Comparison of stress relaxation properties between 3-dimensional printed and thermoformed orthodontic aligners: A pilot study of in vitro simulation of two consecutive 8-hours force application

The study is aimed to examine the stress relaxation of 3d-printed aligners, Tera Hartz TC-85 (Graphy Inc. Seoul, South Korea) and Polyamide (Noxi, Sweden and Martina, Due Carrare, Padova, Italy), in comparison to thermoformed aligners, Zendura FLX (Bay Materials LLC, Fremont, California, USA) and Duran (SCHEU, Iserlohn, Germany). A stress-relaxation test was conducted using a motorized vertical testing bench, TVO-S (AstraLab, Mariano Comense, Italy), applying a constant and controlled pre-set load. Each sample was subjected to a deflection of 0.5 mm while being immersed in water for eight consecutive h at a constant temperature of 37°C. During this period, data were collected at intervals of 1 s. The same test was repeated twice (test 1 and test 2) on the same specimen, in order to simulate the intermittence of deflection force. For both Test 1 and Test 2, a total of 28,800 measurements were recorded for each aligner, with one measurement taken per second over a duration of 8 h/test. A statistically significant difference (P < 0.001) was found for all materials and at all time periods were considered. If the percentage of stress relaxation is considered, a significant difference among the four aligners was found as well. All materials displayed substantial stress decay during the 8-h period of constant load, although significant differences were observed among the various materials under investigation. The Noxi aligner demonstrated the highest force values in both tests, confirmed by low percentages of stress relaxation ranging from 23% to 32%.