Back to Basics: Clinical outcomes with aligners versus fixed appliances
This is the final post in the “Back to Basics” series on clear aligners. In this post, Martyn Cobourne gives a nice summary of the evidence on clinical outcomes of aligner treatment compared to fixed appliances.
There is little doubt that clear aligner treatment continues to increase in popularity, and as these systems evolve, orthodontists are developing increasingly ingenious ways to manage more complex malocclusions with these appliances. However, biomechanically, they remain inferior to conventional fixed appliances, and it is important to understand how effective they are concerning treatment outcomes. Following some helpful discussion with Spyridon Papageorgiou, I have put his summary together.
Effectiveness of Aligners on occlusal outcomes
Current evidence is based primarily upon matched non-randomised studies interspersed with a few small-volume RCTs, which makes it of moderate (to low) quality at best (Papageorgiou et al., 2020):
- Investigators have done studies using the American Board of Orthodontics Objective Grading System (ABO-OGS). They have concluded that aligner treatment has significantly worse occlusal outcomes and more patients with unacceptable results compared to fixed appliances.
- Within the ABO-OGS, variation exists in the success of aligner treatment in managing different aspects of malocclusion (See Figure 1); however,
- Using the Peer Assessment Rating (PAR), there are no differences between aligners and fixed appliances in terms of final PAR score or PAR reduction (this apparent contradiction with the ABO-OGS data is possibly related to the PAR being a less rigorous evaluation of tooth position within the dental arches and a more representative measurement of baseline case severity; indeed, post-treatment ABO-OGS and PAR scores do not generally correlate).
Several systematic reviews have investigated multiple aspects of clinical effectiveness with aligners versus fixed appliances. Investigators have recently synthesised these findings, but we should remember that again, the level of evidence is only moderate to low, and the synthesis quoted here lacks quantitative analysis incorporating risk of bias assessment for these studies (Yassir et al., 2022):
- Aligner treatment is effective for mild to moderate malocclusion requiring simple tooth movement, but is less effective than fixed appliances for more severe crowding and complex cases;
- Treatment duration has been reported to be shorter with aligner treatment compared to fixed appliances for mild to moderate cases; however, treatment duration studies need to be carefully controlled for equivalence, particularly when there is a likelihood of milder malocclusions being treated using aligners and evidence of poorer occlusal outcomes for aligners (Papageorgiou et al. 2020);
- Aligners are associated with a higher tendency for relapse compared to fixed appliances, possibly related to the increased proclination and expansion of teeth that occurs with aligners or possibly to the worse occlusal outcomes. (Angst et al., 2021) or both?;
- Aligner treatment is associated with better periodontal index scores and periodontal outcomes compared to fixed appliances, but the evidence is limited (Oikonomou et al., 2021);
- There is little robust evidence of any differences between aligners and fixed appliances in terms of orthodontic-induced external root resorption (OIRR) (although the data is sparse and probably underpowered); aligners seemingly show less OIRR, but after calculating how much aligners move the teeth (especially the root apex) less OIRR is most likely due to less movement – when you account for this there are no differences between them (Iglesias-Linares et al., 2017; Oikonomou et al., 2021).
- Patients treated with aligners report lower levels of pain during the first few days of treatment compared to those treated with fixed appliances; the placement of attachments seems to be associated with the most pain during treatment with aligners and
- There are no conclusive differences between aligners and fixed appliances regarding Overall Oral Health-Related Quality of Life. However, eating and chewing disturbances and discomfort during treatment seem less with aligners (Zhang et al., 2020). However, the likelihood of speech difficulties appears higher with aligners – although patients adapt quickly and speech rapidly returns to normal (Ali Baeshen et al., 2023).
Finally, a few more points to consider when using aligners:
- At least one refinement is almost always required, and patients often switch to fixed appliances.
- As previously discussed, attachment choice is an important part of treatment planning, and aligners are associated with many problems for different types of tooth movement (see many studies from Tony Weir).
Iglesias-Linares A, Sonnenberg B, Solano B, Yanez-Vico RM, Solano E, Lindauer SJ, Flores-Mir C. Orthodontically induced external apical root resorption in patients treated with fixed appliances vs removable aligners. Angle Orthod 2017;87:3-10.
Oikonomou E, Foros P, Tagkli A, Rahiotis C, Eliades T, Koletsi D. Impact of Aligners and Fixed Appliances on Oral Health during Orthodontic Treatment: A Systematic Review and Meta-Analysis. Oral Health Prev Dent 2021;19:659-672.
Yassir YA, Nabbat SA, McIntyre GT, Bearn DR. Clinical effectiveness of clear aligner treatment compared to fixed appliance treatment: an overview of systematic reviews. Clin Oral Investig 2022;26:2353-2370
Professor of Orthodontics, Kings College, London.