Effect of nanofilled self-adhesive protective coating on color changes and surface roughness of composite resin

Background: Discoloration of composite restorations may affecting aesthetic appearance. The aging-process and surface roughness may influence color changes. A nanofilled self-adhesive protecting coating has been developed for coating tooth restoration and expected to prolong the longevity of restoration. To evaluate the effect of nanofilled self-adhesive protective coating on the surface roughness and color-changes of flowable and packable composite after aging condition. Method: The total of 40 discs (15x2mm) and 60 boxes (20×10× 2mm) specimens from flowable and packable composite were used. A half of the specimens was coated by using protective-coating. The reflectance chromameter was used to measure the color baseline. Afterward, the specimens were subjected into aging process by immersing in artificial saliva and carbonated drink (37 o C,7d). The color changes were calculated based on the [CIE L*a*b*].For surface roughness, box-shaped specimens were measured in fresh condition, after aging conditions. Results: The color change ranged from 0.49 to 2.31. Applying protective-coating was associated with a significant decrease in the color-changes and surface roughness after aging. The changes of three color coordinates resulted in significant differences for both composite, with and without protective coating application. Conclusion: The application of protective-coating decrease the color-changes and surface roughness of flowable and packable composite-resin.


INTRODUCTION
Composite-resin (hereafter: composite) is the most widely used dental material due to its superior physico-mechanical and aesthetic properties.
However, because to the dynamic oral environment, composite may suffer and need to be repaired or replaced after being exposed to the oral cavity. 1 Esthetic failure of tooth restoration that is caused by discoloration is one of the major reason to replace the restoration. 1,2 As a result, the color stability of composite materials is critical, whether after curing or over the life of the restoration.
Discoloration of composite restoration were influenced by intrinsic and/or extrinsic factors.
Absorption of stains comes from food and beverage, incomplete polymerization, chemical reactivity, microorganism, oral hygiene, and restoration's surface topography are categorized as extrinsic factors. [3][4][5] While, intrinsic factors involve the interaction of filler, matrix or silane coupling agent. When the materials are aged under the dynamic oral situation, such as thermal changes, humidity, and ultraviolet exposure, the discoloration will be penetrated in the deeper portion. 6 The study of the aging process of the composite-resin is conducted by imitating a state in the cavity mouth through a number of in vitro assays, namely storage in saliva, [7][8][9] immersion in citric acid, carbonated drink and thermocycling. 10,11 or prone by using biofilm. 12 The aging process can lead to the degradation of composite and may affect the color stability. Water storage and immersion in citric acid resulted in hydrolysis and elution of filler particle and water resorbtion into matrix. 11,13 Carbonated beverages are a type of soft drink that typically contain additional acidity regulators, such as malic acid, citric acid, or phosphoric acid, as well as sweeteners and sugars. These drinks are known to have a significant impact on the risk of dental caries and the degradation of composite restoration compounds. 14 The degradation of composite restorations can be influenced by carbonated soft drinks through the mechanisms of acid attack and water sorption. The higher solubility of composite resin in acidic solutions may lead to surface erosion, which could potentially increase surface roughness and diminish the aesthetic quality of the restoration. 15 Matrix dissolution and the interface failure between matrix and filler particle may result in composite discoloration. 16 It was evaluated that chemical dissociation of the resin matrix itself and/or matrix-filler interface over time are reported to be the main causes of intrinsic discoloration. 17 In addition, the in vitro aging method enhanced the composite's roughness. 18 Matrix : bis-GMA, TEGDMA, and Procrylat K Filler : ytterbium trifluoride filler with a range of particle sizes from 0.1 to 5.0 µm, a non-agglomerated/nonaggregated surface-modified 20 nm silica filler, a nonagglomerated/ non-aggregated surfacemodified75nmsilicafiller, and surface-modified aggregated zirconia/silica cluster filler (comprised of 20 nm silica and 4 to 11 nm zirconia particles). The aggregate has an average cluster particle size of 0. The evaluation of color changes and surface roughness were evaluated prior to-and after aging conditions.

Evaluation of Color Changes
The chromameter were used to measure the color of all specimens, based on the Commission The colors were measure at the baselines (T0) and at a time interval of 7 days (T7) for the corresponding material. A tissue paper was used to wipe the specimens. The dry specimens were placed in the viewing port of the chromameter. L*, a* and b* values, where "L" namely white-black, "a" red-green, and "b" yellow-blue. The spectrophotometer was automatically calculated and recorded the mean values of Δ L*, Δ a*, Δ b* after three measurements. The color difference (ΔE) was calculated from the mean ΔL*, Δa*, Δb* values for each specimen using the following Formula (1) :

Evaluation of Surface Roughness
The roughness of composite surface readout was made over a distance of 5 mm with a cut-off of 0.8 mm, at a speed of 0.25 mm/s. Three measurements were taken at different sites on the specimen surface to calculate the mean of composite roughness (Ra).

Color Changes
The means and standard deviations for values of ΔE*ab of flowable and packable composite with and without nanofilled self-adhesive protective coating are shown in Table 2 and Figure 1.      The changes of composite surface roughness prior-to and after aging were shown in  and degree of conversion. 23,24 In this study, artificial saliva and carbonated drink were used to simulate the intra oral aging process. The water-sorption of resin-monomers resulting the different levels of color-stability. As highlighted by Rinastiti,et al. 19 the water lead to softening of the composite-resin surface due to the penetration of water, resulted in hydrolitic degradation of silane that is coated filler particles. Also, water penetration may cause the swelling of the matrix, resulted in the failure of adhesion between filler particle and matrix. This failure will reduce the hardness of composite matrix, whole the appearance of filler particle on the composite-resin's surface resulted the increasing of surface roughnesses. Instead of water effect, a low pH (2.52) of carbonated drink used in the study owing to the swelling of the matrix as well. 25 Moreover, it is also known that the TEGDMA in the matrix of resin composite is hydrophilic thus may increase the water sorbtion and softening the composite surface. 26 The color changes of flowable composites were founded greater than that packable composite. Analysis of quantitative color values achieved in this study presented that the ΔL, Δa* and Δb*color coordinates of flowable composite was more affected than that packable composite. This could be due to the composition of packable composite that is contained of UDMA that less hydrophilic compared to TEGDMA. Additionally, the size, distribution, and %w or %v of filler particles may affect the discoloration of the compositeresin. 27 Filler particle loading of flowable compositeresin (47 % vol) is lower than the packable composite-resin (60 % vol). This likely resulted in a greater matrix softening that increase the surface roughness so that the coloring substances contained in saliva and carbonated beverage will be trapped between the uneven surface and increase discoloration of composite-resin.

Surface Roughness
The surface roughnesses of composite types prior to-and after aging, except fresh flowable composite, were more than 0.02 μm. Previous studies have been reported that a higher surface roughness (> 0.2 μm) exhibit extensive plaque accumulation on dental materials and known as main contributor to the multifactorial discoloration of composite restorations. 28,29 Based on the results, the surface roughness of the aged packable composite was higher than the flowable one.
Previous study has confirmed that the smoother surface was correlated with the amount of resin. 30 The filler loading of flowable and packable composite in this study was 47 % vol and 60 % vol respectively.
Application of nanofilled self-adhesive protective coating reducing the color change and surface roughnesses of both composite-resin aged in saliva and carbonated drink. The coating material is composed of monomer and nanofilled particles distributed uniformly in the matrix. The coating on the surface of the composite-resin restoration may enhance degree of polymerization and reduce the degradation due to aging process. The previous study showed that the application of nanofilled selfadhesive protective coating on glass ionomer cement would inhibit the penetration of water better than varnish. 31 The protective coating application may inhibit the penetration of water into the composite, reducing the hydrolysis of the adhesion between matrix and filler particle thereby reducing composite degradation. As a result of those phenomena, composite color was more stable, and the surface was smoother even though the composite has aged. The above finding is consistent with such findings revealed that the thin layer of surface coating material might eliminate the irregular or defects surface of inadequately polished composite restorations. 29 In principle, after composite-resin is exposed to testing environment, the color difference will not be detected as long as the material is completely color stable or unstained by colorations (ΔE = 0).
Nonetheless, from the clinical point of view, when ΔE is >1, the color change is considered detectable to the naked eye, and when ΔE is ≤ 3.3, changes are clinically acceptable. 32 The color changes of composites after protective coating were reduced significantly. Even though the color changes of flowable composite still can be detected to the naked eye, it is clinically acceptable. These findings suggest that in general, the nanofilled self-adhesive protective coating may enhance the color stability of composite restoration. To convince the long-term effect of the protective coating agent, a further study is necessary to evaluate the adhesion between protective coating and the surface of compositeresin. Furthermore the degradation of the coating under various intra-oral conditions should be evaluated.

CONCLUSION
Within the limitations of this study, it can be concluded that the color changes and surface roughness of flowable and packable compositeresin after aging condition were decreased by applying nanofilled self-adhesive protective coating.