Comparison of the mechanical properties, color stability, and solubility of various orthodontic retainers at different pH levels

This study investigated the changes in the mechanical, optical, and solubility/absorbance properties of four different vacuum-formed retainer sheets (Atmos–PETG, Erkodur–PETG, Essix ACE–copolyester, Essix C+– polypropylene) after thermoforming and aging under different pH conditions in vitro. A total of 160 specimens—10 specimens for each of the control, pH 4.3, pH 6.7, and pH 8.3 groups and for each test—were prepared in accordance with ISO 527-2 Type 5B standards, aged at 37°C for 14 days, and subsequently evaluated using tensile testing, Shore A hardness measurements, transmittance–absorbance analysis in the 350–1000 nm range, and mass-change assessments. The Kruskal– Wallis test was used for data comparisons, and the brands or specimens causing significant differences were identified with the post-hoc Dunn test. A significance level of P<0.05 was adopted. According to the findings, the Young's modulus was highest for Atmos in the control group, and lowest for Essix C+ in the acidic environment and for Essix C+ and Essix ACE in the basic environment. A significant decrease in the Young's modulus of Atmos and Essix C+ was observed under the applied aging conditions, independent of pH. Shore A hardness showed no significant differences in any material after aging in different pH solutions. In terms of optical properties, the Essix C+ group exhibited the highest transmittance values under nearly all conditions. For mass change, all brands showed an increase in mass in every solution. Mass increase was more pronounced in PETG-based materials, with higher changes observed in Atmos in the neutral environment and Erkodur in the basic environment. The findings indicate that the performance of vacuum-formed retainer materials differs significantly depending on polymer chemistry and environmental conditions, emphasizing that material selection for long-term retention success should be based on scientific evidence.