Background: Maxillary bonded and removable retainers maintain teeth in correct positions following orthodontic treatment. There is insufficient evidence regarding the capacity of the retention methods to stabilize the maxillary teeth both during and after retention.

Objective: To evaluate retention capacity and 1-year post-retention changes in the irregularity of maxillary anterior teeth and single anterior tooth contact point discrepancy (CPD) of two bonded and one removable retention method.

Trial design: Three-arm parallel group single-centre randomized controlled trial.

Methods: Ninety adolescent patients treated with fixed orthodontic appliances were enrolled. After gaining informed consent, the patients were randomized in blocks of 30 by an independent person into one of three groups: A) bonded retainer 13-23; B) bonded retainer 12-22; and C) removable vacuum-formed retainer. The primary outcomes were changes in Little's irregularity index (LII) and single CPD measured on digitalized casts before retention (T1), after 2 years of retention (T2), and 1-year post-retention (T3).

Blinding: The digital casts were blinded for the outcome assessor.

Results: Data on all 90 patients were analysed according to intention-to-treat principles. Changes in LII during retention were 0.3 mm in group A, 0.6 mm in group B, and 1.0 mm in group C. No significant differences between the groups were seen (P > 0.05). Changes during post-retention were 1.1 mm in group A, 0.5 mm in group B, and 0.4 mm in group C. Group A showed more significant changes than groups B and C (P = 0.003). During the whole post-treatment period, no significant differences were shown between the groups (P > 0.05). CPD did not differ significantly between the groups at any point.

Harms: Three patients showed changes of LII over 3 mm or CPD over 2 mm during the post-retention period, and two accepted to be realigned.

Limitations: The trial was a single-centre study evaluating 1-year post-retention changes.

Conclusions: The changes were clinically insignificant during and after the retention period. Thus, all three methods showed equal retention capacity.

Trial registration: www.clinicaltrials.com (NCT04616755).