# A Randomised Controlled Trial Comparing Thermoformed and 3D-Printed Retainers in Young Adults: Evaluation of Post-treatment Stability and Patient Satisfaction

## TL;DR

This study compared thermoformed and 3D-printed retainers in young adults, finding similar long-term dental stability and quality of life improvements after six months.

## Contribution

The study provides a direct comparison of two modern retainer types in terms of post-treatment dental stability and patient-reported oral health outcomes.

## Key findings

- Thermoformed and 3D-printed retainers showed comparable post-treatment stability in most dental measurements.
- Both retainer types led to significant improvements in oral health–related quality of life.
- A minor psychological disability domain difference favored 3D-printed retainers, but it was not clinically significant.

## Abstract

To compare post-treatment stability and oral health–related quality of life (OHRQoL) in young adults wearing thermoformed retainers (TFR) versus direct 3D-printed retainers (3DPR) over a six-month part-time retention period.

This single-centre, two-arm parallel randomised controlled trial allocated 30 debonded orthodontic patients (≥ 18 years) to either TFR (Erkodur PETG, 1.0 mm) or 3DPR (NextDent Ortho Flex, 0.75 mm) in a 1:1 ratio. All retainers were worn part-time (8–12 h/day). Digital intraoral scans were obtained at baseline (T0) and six months (T2) using Trios 3, and post-treatment stability was assessed digitally using Little’s Irregularity Index, intercanine width, intermolar width, arch length, overjet, and overbite. OHRQoL was evaluated at both time points using the OHIP-14(M) questionnaire. Non-parametric tests were applied following the Shapiro–Wilk assessment, and analyses were conducted using an intention-to-treat approach.

One dropout in the 3DPR group at T2. At T2, no significant between-group differences were found in Little’s Irregularity Index, intercanine width, intermolar width, arch length, or overjet (p > 0.05). Overbite values were statistically higher in the TFR group at T2 (p < 0.05); however, the difference was not clinically significant. Changes in post-treatment stability from T0 to T2 (ΔT2–T0) did not differ significantly between groups (p > 0.05). Total OHIP-14(M) scores improved significantly from T0 to T2 in both groups (TFR p = 0.016; 3DPR p = 0.013), with no significant between-group differences at either time point (p > 0.05). A small but significant difference was observed in the psychological disability domain at T2, with slightly higher scores in the 3DPR group (p = 0.020). No harm was observed.

TFR and 3DPR demonstrated comparable post-treatment stability and similar improvements in OHRQoL after six months of part-time wear.

ClinicalTrials.gov (Identifier NCT05968625).

Protocol: The protocol was published before trial commencement

The online version contains supplementary material available at 10.1007/s00784-026-06793-z.