Impact of thermocycling on mechanical properties and discoloration of veneering composite resins after storage in various staining media.

Veneering composite resins (VCRs) are often used to veneer frameworks, but knowledge of mechanical, surface, and discoloring properties is scarce. The purpose of this in vitro study was to evaluate the impact of thermocycling on flexural strength (FS) and the influence of different staining media on the discoloration (ΔE) and surface free energy (SFE) of VCRs. The following VCRs were tested: Ceramage, dialog Vario, Gradia Plus, in:joy, Signum composite, and SR Nexco. FS was tested with enamel and dentin pastes (except SR Nexco: only enamel paste), whereas ΔE and SFE were analyzed for enamel pastes. ΔE was determined by using a spectrophotometer, and SFE was evaluated with contact angle measurements. For FS, rod-shaped specimens (N=660, n=10/subgroup; 25×2×2 mm) were fabricated, thermocycled for 220, 1500, 10 000, 20 000, and 40 000 cycles (5 °C/55 °C). and tested immediately after fabrication (initial) and after aging. For ΔE and SFE, disc-shaped specimens (N=300, n=10/subgroup; 15x15x1.4 mm) were fabricated and stored for 14 days in coffee, red wine, carrot juice, beetroot juice, or curry solution. ΔE and SFE were measured initially after polishing, after discoloration, and after repolishing. Data were analyzed using univariate analysis, 1-way ANOVA followed by the Scheffé post hoc test, Kolmogorov-Smirnov test, Kruskal-Wallis test, Mann-Whitney U test, and Wilcoxon test (α=.05). Ceramage had the highest FS, followed by dialog Vario and SR Nexco. Gradia Plus, and in:joy had the lowest FS, followed by Signum composite. FS was reduced by increasing of thermocycles. The highest ΔE was measured for curry and the lowest for carrot juice and red wine, followed by beetroot juice and coffee. The highest ΔE between the initial polished specimens and repolished specimens were observed for Ceramage. The remaining VCRs showed differences in ΔE between 0.95 (Signum composite) and 1.30 (SR Nexco). The SFE of the VCRs was similar. After storage in discoloring media, all VCRs had higher SFE than directly after polishing (initial). Repolishing decreased the SFE values compared with those of specimens measured after storage. FS, ΔE, and SFE differed between the VCRs tested. Discoloration of the VCRs depended on the food and beverage and could be corrected to a clinically acceptable range by repolishing.

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