Long-term colour stability of tooth-coloured restorative materials is important not only in terms of aesthetics but also in terms of reducing the additional costs of treatment, which is related to frequent replacement of dental restorations. Colour stability of restorations during their functional life is critical to the acceptability of restoration. Discoloration in dental composites is due to multiple reasons and depends on inherent factors such as chemical changes in materials (resin matrix filler particles) and particle–matrix boundaries, and external factors such as adsorption or absorption of stains, diet, smoking habits, and water absorption of resin monomers . One of the problems we may encounter when using composites is their lack of complete polymerisation due to insufficient light intensity and insufficient light exposure time. Composites that are not fully polymerised have more water absorption and solubility, which is evident in the clinic as early colour instability . In this study, the polymerisation time of all specimens was sufficient and equal. When the composite surface is cured against a transparent celluloid strip and subsequently it is not finished, a resin-rich surface is formed, which due to low physical properties, the surface layer undergoes discoloration more than the finished surfaces; however, a finished surface creates a filler-rich surface with higher Knoop hardness values and less prone to chemical solubility [1, 15]. Therefore, all specimens were finished in a standard and uniform form. When the composite is adjacent to liquids, most of the water absorption occurs by organic polymer matrix in the first 4 days and the highest water absorption occurs during the first week . Composite resins that can absorb water are able to absorb other liquids with pigments that lead to discoloration. It is assumed that water acts as a means of penetrating the dye into the resin matrix . Because colour is a physical-psychological phenomenon that varies from person to person and even in a person at different times, measuring it with precision instruments eliminates the subjective errors of evaluation. Therefore, in this study, colour measurement was performed using a reflective spectrophotometer, the accuracy of which has been confirmed in various studies [1, 17]. In the present study, common natural and commercially-produced juices including natural and industrial orange juice, natural and commercially-produced pomegranate juice, and distilled water (control group) were used; due to the fact that the highest absorption rate occurs during the first 7–10 days, the period for determining the colour of the specimens was determined 10 days after the specimens were placed in the solutions.
The findings of this study showed that microhybrid composite in natural pomegranate juice solution had an unacceptable discoloration (∆E = 4.79) and nanohybrid composite in three solutions of industrial orange juice (∆E = 13.03), natural pomegranate juice (∆E = 8.54), and industrial pomegranate juice (∆E = 4.66) had unacceptable discoloration, which was visually noticeable. Discoloration rate of nanohybrid composite exposed to commercially-produced orange juice, natural pomegranate juice, and commercially-produced pomegranate juice was significantly higher than microhybrid composite (P < 0.01) and there was no significant difference between mean discoloration of microhybrid and nanohybrid composites exposed to natural orange juice and water (P > 0.05). In this regard, studies have examined the effect of different beverages on the colour stability of composites.
Al-Haj Ali et al. examined the effect of common soft drinks (iced tea, sports drink, orange juice, Cola, and distilled water) on the colour stability of microhybrid composites and nanocomposites. The results showed that microhybrid composite has higher colour stability in all soft drinks . Kheraif et al. examined the effect of coffee, tea, cola, and distilled water on colour stability and conversion degree of nano- and microhybrid composites. The results showed that nanohybrid composites with a high conversion degree had the lowest colour stability and had a significant discoloration compared to microhybrid composites . Bansal et al. examined the effect of alcoholic and non-alcoholic beverages on the colour stability of nanofield and microhybrid composites. The results showed that microhybrid composite has higher colour stability in different beverages . The results of these studies are consistent with the current study and suggest that microhybrid composite has higher colour stability than nanohybrid composite.
Effective factors on the sensitivity of composites to discoloration include filler type, resin type, and staining agent. Microhybrid composite is a glass-ceramic composite with different sizes and distributions of filler particles. This composite has 77% by weight of microfillers and very fine particles whose size is about 0.05 μm. This glass structure must provide optimal stability. It has also been reported that increasing the particle size causes less discoloration due to the reduced matrix filler . Nanohybrid composites, on the other hand, contain agglomerate particles called nanoclusters. These particles are less resistant to discoloration than silicon-zirconia micron-sized fillers in microhybrid composite, which can be due to their high water absorption properties .
It has been reported that smaller filler particles are removed during polishing and finishing operations in nanohybrids, and small voids remain at the surface of the restorative material compared to microhybrids. This advantage of nanohybrids does not seem to make them resistant to staining . On the other hand, some studies contradict the results of the current study.
Kumar et al. stated that the colour stability of microhybrid composite after 24 and 48 h of exposure to red wine and cola is less than that of nanohybrid composite . Reddy et al. examined the effect of cola, coffee, and tea on the colour stability of nano, microhybrid, and hybrid resin composites. The results showed nanofilled composites have less colour change than microhybrid and hybrid composite resin . Erdemir et al. examined the effect of sports drinks on the colour stability of microhybrid and nanofilled composites after 1 month and 6 months. The results showed that the highest discoloration occurred during 1 month in microhybrid composite and the lowest discoloration occurred during 6 months in nanofilled composite . Contrary to previous studies, some scientists believe that due to the large size and high stiffness of filler particles in microhybrid composites, the resin matrix tends to wear harder than fillers, and this clinically leads to rough surfaces after finishing, restoration wear, after long-term function and increased sensitivity to discoloration in microhybrids .
In both groups of microhybrid and nanohybrid composites, there was a logical relationship between pH of natural and industrial juices and discoloration of the composites, in the way that commercially-produced orange juice with pH = 3.7 and natural pomegranate juice with pH = 4 compared to natural orange juice (pH = 5.3) and commercially-produced pomegranate juice (pH = 4.8) lead to more discoloration in both groups. Bansal et al. also found a logical relationship between pH and discoloration of composites. Coca-Cola, with the lowest pH (pH = 1.57) among beverages, led to the highest discoloration in composites . In the end, it is worth mentioning that the oral cavity is a complex environment in which several factors are involved. Saliva dilutes the solutions used and changes their pH; in addition, it contains various enzymes and effective salts. Composites are exposed to a wide range of thermal changes following consumption of hot and cold foods and beverages, and their physical properties change over time [1, 17].