Do aligner materials matter?

Clear aligners are all the rage these days. But behind their sleek and nondescript appearance is a complex interplay of materials and attachments that inadvertently allow these super-braces to move teeth quickly and effectively.

Two factors have a significant impact on the aligner’s performance – aligner material and attachments. Most aligners today are constructed using polymers, the most used of which are polyester, polyurethane, and polypropylene. An ideal aligner should be able to withstand the forces applied to it while also fulfilling its physically desirable qualities. As high as 50% of the original stress value can be released in the first few hours after wearing the aligner. Therefore, the composition and thickness of the aligners can affect load and stress changes.

Clear aligners also use special implements called attachments to apply and distribute force systems within the dentition. Based on the various types of attachments, the aligners can improve retention and enable orthodontic tooth movement. Bevelled attachments are typically preferred over ellipsoid attachments because they have been found to greatly boost retention.

This systemic review and network meta-analysis examined the various market-available thermoplastic materials and attachment forms and recorded their different impacts on the orthodontic tooth movement of patients. The question resounded, do aligner materials and attachments really matter?

This study was conducted by a team of researchers from Saudi Arabia, UAE, and Malaysia and published in the Journal of Functional Biomaterials.

 

A Systematic Review and Network Meta-Analysis on the Impact of Various Aligner Materials and Attachments on Orthodontic Tooth Movement

by Mohammad Khursheed Alam, Bushra Kanwal, Abedalrahman Shqaidef, Haytham Jamil Alswairki, Ahmed Ali Alfawzan, Abdulilah Ibrahim Alabdullatif, Abdulaziz Naser Aalmunif, Sattam Hamad Aljrewey, Thamer Abdullah Alothman, Deepti Shrivastava, Kumar Chandan Srivastava

J. Funct. Biomater. 2023, 14(4), 209; https://doi.org/10.3390/jfb14040209

Therefore, this systematic review and network meta-analysis assessed studies that examined different aligner materials and attachments and their impact on the orthodontic tooth movement of patients who were undergoing any form of orthodontic treatment and determine whether one type of material was functionally better than the other.

 

Objective of the study

The authors did this study with the goal:

“To examine various aligner materials and attachments and their impacts on the orthodontic tooth movement of patients and to determine whether one type of material was functionally better than the other”

 

Materials and methods

They carried out a  systemic review as per the Preferred Reporting Items for Systematic Review and Meta-analysis (PRISMA) strategy and rules from the Cochrane group as well as rules from the book Orderly Reviews in Health Care: Meta Examination.

 

Inclusion criteria:

They included articles and review studies that reported clinical trials, in vitro investigations, randomized/non-randomized studies, systematic literature reviews with sizable sample sizes, and comprehensive case reports.

 

Search Strategy:

Databases such as Cochrane, PubMed, and Web of Science were searched using keywords like “Aligners”, “Orthodontics”, “Orthodontic attachments”, “Orthodontic tooth movement”, and “Polyethylene” as well as reference searches, and citation searches.

The PICO was:

• Participants: Patients undergoing orthodontic treatment with clear aligners.
• Intervention: Different types of aligner materials and attachments
• Comparison: Conventional clear aligners or different types of aligner materials and attachments
• Outcomes: Orthodontic tooth movement, including rate, amount, and direction.

 

Data selection:

After searching through all relevant databases using specific keywords, two separate reviewers were able to find the pertinent papers. A third reviewer was consulted to relieve disagreements about the selected articles. The same two reviewers then extracted the data that matched the goals of the investigation. Finally, the third reviewer conferred about any discrepancies.

 

Risk of bias assessment:

The authors used the AMSTAR-2 approach to evaluate studies and assess them for bias. This showed that in selecting each case, a choice was made followed by input from more than 30 methodological experts.

 

Results

Initially, a total of 634 documents were found after searching through online journals whereas another 234 were identified using other methods such as citation searches. Of these 169 papers were chosen. Then, they eliminated 153 studies based on duplicacy and irrelevancy which left them with a total of 16 studies.

Out of the 16 studies, 1 was a comparative clinical trial, 2 were prospective studies, and the rest were in-vitro studies which investigated various types of aligner materials.

Materials such as Polyethylene (PE), Polyethylene Terephthalate (PT), Invisalign (Inv), Polyethylene TerephthalateGlycol (PTG), Polypropylene (PP), Thermoplastic Polyurethane (TP), and PolyvinylSiloxane (PS).

The mean values were calculated for each material. This represented the central tendency of the posterior distribution for each plastic material. These values provided an estimated impact of these materials on orthodontic tooth movement. Additionally, variance values were also calculated for each plastic indicating the spread of the posterior distribution. Higher variance values indicated that the data points were more spread out from the mean, suggesting that there was a greater level of uncertainty in these results. PT and PTG were found to have greater uncertainty than other materials.

Invisalign had a high mean value of 642.467 which was greater than the other materials. This suggests that Invisalign produced a greater impact on orthodontic tooth movement compared to other plastics. However, its variance value (158,783.189 with 95% confidence level) was also high and indicated a greater uncertainty.

They also evaluated the impact of attachment geometry on tooth movement using the same thickness of the aligner material. To compare the impact of attachments, the PTG and TG group aligners with and without attachments were compared. The result was no differences in tooth movement outcomes between the two aligner material groups.

There was more crown tipping associated with bar-type attachments. However, with both ellipsoid and bar-shaped attachments, the PTG or TP aligners were successfully able to correct canine crown distal tipping and root mesial torque.

Finally, the systemic review found that there was an interconnectedness of all the types of aligner materials, indicating that all the materials functioned with a very noticeable difference separating their impacts on orthodontic tooth movement.

 

Final thoughts

The biggest limitation of the study was that the authors needed to include a good number of randomized clinical trials. The overall number of investigations selected and their sample sizes were produced in less-than-ideal conditions with a high level of bias.

While the study highlighted that there was a significant association between aligner materials and attachments and orthodontic toothment, they could not come to a robust conclusion and determine the differences between the materials. Instead, they suggested that the aligner materials had a significant impact on tooth movement based on the material used and not based on their attachment size or shape.

The range of values and high uncertainty levels of the findings conclude that further research is needed to confirm the impact of various types of aligner materials and attachments on orthodontic tooth movement.