Impact of various cleaning protocols on the physical and aesthetic properties of 3D-printed orthodontic aligners.

The surface similarity phenomenon describes how users apply maintenance habits from one material to another based on perceived similarities. Cleaning protocols for thermoformed aligners are often used for three-dimensional (3D) printed aligners, despite their different material properties. This study aimed to evaluate how such practices impact the physical and aesthetic properties of 3D-printed aligners. A total of 84 polyurethane-based specimens were fabricated using direct light processing. They were divided into three groups: brushing only, brushing with toothpaste, and immersion in an alkaline solution (Corega). Each protocol simulated two weeks of daily cleaning. Surface roughness was assessed via profilometry and AFM, hardness via the Ball Indentation Method, and color change via VITA Easyshade before and after cleaning. Water absorption was measured daily over 14 days during immersion in either Corega or saline. Statistical analyses included Wilcoxon signed-rank tests, paired t-tests, one-way ANOVA with LSD post-hoc tests, and Kruskal-Wallis tests with post-hoc Dunn test. Brushing with toothpaste caused the greatest increase in surface roughness (ΔSa = 37.45 nm), followed by brushing alone (ΔSa = 10.55 nm); Corega slightly reduced roughness (ΔSa = -4.86 nm). Surface hardness increased after brushing but decreased with Corega (p < 0.05). No significant ΔE color differences were found, though "b" values decreased with brushing (p = 0.012). Water absorption stabilized at 8.47% in saline, while Corega immersion led to peak absorption on day 8 before degradation (p < 0.001). Cleaning methods designed for thermoformed aligners can alter surface properties and degrade 3D-printed aligners. Tailored cleaning protocols are essential to preserve their integrity and functionality.