Creating an aesthetic anterior composite restoration has been an exacting challenge for a long time because of the limitations of many materials that affect either shade integration or surface quality and probably color stability. In addition to the drawbacks in technology, a lack of predictability and certain complexity in the clinical application were inherent to the technique and produced its elitist for a long time. Universal shade composites claimed to have a breakthrough in dentistry that has an impact on treating all these problems.16
Universal shade composites are based mainly on structural color phenomena. Structure colors are the result of the fundamental optical processes of interference, scattering or diffraction. Subsequently, unlike traditional pigmented color that comes from the light absorption of pigments, structural color claimed to be more adequate and stable. In this study, two omni-chromatic universal shade composites were chosen with different organic matrix structures, as the matching between refractive indexes between the organic matrix and the fillers is one of the fundamental issues to achieve structural color.17 Moreover, the OBC used some pigments in its structure, while RBC color was purely based on the structure color concept.
In this study, the employed methodology was in accordance with previous studies. Every effort made to ensure the standardization of the methodology, as well as all the steps, was performed by a single operator. The extracted human teeth were used as the studied materials to gain their color by induction from the surroundings. In addition, natural teeth have different optical properties, so they were used to reveal the clinical conditions. The series of Sof-lex polishing discs was the system of choice. Aluminum oxide discs have been suggested as a standard protocol because of their capability to form smooth, nondestructive polished surfaces that are less susceptible to chemical solubility.18 The studied liquids were chosen as colorant agents because of their constant consumption in daily life. A four-week immersion period was chosen, which is equal to approximately 2.5 years of clinical aging (24 h of staining in vitro corresponds to about 1 month in vivo, and 3,500 thermal cycles were performed to mimic the oral environment during this period).12
Spectrophotometry and the CIE L*a*b* coordinate systems are recommended methods for dental purposes. The CIE L*a*b* coordinate system was selected to evaluate the color variation (ΔE) because it is well suited to determine the small changes in color and has advantages such as objectivity and sensitivity. Additionally, it enhances the reliability of shade matching and shade communication.19 A noncontact digital profilometer microscope was used because of its ability to scan the surface with a type of laser and provide a 3D surface map without damaging the specimens, thereby proving to be a fast and easy evaluation method.20 The surface roughness over the roughness threshold (Ra = 0.2 µm) causes a simultaneous increase in biofilm accumulation, and no further decrease in bacterial adhesion could be observed under the threshold value. 15 Smooth surfaces add to the comfort of the patient as a surface roughness change of 0.3 µm can be identified by the tip of the tongue.21
Based on this study’s results, there were no significant differences at baseline color measurements between OBCs and RBCs. The ∆E values for OBC were 3.2 and 3.1 for RBC, which are considered clinically acceptable. This finding may be attributed to the unique pure silicate technology of OBC restoration, as manufacturers claimed that its nanoparticulate amplifies the chameleon effect, further reinforcing its ability to blend and adapt to the surrounding tooth structure because it is smaller than the wavelengths of visible light. Therefore, its nanoparticulate neither diffracts nor refracts light, but allows the light to pass through uninterrupted and bounce off the surrounding tooth structure.
Moreover, the smart chromatic technology of RBCs, a unique technology based on fillers (uniform supra-nano spherical fillers and round fillers, fabricated with zirconium dioxide and silicon dioxide) that are claimed to generate red to yellow structural color, as light passes through the fillers, reflects the red to yellow range of colors found in all the teeth. These colors then combine with the surrounding tooth color, thus permitting the unprecedented ability of color matching. Consequently, the cured composite blends with the surrounding tooth structure.
These results agreed with those of Bakti I et al., 22 who concluded that nanofilled RBCs exhibit a chameleon effect whereby they can adjust their color to suit that of their surroundings. They also concluded that the chameleon effect has a limitation in its color adjustment. Additionally, Abdel Rouf R et al 23 assessed the visual color matching and blending effect of universal RBCs and concluded that universal composites showed an acceptable color matching, but it may not be the optimal selection when esthetic is the patient’s prime concern. These results may occur due to the differences in the materials used. In contrast, de Abreu JLB et al 24 and Iyer RS et al 25 reported that the color matching of single-shade composite is inferior to that of multishade composite, which may limit their clinical use in the cases of high esthetic demand.
After the staining process, the null hypothesis stating that color matching measurements would not be affected after storage was rejected, as all the tested groups of restorations showed a significant increase in ∆E, which was clinically unacceptable. In OBC (mean ∆E for saliva = 8.14 ± 1.14 < for tea = 10.37 ± 1.13 < for cola = 13.19 ± 1.70) which were significantly higher than those for RBC, the mean ∆E for saliva = 4.87 ± .77 < for tea = 7.62 ± 1.17 < for cola = 9.87 ± .79). Cola subgroups had the highest significant difference.
Color changes can be attributed to the combination of matrix degradation by acids, penetration/absorption of colorants into the material as well as the surface adhesion/adsorption of colorant.26 Immersion in saliva may lead to a yellowish color for restoration because it contains mucin.27 Additionally tea is rich in tannins, which promotes yellowish staining as it enhances the chromogens’ ability to adhere to the materials’ surfaces; moreover, immersion in tea increases the surface roughness, hence causing further stain updates.28 Studies have shown that black tea and tannin-containing compounds cause chemical reactions due to the presence of denaturing factors that lead to stable discoloration.29 Cola is a yellow-brown carbonated beverage that causes staining due to sulfite ammonia caramel. It also has a decolorizing effect that affects the sorption and solubility of RBC material.30, 31
Additionally, the effect of thermocycling, is a combination of thermal and hydrolytic degradation, and is considered a method that simulates temperature-related breakdown by sudden repeated changes in temperature, thereby affecting the durability of the material. Water absorption impacts the mechanical characteristics of composites toward hydrolytic degradation. It can also lead to microfractures in the interface between the resin matrix and the fillers and induce superficial stress due to high temperature gradient differences, which are close to the surface and affect its roughness and the ability to gain stains.32
These results were in accordance with those of Reddy PS, et al, 33 and Ozkanoglu S et al,34 who reported that in vitro staining affects the color match of esthetic restorations. They also concluded that the staining intensity of cola is greater than that of tea. Additionally, Pordan et al, 35 assumed that specimens immersed in saliva exhibited color changes compared to baseline, and these changes were attributed to the water sorption characteristics of the restorations. In contrast, the results of previous studies were not in agreement with this study, 29, 30 that is; tea had a higher staining ability than that of cola.
There were no significant differences in surface roughness between the two studied composites either immediately or after storage is accepted, as the average Ra for OBC was 0.253 and 0.254 for RBC which is considered clinically acceptable in terms of bacterial adhesion and patient comfort. These results may be attributed to the fact that the manufacturing of the nanoparticles in both materials was the same which is called the sol-gel process. This process is a controlled reaction between different chemistries that results in the creation and growth of uniform nanospheres (nanoparticles) that are harvested once they grow to the desired diametrical size, (the nanoparticles’ diameter in OBC = 20–40 nanometers), whereas RBC fillers were based on their own patented “Sub-Micro-Pearl-Technology”. In this process, the Sol-Gel method is used to progressively coat the spherical fillers in an organic solution. After several weeks, the fillers have “grown” evenly in a spherical shape and are exactly 0.26 µm in size. This feature results in a highly smooth polished surface.13
This result was in accordance with that of Cunha LG et al and Gurbuz A et al, 36, 37 who concluded that OBC did not present significant differences compared to the surface roughness of the conventional composites because of the comparable filler size and load between them. However, Tagtekin DA et al,38 concluded that ormocer had a higher surface roughness than conventional hybrid RBC as the filler particles in the used ormocer are harder than the matrix, causing preferential loss through finishing and polishing, as well as leaving the filler phase in a positive surface and causing more surface roughness.
After staining, there was no significant difference between the control groups (distilled water) and those stored in saliva; however tea and cola caused a significant increase in the surface roughness of both composites. These results were in agreement with previous studies39 and may be ascribed to the chemical erosion from tea as it contains oxalic, malic, and citric acid with a pH (value of 5.4), which is acidic in nature. Additionally, cola has a low pH (value of 2.5) that influences the surface integrity of the RBC, thereby leading to an increasing surface roughness. The lower pH of cola with respect to tea can explain the significant difference between their RBC results. 33, 40
All clinical conditions are difficult to replicate with high precision in a laboratory study; subsequently, further clinical investigations are still necessary to predict the acceptability and longevity of universal shade composite restorations.