Evaluation of aging effects on the physical, chemical, and mechanical properties of thermoplastic materials for orthodontic clear aligners: an in vivo study

Abstract Background Due to the viscoelasticity of orthodontic thermoplastic materials, the force generated by the orthodontic device for the same deformation will decline over time. Aligners are continually exposed to saliva at a constant body temperature, which can have an adverse effect on the chemical composition of the polymers. More studies are needed to investigate the in vivo aging of other materials, to determine the Best material that could meet treatment needs. We show the comparison of physical, chemical, and mechanical changes of clear aligners before and after 10 days of intraoral usage. Methods Twenty-five unused aligners and twenty-five aligners used in the oral environment were first gathered from five patients requiring clear aligners and then subjected to water absorption and solubility, tensile strength, modulus of elasticity, and Chemical properties testing. The Kolmogorov-Smirnov (K-S) test was used to compare the equality of distributions. The Wilcoxon test was used to determine the median difference by time. Data were analyzed in R-4.4.1 using the ggstatsplot package and ggwithinstats function. The results with p < 0.05 were considered statistically significant for all tests. Results The mean water solubility (Wsl) was zero (before using aligners) and 0.00013 (after using aligners), indicating an insignificant difference between the two time periods (p = 0.150). The mean Max Force (N) was 97.105 (before using aligners) and 111.18 (after using aligners), implying an insignificant difference between the two time periods (p = 0.640). Similarly, the mean normalized displacement (%) was 123.56 (before using aligners) and 122.08 (after using aligners), indicating an insignificant difference between the two time periods (p = 0.290). The chemical composition analysis revealed the hydrolysis of ester bonds to carboxylic acid and alcohol functional groups post-treatment. Conclusion The aligners’ mechanical and physical properties are not significantly affected, but their chemical properties can be altered with aging.