019 Nov;22(4):248-258. doi: 10.1111/ocr.12333. Epub 2019

Jul 9.

 

What they asked

The authors aimed:

“To systematically appraise the evidence on aligner mechanics and forces and moments generated across different types of aligners.”

 

What they did

• Study design: In vitro/laboratory studies, studies related to the forces/moments exerted by aligners, and any clinical trial/retrospective cohort study with at least two groups for comparison.
• Participants: Models for simulated tooth movement with aligners were considered for in vitro studies. Participants undergoing orthodontic treatment with aligners (irrespective of age), if applicable, were considered.
• Intervention: All types of aligners used for orthodontic tooth movement were considered irrespective of material type, thickness, and activation.
• Comparator: Any type of comparator was considered, either non-aligner orthodontic devices or different types of aligners
• Outcome: Forces and/or moments generated, complying with any type of tooth movement produced (i.e. rotation, intrusion, torque)

They performed electronic searches on these databases: Medline via Pubmed, Cochrane Central Register of Controlled Trials (CENTRAL), LILACS via BIREME Virtual Health Library. Unpublished literature was also searched in Open Grey, ClinicalTrials.gov (www.clinicaltrials.gov), the National Research Register (www.controlled-trials.com) and Center for Open Science (Open Science Framework), using the terms “aligner” AND “orthodontic”.

Data extraction was performed by one reviewer (AI) in pre-piloted forms. The risk of bias assessment was done based on a modified version of the Cochrane Risk of bias tool. Random effects meta-analyses were conducted.

 

What they found out

They were able to identify a total of 447 studies through the electronic search and after a full assessment, 13 studies were considered eligible for this review. All 13 studies were in-vitro studies.

The studies reviewed six different aligner materials ((Biolon, Erkodur, Ideal Clear, Duran, All-In, and Invisalign) with foil thicknesses from 0.3 to 1 mm. They also described six distinctive types of tooth movement with the use of the aforementioned aligner combination thickness.

The risk of bias in 11 of the studies was considered unclear due to the unclear blinding of the assessors. In two of the studies, the risk of bias was identified to be high.

The studies were examined from 3 different perspectives: (i) aligner thickness, (ii) generated tooth movement, and (iii) aligner type.

1. Aligner thickness:

The quantitative analysis was only possible between two of the studies and was in regard to the palatal tipping movement of the maxillary central incisor, generated by PET-G aligners trimmed to a gingival edge width of 3-4 mm.

In this case, there was no difference between any of the aligner thicknesses (0.5, 0.625, or 0.75 mm) with regard to moment-to-force (M/F ratio). It was found that no significant role was played by aligner thickness in forces and moments generated by clear aligners.

 

1. Generated tooth movement:

The most commonly examined tooth movements are tipping and rotation.

 

1. Aligner type:

They found that the aligners vacuum-formed with Biolon (Dreve Dentamid GmbH, Unna, Germany) delivered the highest forces and moments ranging from 1.15 to 6.19 N during tipping and  35.3 to 71.8 N during rotation depending on activation magnitude.

The lowest forces and rotational moments were reported for Erkodur (Erkodent Erich Kopp GmbH, Pfalzgrafenweiler, Germany) at all activation ranges.

 

What we can conclude

The use of fabrication material for aligners is typically restricted to different types of PET-G. We learned through this study that the aligner thickness plays no significant role in the forces and moments generated by thermoplastic aligners in terms of moment-to-force ratio.

Foils have been typically reported to range between 0.5 and 1 mm. The most widely examined tooth movements are tipping and rotation. However, as the authors mentioned, the findings of this review may only be applicable to specific conditions and tooth movements performed in laboratory settings.

Overall, the study concludes with: “There is a need for standardized protocols, types of movements or designs of the aligners in order to inform the existing evidence with more conclusive outcomes.”