Integrated Aesthetic Longevity Assessment of Structural-Color and Nanohybrid Composites: A Multi-Factorial Analysis Under Chemo-Mechanical Stress

Achieving predictable long-term aesthetic stability for direct composite restorations is crucial, yet challenging, due to continuous exposure to mechanical and chemical stresses in the oral environment. Monochromatic or "structural-color" composites (e.g., Omnichroma, Vittra APS Unique) have been developed to simplify shade selection by blending seamlessly with tooth structure. However, their comparative structural and optical stability against conventional multi-shade nanofilled composites (e.g., Filtek Z350XT, Tetric N-Ceram, Estelite Sigma Quick) under clinical conditions remains a primary concern. This study provides an integrated, multi-factorial assessment of aesthetic longevity, moving beyond initial shade matching to investigate material performance under simulated chemo-mechanical stress. The research encompassed multiple in vitro analyses involving the evaluation of color stability (Δ E, Δ E₀₀), surface roughness (Ra), and microhardness (VHN). Specimens from various single-shade and multi-shade composite materials were subjected to: 1) staining by highly pigmented and acidic media, including coffee, tea, kombucha, turmeric, cola, and orange juice 2) accelerated aging via thermocycling (up to 5000 cycles) and 3) the effects of different polishing systems (e.g., Sof-Lex, Twist Dia).Data was analyzed using spectrophotometry (CIE L*a*b, CIEDE2000), profilometry, Vickers testing, and appropriate statistical methods (ANOVA, t-tests). All tested composites exhibited clinically unacceptable discoloration (Δ E > 3.3$ or Δ E₀₀ > 1.8) after prolonged exposure to staining media and aging, rejecting the null hypothesis of acceptable stability. Turmeric consistently demonstrated the most detrimental staining effect compared to coffee and kombucha. Furthermore, thermocycling caused a significant reduction in Vickers microhardness for both universal-shade and conventional materials. While certain structural-color composites (e.g., Omnichroma) showed low roughness values, material performance remained inconsistent and dependent on the specific combination of chemo-mechanical challenges. These results emphasize that the long-term aesthetic longevity of both universal and conventional composites is highly vulnerable to dietary habits and necessitates regular patient recall for professional maintenance and re-polishing.