Does additional light curing improve the color stability of resin composites?

Aims: The influence of additional light curing protocols on the color stability (ΔE₀₀) of single-shade resin composites following coffee exposure was evaluated in comparison with a conventional universal composite at baseline, 7, and 30 days. Methods: Five resin-based composites were tested: four single-shade composites (Omnichroma, Vittra APS Unique, Zenchroma, and Charisma Diamond ONE) and one conventional universal composite (Filtek Universal Restorative). A total of 150 disc-shaped specimens (8 mm diameter×2 mm thickness) were prepared (n=10 per group). Three light-curing protocols were applied: 20 s (control), 20 s+10 s additional curing, and 20 s+20 s additional curing. After finishing and polishing, specimens were maintained in distilled water for 24 hours before baseline color evaluation. The specimens were then subjected to coffee immersion, and color measurements were recorded at baseline, 7, and 30 days using a spectrophotometer (VITA Easyshade V). Color differences were determined using the CIEDE2000 (ΔE₀₀) formula. Data were analyzed using generalized estimating equations (GEE) with Bonferroni-adjusted post-hoc testing (α=0.05). Results: Material type, curing protocol, and immersion time significantly affected ΔE₀₀ values (p<0.001). The lowest overall mean ΔE₀₀ value was observed for Omnichroma (2.01), whereas the highest value was recorded for Filtek Universal Restorative (2.95). Additional light curing significantly reduced discoloration, with the lowest ΔE₀₀ values observed in the 20 s+20 s group (1.89) and the highest in the 20 s group (2.99). Immersion time significantly increased color change, with the highest ΔE₀₀ values observed after 30 days (3.01). No significant interaction was found between material and curing protocol (p=0.150), whereas significant interactions were observed between material and immersion time and between curing protocol and immersion time (p<0.001). Conclusion: Additional light curing significantly improved the color stability of resin composites after coffee staining. Extended curing protocols reduced discoloration, although the magnitude of this effect may vary depending on the composite material. Among the tested materials, single-shade composites generally exhibited lower color change values than the conventional universal composite.