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A randomised controlled trial comparing thermoformed and 3D-printed retainers in young adults: evaluation of surface
Hui Shan Boo1, Norhidayah Nor Zahidah Mohd Tahir2, Mohd Shahriman Adenan3
1Ministry of Health, Government Dental Clinics, Putrajaya, Malaysia.
Background:
This study aimed to evaluate and compare the surface roughness, surface morphology, and mechanical properties of direct 3D-printed retainers (3DPR) and thermoformed retainers (TFR) after six months of intraoral use.
Methods:
This study reports a secondary laboratory objective of a previously published randomised controlled trial (Boo et al. 2026) and included a subset of 10 participants (n = 5 per group). TFRs were fabricated using 1.0 mm Erkodur blanks, while 3DPRs were designed with a uniform thickness of 0.75 mm and printed using NextDent Ortho Flex resin. For each participant, unused upper and lower retainers served as controls, while the corresponding retainers were retrieved after six months of intraoral use. Surface roughness parameters (Ra and Rq) were measured on the cameo and intaglio surfaces using contact profilometry. Surface morphology was evaluated using scanning electron microscopy. Mechanical properties were assessed by instrumented indentation testing. Upper- and lower-arch measurements were averaged at the participant level for statistical analysis.
Results:
The 3DPR group generally exhibited higher surface roughness values than TFR, with statistically significant differences observed at the intaglio surface at both time points and at the cameo surface after six months (Ra and Rq, p < 0.05). No significant within-group changes in Ra or Rq were observed over time. Surface morphology analysis demonstrated more pronounced surface irregularities in 3DPR compared with TFR, which showed relatively smooth surfaces. For mechanical properties, TFR demonstrated statistically significantly higher Martens hardness, indentation modulus, and indentation hardness than 3DPR at baseline (all p < 0.05), with further increases after six months, whereas 3DPR showed declines in these properties (p < 0.05).
Conclusions:
In this pilot laboratory study, 3DPR showed higher surface roughness and more significant decreases in indentation-based mechanical properties after six months of intraoral use compared to TFR, indicating potentially lower mechanical durability under these conditions. However, these findings should be interpreted cautiously given the small sample size.