Influence of Water Storage on the Mechanical Properties of 3D-Printed Aligners: An In Vitro Study

Directly printed aligners represent a promising alternative to conventional thermoformed aligners. The aim of this in vitro study was to compare the effects of water on the mechanical properties of directly printed aligners with those of conventionally manufactured thermoformed PET-G foils. Dental LT Clear V2 (LT), V Print Splint Comfort (VP), and TC-85 DAC (TC) were examined. Biolon (BL), a conventional PET-G material, served as the thermoplastic reference material. All samples were tested before and after 14 days of water storage at 37 °C. We performed a three-point bending test and an indentation test, and examined changes in the abrasion resistance and hygroscopic volume. The resistance of all printed specimens decreased significantly after water storage. VP and TC were less resilient than BL overall. LT and BL exhibited the lowest indentation creep (BL: 0.08 ± 0.01, LT: 0.13 ± 0.02, VP: 0.21 ± 0.02, TC: 0.24 ± 0.02). Furthermore, the abrasion of LT (0.72 ± 0.21 mm³) was significantly lower than that of BL (1.12 ± 0.37 mm³). In conclusion, the water sorption of the printed test specimens had a significant influence on the mechanical properties, with a reduction in the flexural modulus, Martens hardness, and plastic hardness.