Effect of natural and commercially produced juices on colour stability of microhybrid and nanohybrid composites

The study addresses the colour stability of resin composites, a critical factor for the aesthetic longevity of restorations. Despite improvements in composite technology, both intrinsic (e.g., resin matrix composition, incomplete polymerisation, photo-initiator/amine discoloration) and extrinsic factors (e.g., staining from dietary sources, water absorption, oral hygiene) can lead to colour changes. Given the widespread consumption of fruit juices and their potential to chemically interact with and stain resin surfaces, the research aims to compare the discoloration of microhybrid versus nanohybrid composites when exposed to natural and commercially produced orange and pomegranate juices, using instrumental colour measurement to eliminate visual bias.

Prior work has shown beverages like coffee, tea, cola, and sports drinks can cause varying degrees of staining in resin composites, influenced by beverage composition and pH. Studies (Al-Haj Ali et al.; Al Kheraif et al.; Bansal et al.) generally reported higher colour stability for microhybrid compared with nanohybrid/nanofilled composites after exposure to common drinks. Other studies reported contrary findings, with some indicating greater discoloration in microhybrids under certain conditions (Kumar et al.; Reddy et al.; Erdemir et al.). Factors implicated include filler type and size, resin matrix composition, water sorption, and surface characteristics post-finishing. Lower pH beverages tend to increase discoloration.

Design: Experimental laboratory study. Sample size: 45 specimens per composite determined by mean comparison formula (α=0.1, β=0.1) based on prior data; minimum calculated n=43, set to 45 for assurance. Materials: Microhybrid composite P4 (Kerr, Italy) and nanohybrid Filtek Z250XT (3M-ESPE, USA), shade A2. Specimen preparation: Disc-shaped samples (2 mm thickness, 10 mm diameter) made using cylindrical molds, pressed between glass slabs to ensure smooth surfaces and avoid oxygen-inhibited layers; packing to minimize bubbles; 5 kg load for 3 min; light-cured 60 s each side (total 120 s) with 550 mW/cm² device; finished/polished with silicon carbide paper discs (Sof-Lex, 3M-ESPE) to standardize surface smoothness; final thickness verified with caliper. Preconditioning: Stored in distilled water for 48 h to allow primary water absorption and further polymerisation. Grouping: For each composite, specimens randomly assigned to five subgroups (n=9): natural orange juice, commercially-produced orange juice (Sunich, Iran), natural pomegranate juice, commercially-produced pomegranate juice (Sunich, Iran), and distilled water (control). Immersion protocol: For 10 consecutive days, specimens immersed 4 h/day in the assigned beverage and 20 h/day in distilled water at 37 °C; beverages at ~4 °C, replaced daily; pH of each beverage measured prior to immersion; after each immersion, specimens rinsed and brushed with a soft toothbrush for 30 s. Colour measurement: Baseline and post-immersion colour measured with a reflective spectrophotometer using CIE L*a*b* coordinates. Colour difference computed as ΔE = [(L−L0)² + (a−a0)² + (b−b0)²]^(1/2). Data analysis: Descriptive statistics (mean, SD); assumptions checked with Kolmogorov–Smirnov (normality) and Levene’s test (homogeneity of variances); independent t-tests used for pairwise comparisons (e.g., natural vs commercially produced within beverage type; between composites within each beverage); LSD post hoc used for mean comparisons; analysis performed in SPSS 20.

- Microhybrid composite (P4): Mean ΔE after 10-day protocol—natural pomegranate juice: 4.79; commercially-produced pomegranate juice: 3.10; commercially-produced orange juice: 3.02; natural orange juice: 2.16; water: 1.79. No significant differences between natural vs commercially produced juices within pomegranate (P=0.088) or orange (P=0.447). - Nanohybrid composite (Filtek Z250XT): Mean ΔE—commercially-produced orange juice: 13.03; natural pomegranate juice: 8.54; commercially-produced pomegranate juice: 4.66; natural orange juice: 3.14; water: 1.27. Within-type comparisons: natural pomegranate > commercially-produced pomegranate (P=0.001); commercially-produced orange > natural orange (P=0.010). - Between composites (Table 3): Nanohybrid showed significantly higher ΔE than microhybrid in natural pomegranate juice (8.54 vs 4.79; P=0.006), commercially-produced pomegranate juice (4.66 vs 3.10; P=0.004), and commercially-produced orange juice (13.03 vs 3.02; P=0.010). No significant differences in natural orange juice (3.14 vs 2.16; P=0.079) or water (1.27 vs 1.79; P=0.100). - Highest discoloration observed: nanohybrid in commercially-produced orange juice (ΔE=13.03); microhybrid in natural pomegranate juice (ΔE=4.79). - Authors noted several conditions produced visually noticeable and clinically unacceptable discoloration (e.g., ΔE ≥ ~4–5) for nanohybrid in industrial orange, natural pomegranate, and industrial pomegranate juices, and for microhybrid in natural pomegranate juice.

Findings indicate microhybrid composite exhibits greater colour stability than the nanohybrid under exposure to tested fruit juices. Proposed reasons include differences in filler systems and resin matrices: microhybrids with larger glass-ceramic fillers and higher filler loading may have reduced resin matrix exposure and water sorption, lowering stain uptake. Nanohybrids include nanoclusters that may absorb more water and be more prone to pigment penetration; finishing may leave smaller voids in nanohybrids that facilitate staining. Beverage acidity correlated with staining: commercially-produced orange juice (pH≈3.7) and natural pomegranate juice (pH≈4.0) induced more discoloration than natural orange juice (pH≈5.3) or commercially-produced pomegranate juice (pH≈4.8). Results align with several prior studies reporting higher colour stability in microhybrids than nanohybrid/nanofilled materials, though conflicting reports exist, highlighting that staining outcomes depend on material formulation, finishing, exposure medium, and duration. Instrumental spectrophotometry minimized subjective bias, and standardized curing/finishing protocols were used to reduce confounders like incomplete polymerisation and surface roughness. Nevertheless, differences between in vitro conditions and the complex oral environment (saliva, enzymes, thermal cycling) may affect generalizability.

Microhybrid composite (P4, Kerr) demonstrated significantly better colour stability than nanohybrid composite (Filtek Z250XT, 3M-ESPE) under exposure to selected natural and commercially produced juices. Clinically, patients should be advised to avoid or limit highly staining beverages—particularly commercially-produced orange juice, natural pomegranate juice, and commercially-produced pomegranate juice—during the first few days after composite restorations. Future research should include clinical studies evaluating the effects of natural and commercially produced juices on colour stability of microhybrid and nanohybrid composites over longer periods and under oral conditions.

In vitro laboratory design may not replicate the complex oral environment (saliva dilution, enzymatic activity, fluctuating pH, and thermal changes). Only two composite materials and specific juices were tested over a 10-day protocol; long-term ageing, thermal cycling, and broader beverage types were not evaluated. Thus, findings should be extrapolated to clinical conditions with caution.