Effect of thermal cycling on the optical stability of clear aligner attachment composites: an in vitro study

The primary aim of this in vitro study was to evaluate the effect of thermal cycling on the color change (ΔE₀₀) and relative translucency parameter change (ΔRTP₀₀) of clear aligner attachment composites; the secondary aim was to assess changes in surface roughness. Three composites were investigated: GC Aligner Connect (GAC), Filtek Ultimate Universal Restorative (FUU), and Estelite Sigma Quick (ESQ). Specimens were standardized square prisms polymerized through thermoformed templates. For optical evaluation, 75 specimens were prepared ( n = 25 per group); 74 were analyzed after one ESQ specimen was excluded due to handling-related loss during thermal cycling. Color coordinates were measured using a spectrophotometer before and after 5,000 thermal cycles (5–55 °C). ΔE₀₀ and ΔRTP₀₀ were calculated using CIEDE2000-based formulas and interpreted against established perceptibility and acceptability thresholds (ΔE₀₀: 0.8/1.8; ΔRTP₀₀: 0.6/2.6). Surface roughness (72 additional specimens) was assessed by contact profilometry. ΔE₀₀ was analyzed using Welch’s ANOVA and Games–Howell tests; relative translucency parameter (RTP₀₀) using mixed-design repeated-measures ANOVA; surface roughness using Wilcoxon signed-rank and Kruskal–Wallis tests; and threshold-category distributions using the Fisher–Freeman–Halton exact test. ΔE₀₀ differed significantly among composites, with GAC showing the highest value, followed by ESQ and FUU; all pairwise differences were statistically significant. A significant time × composite interaction was observed for RTP₀₀, which decreased significantly only in GAC; GAC nonetheless differed significantly from FUU and ESQ in overall translucency and remained the most translucent material at both time points. Specimen distribution across perceptibility/acceptability categories differed significantly among materials, with unacceptable changes concentrated in GAC. Surface roughness increased significantly only in FUU, without significant differences in the magnitude of change among materials. Thermal cycling affected the optical and surface properties of the composites in a material-dependent manner. FUU and ESQ showed greater color and translucency stability than GAC, whereas GAC retained the highest translucency overall; FUU, in turn, was the only material with a significant increase in surface roughness. No material was therefore superior across all properties, indicating that composite selection for clear aligner attachments should balance optical stability, translucency, and surface characteristics.