Design Systematic review
Study population Orthodontic patients undergoing treatment with removable thermoplastic retainers compared with conventional fixed orthodontic appliances. The mean age of patients was between 15 and 33 years.
Data sources Six electronic databases including: Scopus, Web-of-Science, PubMed, Cochrane, Clinical Trials and Grey Literature were searched in addition to a search of references in eligible studies with no restriction of language. Out of 559 studies, 55 were eligible. Only four articles were included; three non-randomised and one randomised controlled trial (RCT) involving 96-182 patients. The studies included were completed within 2005-2016.
Study selection Clinical trials involving orthodontic patients undergoing treatment using thermoplastic removeable aligners, namely Invisalign, in comparison to conventional fixed orthodontic appliances of either self-ligating or tip-edge bracket systems were included.
Data extraction and synthesis Two members of the research team independently selected articles which fulfilled the inclusion criteria and compared results prior to data collection. A third party examiner was available to resolve disagreements.
The study quality was analysed for presence of bias; three non-randomised studies were evaluated and scored using the Methodological Index for Non-Randomised Studies (MINORS), one study was considered high methodological quality, two were considered moderate. The RCT was assessed using the Cochrane Collaboration Common Scheme for Bias and was deemed to be low risk of bias.
Results The results suggest that Invisalign can provide successful alignment, however, it poses difficulties in achieving occlusal contacts, buccolingual inclination and vertical movement of teeth. One study found that Invisalign cases are more susceptible to relapse, which can be explained through the tipping mechanism compared to bodily movement, of conventional fixed appliances. Furthermore some studies show that although using aligners can result in overall shorter treatment time the final occlusion was not favourable. The outcome of the RCT, showed that aligners can be successful in treating class I malocclusions in extraction cases.
Conclusions This systematic review found that the use of clear aligners made no significant difference to treatment time compared with conventional appliances. Additionally, removable aligners were effective in crowding cases but had limitations in achieving favourable outcomes in the anterior-posterior position and final occlusion, as well as being more likely to relapse post treatment.
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Dijkers M. Introducing GRADE: a systematic approach to rating evidence in systematic reviews and to guideline development. KT Update 2013; 1: 1-9.
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Kassam, S., Stoops, F. Are clear aligners as effective as conventional fixed appliances? . Evid Based Dent 21, 30–31 (2020). https://doi.org/10.1038/s41432-020-0079-5
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