In Vitro Aging Effect on Young's Modulus of Orthodontic Aligners

Objectives: The aligner system technology offers an orthodontic treatment option with transparent thermoplastic polymers for patients who prioritize aesthetics. To maximize their therapeutic usefulness, it is crucial to understand how their mechanical properties alter with time.
Methods: The 4mm x 10mm common dimensions for aligner material testing were used to create specimens of four brands: Duran+ (0.75mm PETG), CA® Pro (0.75mm 3-layer copolyester and elastomer blend), Zendura A (0.76 mm PU) and Zendura FLX (0.76 3-layer PU). Each brand's 24 samples were split into three groups as follows: G1 being thermoformed, G2 being thermoformed and underwent 500 thermocycles (simulating 7 days), and G3 being thermoformed and underwent 1000 thermocycles (simulating 14 days). Three-point bend test was performed to investagate Young's modulus. The samples were submerged in distilled water for 24 hours prior to thermocycling.
Results: Duran+ and Zendura A had the highest modulus of elasticity values in the control group, and they differed statistically significant from Zendura FLX, which had the lowest values. After 500 thermocycles, Zendura FLX and CA Pro showed the lowest Young's modulus (171.66 MPa and 109.41 Mpa) and were significantly different from single-layer PETG (Duran+) and PU (Zendura A). While after 1000 thermocycles, Duran+ had the highest modulus of elasticity and differed statistically from all other groups. The values from highest to lowest were: Duran+ > Zendura A > CA Pro > Zendura FLX). Intragroup comparison showed that only Duran+’s elastic modulus significant changed after 1000 thermocycles in comparison with control group.
Conclusions: One-layer PETG failed to demonstrate stability after in vitro aging, suggesting clinicians should be aware of the change in mechanical properties when using onelayer PETG in 2 weeks regime.