Effects of simulated intraoral temperatures and wet environments on the stress relaxation properties of thermoplastic aligner materials.

Introduction: Thermoplastic aligner materials are made from copolymers, and in the oral environment, their mechanical properties change over time. The effects of intraoral temperatures and the wet environments on the stress relaxation properties of these materials remain poorly understood. The aim of this study is to investigate the separate effects of the temperature and wet environment on the stress relaxation behavior of five available commercial orthodontic thermoplastic materials consisting of three chemical compositions.

Method: A modified temperature-controlled water bath system was used to eliminate the confounding effect of water. The residual stresses of five commercial orthodontic thermoplastic materials with different chemical compositions (Biolon, Duran, and Erkodur (PETG), Essix ACE (copolyester), and Essix C + (PP/PE)) were examined at room temperature (22 °C), 37 °C, and 55 °C. After the materials were immersed in deionized water and artificial saliva for two weeks (37 °C), the 30 min stress relaxation curves of the five materials were measured.

Results: Compared with those at room temperature (22 °C), the stress relaxation rates of the five materials increased and ranged from 0.7% to 18.11% at 37 °C and from 20.54% to 88.31% at 55 °C, and Ekodur and Essix ACEs exhibited relatively smaller increases. After two weeks of immersion in deionized water and artificial saliva, the stress relaxation rate of Essix ACE significantly decreased (p < 0.05), whereas that of the other four materials did not significantly change.

Conclusion: Elevated intraoral temperature accelerated the stress relaxation of thermoplastic aligner materials. The intraoral liquid immersion had no accelerating effect on the stress relaxation of any of the tested materials and even had a significant decelerating effect on that of Essix ACE.