Color stability of nanohybrid and microhybrid composites after immersion in common coloring beverages at different times: a laboratory study

Resin composites are widely used due to esthetics and bonding capabilities but are prone to discoloration, a common reason for restoration replacement. Discoloration mechanisms involve resin matrix hydrophilicity, water sorption, filler size/distribution, resin composition, and photoinitiator system. Extrinsic factors include dietary habits (coffee, tea), smoking, and qat chewing—a prevalent Yemeni habit that can stain teeth and restorations. Traditional Yemeni beverages such as coffee and qishr are also widely consumed. There is limited local data on the effects of Yemeni beverages and qat on composite color stability. This study aimed to assess the effects of these staining agents on microhybrid (Te-Econom Plus) and nanohybrid (Tetric N-Ceram) composites over up to 1 week, measuring ΔE and solution pH at specified intervals. The null hypothesis was that beverage type, immersion time, and pH would not affect color stability of the composites.

Prior studies indicate composite color stability is influenced by material type, storage time, and staining medium, with many composites showing unacceptable changes over time. Studies often report nanohybrids to be more color-stable than microhybrids upon exposure to coffee, tea, cola, and wine. Tannins in coffee and tea are implicated in discoloration. Acidity can affect surface roughness and degradation but does not always correlate directly with staining magnitude; the nature and strength of colorants also matter. There is a research gap regarding Yemeni beverages (Yemeni coffee, qishr) and qat on commonly used local composites, which this study addresses.

Design: Laboratory-based in vitro study of color stability in two direct restorative composites. Materials: Nanohybrid Tetric N-Ceram and microhybrid Te-Econom Plus (both shade A2). Specimens: 60 total disc-shaped specimens (30 per material), 10 mm diameter, 2 mm thickness. Preparation: Composite inserted into metallic molds, covered with nylon strip between two 1.5 mm glass slides, compressed (500 g, 20 s), LED light-cured (MaxCure 9; TURBO P1, 2300–2500 mW/cm²) with 1.5 mm glass spacer to standardize tip distance; device output checked with radiometer after every 10 cures. Post-cure: Surfaces further light-cured 1 s each side. Finishing/polishing: Brush, cup, and Polidont kit with aluminum oxide discs (medium, fine, superfine) with paste for 10 s per step at slow speed. Storage: Specimens stored in distilled water at 37 °C for 24 h to complete polymerization. Grouping: For each composite, six groups (n=5) were immersed in: qat solution, Yemeni coffee, qishr, red tea, Dilsi cola, or distilled water. Immersion conditions: Individual closed tubes, 37 °C; solutions stirred 3×/day at 30 rpm using orbital shaker; solutions renewed daily; lids closed to maintain carbonation. Immersion Media Preparation: Red tea: 2×2 g bags in 300 mL boiling mineral water. Yemeni coffee: 2 g in 100 mL boiling water. Qishr: 2 g husks in 100 mL boiling water for 10 min, no additives. Dilsi cola and distilled water used as received (100 mL). Qat solution: 20 g minced dried leaves/twigs in 500 mL mineral water, stirred 200 rpm for 5 h at 37 °C, filtered; prepared fresh daily to mimic chewing. Measurements: Time points at baseline (post-24 h water storage), and after 1, 3, and 7 days immersion. Color measurement: Portable colorimeter (PCE TCD100), white background, CIELAB L*, a*, b*; ΔE calculated as [(ΔL*)² + (Δa*)² + (Δb*)²]½. Two readings per specimen position averaged. pH: Measured with Hach HQ411D pH meter at each time point. Pilot Study: Conducted on six specimens per composite to familiarize with procedures and device. Statistical Analysis: Shapiro-Wilk for normality; reliability via paired sample t-test; three-way ANOVA to assess effects and interactions (composite type × medium × time); one-way ANOVA with Scheffé for multiple comparisons; significance at p < 0.05. Additional observations: Visual documentation after 168 h and surface roughness noted post-immersion in Dilsi cola.

- Normality confirmed (Shapiro-Wilk p > 0.05); high internal consistency (paired t-test reliability = 0.97). - Overall, Tetric N-Ceram (nanohybrid) exhibited less discoloration than Te-Econom Plus (microhybrid). - Ranking of discoloration (ΔE) across media for both composites: qat > coffee > red tea > qishr > Dilsi cola > distilled water. - Only qat, coffee, and red tea produced clinically perceptible color changes (ΔE > 3.3) at all measured times. - Qat produced the highest discoloration among all media; distilled water the least. - Significant three-way interaction (composite × staining medium × immersion time) by ANOVA (p < 0.05). - No significant difference in ΔE between 3 and 7 days for either composite when material and medium were fixed (p > 0.05), though discoloration increased over time from 24 h to 3 days and generally reached a plateau thereafter. - The b* axis (yellow-blue) was most affected; changes on a* and b* were more pronounced than L*. - Media pH were acidic (except water). Example pH values over time (before, 24 h, 3 d, 7 d): Yemeni coffee 6.4, 5.9, 5.7, 5.4; red tea 6.6, 6.1, 5.7, 5.8; Dilsi cola 3.1 to 2.8 (most acidic); water 7.1 (neutral); qat ~5.1. Solutions tended to become more acidic with immersion time. - Despite higher acidity, Dilsi cola produced relatively low ΔE due to weaker colorant (caramel) but caused notable surface roughness and physical changes on both composites. - Reported ΔE ranges: coffee 6.4–11.4 (yellowish), red tea 6.6–10.2; qat mean ΔE (AE) 18.1–28.0. - Qat and qishr media generated gas/foam; pronounced acidulous odor noted upon opening tubes prior to 24 h.

The study demonstrates that composite discoloration is influenced by composite type, staining medium, and immersion time. Nanohybrid Tetric N-Ceram was more color-stable than microhybrid Te-Econom Plus, consistent with reports that smaller filler particles, higher filler volume, and optimized resin matrices reduce water sorption and stain penetration. Qat and coffee exerted the strongest staining effects, attributed to potent chromophores (e.g., polyphenols/tannins) and potentially prolonged exposure patterns mimicked by daily fresh qat solutions. Although low pH can degrade resin surfaces and enhance roughness (as with cola), staining magnitude did not correlate strictly with acidity; the chemical nature and strength of the colorant played a larger role. Time significantly increased discoloration up to 3 days, with limited additional change by 7 days, indicating early rapid uptake followed by a plateau. The most affected chromatic parameter was b*, reflecting yellowish/greenish shifts typical of tannin-rich beverages and qat extracts. Surface degradation from acidic media may facilitate future discoloration even when colorant strength is low. These findings address the hypothesis by rejecting the null: beverage type and immersion time affected color stability, and pH exerted an indirect, colorant-dependent role.

Nanohybrid Tetric N-Ceram exhibited superior color stability compared to microhybrid Te-Econom Plus. Smaller filler particle sizes and higher filler content likely enhance resistance to discoloration. Qat caused the greatest and most undesirable discoloration, followed by coffee and red tea; qishr, Dilsi cola, and distilled water had progressively lesser effects. Dilsi cola, although highly acidic, primarily induced surface roughness rather than pronounced staining due to weaker colorants. Discoloration generally increased with time, especially within the first 3 days. Practical implication: material selection and patient counseling should consider local habits (qat chewing) and beverage consumption. Potential future research: simulate oral conditions with artificial saliva, thermocycling, and abrasion; evaluate a broader range of composites and beverage preparations/concentrations; include longer observation periods; compare Yemeni coffee/qishr with international variants; and assess the impact of surface treatments/sealants on stain resistance.

- Flat, polished disc specimens do not replicate clinical anatomy (grooves, pits, convex/concave surfaces) which can complicate polishing and alter staining patterns. - Beverage concentrations vary in real-life use; standardized preparations here provide only general estimates. - No artificial saliva, thermocycling, or abrasion were used to simulate oral environment and mechanical wear. - No comparisons between Yemeni coffee and other global coffee types; detailed variables (bean type, roast, processing, concentration, sugar/additives) were not controlled. These factors limit generalizability to clinical settings.