Vacuum-formed retainer versus bonded retainer for dental stabilization in the mandible-a randomized controlled trial. Part I: retentive capacity 6 and 18 months after orthodontic treatment

Abstract

Background: Evidence concerning the most appropriate retention strategy after orthodontic treatment is still inconclusive.

Objective: This trial compares the retentive capacity of vacuum-formed Essix C-retainers (VFR) and bonded cuspid-to-cuspid retainers (CTC) in the mandible 6 and 18 months after orthodontic treatment.

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

Methods: This study included 104 adolescent patients, computer-generated randomized with sequentially numbered, opaque, and sealed envelopes into two groups and stratified by gender (52 females and 52 males). The patients were treated with fixed appliances with and without tooth extractions in both jaws and were ready for debond. In the intervention arm, patients received a VFR (n = 52); in the active comparator arm, patients received a CTC (n = 52). Dental casts were obtained at debond (T1), after 6 months (T2), and after 18 months (T3). The casts were digitized. The retentive capacity was evaluated on digital three-dimensional models using Little's Irregularity Index (LII), overbite, overjet, arch length, and intermolar and intercanine width.

Results: Baseline values were similar for both groups. Statistically significant increases were noted in LII and overbite, mainly between T1 and T2, but also between T1 and T3 in the VFR group (LII = 0.52 mm, P < 0.001, overbite = 0.51 mm, P < 0.001) and in the CTC group (LII = 0.45 mm P < 0.001, overbite = 0.36 mm, P < 0.001). There were no significant differences between groups. Overjet showed small variations during the observation periods but was overall stable within and between groups after 18 months. Arch length decreased slightly in both groups after 6 and 18 months. Intermolar and intercanine widths remained stable after debond.

Limitations: Patients, operator, and outcome assessor could not be blinded due to the study design.

Conclusions: VFR and CTC have the same retention capacity in the mandible after 6 and 18 months. Relapse mainly occurs during the first 6 months of retention, but the post-treatment changes are generally small. Shorter VFRs in the mandible do not cause negative vertical effects. Part-time wear regimen is not associated with increased relapse.

Trial registration: NCT03070444.