Degradation of Orthodontic Clear Aligners from Outdoor Weathering Exposure

Orthodontic clear aligners are used to straighten teeth, and after each tray is used it is disposed. Most plastics do not make it to recycling, and end up in either a landfill or as litter (Geyer et al., 2017). This begs the question; how long does it take for these materials to degrade when exposed to outdoor weathering? The purpose of this study is to determine the degradation rate of different aligner brands and how they differ from one another. Four orthodontic clear aligner materials (Zendura 0.75 mm (thin) and 1.0 mm (thick), Zendura FLX, Great Lakes Biocryl, and Essix Ace), n =12, were thermoformed, cut into rectangular test strips, and then exposed to outdoor weathering conditions for 2000 hours. Change in color, weight, mechanical properties, and surface roughness were recorded. All materials exhibited weight loss (p<0.05). Degradation rates and total degradation times were calculated for each of the materials, and only Zendura FLX had a significantly different degradation time when compared with the other aligner groups. The average total predicted degradation times were as follows: Essix Ace, 24.99 ± 1.34 years; Great Lakes Biocryl, 26.20 ± 1.33 years; Zendura Thin (0.75 mm), 28.95 ± 0.87 years; Zendura Thick (1.0 mm), 36.90 ± 0.86; Zendura FLX, 38.58 ± 0.77 years. This study gained insight into degradation rates for the selected aligners in the conditions set, and future studies could incorporate variations in climate, increased exposure times, in-vivo use of test strips prior to exposure, and exposure to soil.