The Stress Relaxation Properties Of Orthodontic Thermoplastic Materials

The effect of intraoral temperatures and liquid environment on the stress relaxation properties of Orthodontic thermoplastic materials remain inadequately understood. This study aims to investigate the separate effects of temperatures and liquid environments on the stress relaxation behavior of five available commercial orthodontic thermoplastic materials, composed of three chemical compositions. A modified temperature-controlled water bath system was used to eliminate the confounding effect of water. The residual stress of five commercial orthodontic thermoplastic materials with different chemical compositions—Biolon, Duran, and Erkodur (PETG), Essix ACE (copolyester), and Essix C+ (PP/PE)—were tested at room temperature (22°C), 37°C, and 55°C. After immersing the materials in deionized water, artificial saliva for two weeks (37 ℃), the 30min-stress relaxation curve of the five materials was measured. Compared to room temperature (22°C), the stress relaxation rate of five materials was increased, ranging from 0.7% to 18.11% at 37°C and from 20.54% to 88.31% at 55°C, with Ekodur and Essix ACE exhibiting relatively smaller increases. After two weeks immersion in deionized water and artificial saliva, the stress relaxation rate of Essix ACE showed a significant decrease (p<0.05), and that of the other four materials had no significant change. The elevated intraoral temperature accelerates the stress relaxation of the orthodontic thermoplastic materials. The intraoral liquid immersion has no accelerating effect on stress relaxation of all tested materials, and even has a significant decelerating effect on that of Essix ACE.