Impact of Pre-Polymerization Thermal Modification on the Optical Resistance of Anterior Composites Against Thermal Cycling and Coffee Staining: An In Vitro Study

Color instability in anterior composite restorations remains a primary cause for clinical replacement. While pre-polymerization thermal modification is increasingly utilized in daily practice, its precise impact on phase-specific and long-term optical resistance against continuous thermal aging and aggressive dietary staining remains unclarified. This in vitro study evaluated the impact of pre-polymerization thermal modification (4 °C, 23 °C, and 55 °C) on the phase-specific and cumulative color stability (ΔE00) of four anterior composite resins subjected to sequential thermal aging and prolonged coffee immersion. One hundred twenty specimens (n = 10) were prepared, and ΔE00 was assessed using the CIEDE2000 formula at baseline (T0), post-thermal cycling (T1), and post-coffee immersion (T2). Mixed repeated-measures ANOVA revealed a significant three-way interaction (phase × material × temperature, p < 0.001), demonstrating that thermal conditioning effects vary by material formulation and aging dynamics. During thermal aging (ΔE00 T0–T1), pre-polymerization cooling (4 °C) induced significantly higher discoloration in Estelite Sigma Quick compared to preheated conditions (p < 0.001). Following coffee immersion (ΔE00 T1–T2), refrigeration at 4 °C significantly increased staining in Enamel Plus HRI (p = 0.026) and Estelite Sigma Quick (p = 0.007) compared to room temperature and preheated groups. For cumulative color change (ΔE00 T0–T2), the material type was the primary determinant (p < 0.001), while the independent effect of temperature was not statistically significant (p = 0.395). In conclusion, preheating (55 °C) provides no significant cumulative advantage against staining, whereas refrigeration (4 °C) increases susceptibility to physical and chemical discoloration. The intrinsic chemical composition remains the critical factor driving long-term color stability.