The discoloration of restorative materials occurs due to multifactorial reasons, such as the DC, titratable acidity, food colorant absorption, and penetration [1, 35]. The dental substrate and surface properties of the materialscan also be included, as well as the type of polishing performed [5, 8]. In the present study, for both CIELab and CIEDE2000 equations, the S-PRG-based nanohybrid composite showed the highest values for color alteration, followed by nanofilled, microhybrid, and nanohybrid, which presented the lowest ΔE values. Therefore, the first null hypothesis was rejected.
The mechanism of action of S-PRG fillers depends on the diffusion of water [40], which requires higher amounts of hydrophilic monomers and higher water sorption than materials without this technology [24–28]. Previous studies have shown a positive correlation between water sorption and staining [24, 29], which could justify the higher staining observed for the S-PRG-based composites. Nonetheless, as water is absorbed, expansion and plasticization of the resin and hydrolysis of the silane bonds occur, which can generate microcracks and the release of residual monomers [10]. This could also favor staining and compromise the esthetic appearance of the restoration [31, 35]. Moreover, as silane bonds are degraded, the filler particles of the resin can start to detach from the organic matrix, contributing to the increase in their staining potential due to increased roughness and pigment retention [31, 35, 40].
Studies have shown that the solubility of BisGMA-based composites increases when associated with triethylene glycol dimethacrylate (TEGDMA) and decreases with urethane dimethacrylate (UDMA) [24, 32]. These findings could justify the results of the present study, in which greater staining was seen for the BisGMA and TEGDMA-based composites (S-PRG-based and nanofilled). The high-water sorption promoted by the association of BisGMA and TEGDMA, combined with the mechanism of S-PRG fillers, could justify the staining of the materials shown in this study. The manufacturers did not specify the dimethacrylates of the tested nanohybrid and microhybrid resins, except for the UDMA diluent used in the microhybrid composite. The presence of UDMA in the organic matrix could support the higher resistance to staining of the microhybrid resin, but the lack of this information prevents a more robust interpretation of the results.
In this case, limitations can also reach other properties, and thus, the balance among the main properties needs to be considered to indicate each material in more appropriate situations [3, 4].
The color stability of resin composites has also been strongly associated with the DC of these materials [34]. In this study, S-PRG-based nanohybrid and microhybrid materials presented the lowest DC, while nanohybrid and nanofilled materials showed the best percentages of monomer-to-polymer conversion. Therefore, the second null hypothesis is rejected. Incomplete polymerization generates a greater quantity of residual monomers, which are easily degraded, resulting in greater susceptibility to pigmentation [36].
Differences in the organic matrix composition of methacrylate, size, type, and volume of particles can affect the depth of polymerization and light scattering, and consequently, the DC [33]. Hence, the volume and size of fillers in the composition of the S-PRG-based resin composite used in this study could have influenced the DC results. Ilie and Fleming [37] compared different materials, including one with S-PRG fillers similar to this study, and explained its poor performance due the presence of the filler [38]. This technology shows larger particles when compared with other technologies, which is associated with more filler percentage and can impair the light penetration in deep layers, decreasing the DC [37]. S-PRG nanohybrid presents a percentage of inorganic fillers of 83.3wt% against 73.0wt% of microhybrid and 78.5wt% of nanofilled. The percentage of the nanohybrid resin was not provided by the manufacturer.
Additionally, it is known that TEGDMA increases the DC [38], which could have favored the nanofilled resin and explained the best values. The similar performance of the S-PRG nanohybrid, which also contains the TEGDMA monomer, can be limited by these fillers, as mentioned above. Nevertheless, the presence of TEGDMA can impair the mechanical properties [38] because the monomers used in the formulation of these composites are strongly related to the staining potential and DC of the material, future studies addressing the physical-chemical properties of these resins should be conducted.
Therefore, based on the analyses in the present study, microhybrid and nanohybrid resin composites could be used as an excellent esthetic treatment alternative. In contrast, S-PRG-based nanohybrid resins should be indicated in precise situations and should be followed up more frequently to guarantee proper longevity. However, as reported by other studies, the clinical circumstances, such as diary brushing mode [9] and polishing protocol accomplished [11], are parameters that may also play important roles and should be analyzed in future studies, particularly because repolishing can re-establish the original color of the restorative material, reaching clinically acceptable levels for esthetic longevity [11].