Do orthodontic fixed retainers guarantee the stability of dental alignment at the end of orthodontic treatment?

Data sources Electronic searches of studies on orthodontic retainers were conducted up to 12 February 2021 in four electronic databases including Scopus, Web of Science, Embase and PubMed. Only studies in English language were included.Study selection Only clinical studies were included. The inclusion and exclusion criteria were reported. The initial search identified 117 results. After removing duplicate studies, studies were evaluated against the inclusion criteria. Finally, 21 papers were included. Selection of the studies was performed independently by two reviewers. The included studies assessed the effects of the type of orthodontic wire or fibre splint, the material used to bond it to the teeth and the procedure for bonding on the failure rate of the fixed orthodontic retainers.Data extraction and synthesis Data extraction was performed independently by two reviewers. They followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) reporting guidelines. The following items were extracted: authors, year, type of study, study objective, number of subjects, comparison made, outcome measured and results. Three tools were used to assess the risk of bias of the included studies including the revised tool for assessing risk of bias in randomised trials (RoB 2), the Newcastle-Ottawa Scale for case-control studies and the Newcastle-Ottawa Quality Assessment Form for Cohort Studies. Meta-analysis was performed using the random-effects model.Results Twenty-one studies were included in the qualitative analysis while seven studies were included in the quantitative analysis. The included studies were three retrospective cohort studies, two case-controlled studies and 16 randomised clinical studies. The results showed that the failure rate of orthodontic fixed retainers ranged from 7.3% to 50%. Failure rates of fixed retainers placed in the maxilla were higher than those placed in the mandible. Previous failure increased the risk of repeat failure. Adhesive failure was considered the most common type of bond failure that was observed in fixed lingual retainers. The type of wire or splint has no effect on the failure rate. Results showed that retainers bonded to all teeth were more efficient in maintaining alignment when compared to retainers bonded on the teeth on the ends only. Indirect bonding techniques have no advantage over direct bonding techniques except for shorter chair time. Bonding fibre-reinforced composite retainers were more sensitive to operator skills, hence failure rates would be high if an incorrect technique was used when bonding.Conclusions No fixed retainer can guarantee the stability of alignment stability after orthodontic treatment. Retainers that are bonded to all teeth are preferable to those bonded only at the ends of the wire. Wires and fibre splints were similar regarding failure rates and stability of alignment.