Six-month color change and water sorption of 9 new-generation flowable composites in 6 staining solutions

Color match and water sorption are two factors that affect restorative materials. Discoloration is essential in the lifespan of restorations. The aim of this study was to evaluate color change and water sorption of nine flowable composites at multiple time points over 6 months. 60 samples of each composite were divided into two groups (Color Change and Water Sorption/Solubility). Each Color Change group was divided into six subgroups, which were immersed in distilled water (DW), coffee (CF), Coca-Cola (CC), red wine (RW), tea (TE) and orange juice (OJ). The color was measured at the baseline, 1, 2, 3 and 4 weeks, and 3 and 6 months and color change values (ΔE) were calculated. Each Water Sorption [WS]/Solubility [WL] group was tested according to ISO 4049:2009. The data were evaluated using two-way ANOVA, Fisher’s post-hoc test and Pearson’s correlation test. The composite with the lowest ΔE differed for each solution: FiltekTM Bulk Fill in DW (∆E = 0.73 (0.17–1.759)); Vertise Flow in CF (∆E = 14.75 (7.91–27.41)), in TE (∆E = 7.27 (2.81–24.81)) and OJ (∆E = 3.17 (0.87–9.92)); Tetric EvoFlow® in CC (∆E = 1.27 (0.45–4.02)); and FiltekTM Supreme XTE in RW (∆E = 8.88 (5.23–19.59)). RW caused the most discoloration (∆E = 23.62 (4.93–51.36)). Vertise Flow showed the highest water sorption (WS = 69.10 ± 7.19). The Pearson test showed statistically significant positive correlations between water sorption and solubility and between water sorption and ∆E; the positive solubility-∆E correlation was not statistically significant. The findings suggest that water sorption is one factor associated with the ability of composites to discolor; however, discoloration is a multifactorial problem.


Introduction
Composite resin-based materials are widely used to meet the growing demand for aesthetic and cosmetic dental treatments. 1,2These composites have a natural appearance and are more conservative and less costly than other materials, such as ceramics. 3,4Several types of resin-based composites are available with different physical properties and are classified according to resin matrix, 5 particle size, filler distribution 6 and flow ability. 6,7he first generation of flowable composites was introduced in 1996. 7These low-viscosity resin-based restorative materials differ from conventional resin composites in their filler load 8 and in their formulation, which contains a higher proportion of diluent monomers. 8The novel flowable composites were developed in 2000 with the aim of improving their mechanical properties. 9These flowable composites are available in two forms: self-adhesive and bulk-fill flowable composites.Self-adhesive flowable composites contain acidic monomers 10,11 and require no adhesive bonding agent, necessitating fewer clinical application steps than conventional composites. 12Bulk-fill flowable composites can be placed in bulk up to 4 mm thick, eliminating the incremental placement technique required with other currently available composites. 13 The differences in composition and filler content are key to the optical properties of resin composites: flowable composites exhibit different optical and color properties than conventional composites. 14These differences, which are related to minor pigment additions 15 and their major levels of translucency, 14 have a greater effect on the color change of flowable composites than on universal composites. 14One of the main drawbacks of resin composites is their tendency to change color due to intrinsic and extrinsic factors 1,2,4,16,17,18 after long periods in the oral environment and this discoloration could make the color of the resin composites perceptible to the human eye, as they would no longer match the color of the tooth. 4,16,17ased on the human eye's ability to perceive color differences, three intervals were used to distinguish changes in color: ∆E < 1 (imperceptible to the human eye), 2 ∆E from 1-3.3 (visible only to the skilled observer, clinically acceptable), 1,2,3 and ∆E > 3.3 (easy to discern, not clinically acceptable). 1,3,19umerous in vitro studies have demonstrated that common drinks such as coffee, 19,20,21,22,23 tea, 2,3,18,23,24 red wine, 1,3,4,24 orange juice, 1,3,24 and cola drinks 1,2,3,4 can cause significant discoloration of composite resin materials.
In a wet oral environment, composites may absorb water or other substances such as saliva, food components or beverages, which can have an important influence on the degradation of dental composites. 25Since water sorption is a diffusion-controlled, time-dependent process, 11 it may decrease the lifespan of the restoration by expanding and plasticizing the resin component and hydrolyzing the silane. 4,25Water sorption is associated with solubility, which consists of the release of residual products such as monomers and oligomers. 25These leached products alter the microstructure of the matrix, creating voids and microcrack formations 10 that allow stain penetration and discoloration. 2,4n response to the lack of research on the color instability of conventional flowable composites 14,15 and the apparent absence of literature on new flowable composites, this study aimed to evaluate the effects of immersion in a range of beverages on the color change and water sorption of nine flowable composites at multiple time points over 6 months.The first null hypothesis was that increased color change is not related to water sorption in composites maintained at 37°C for 6 months.The second null hypothesis was that increased color change is related to staining solutions in composites maintained at 37°C for 6 months.

Methodology Disk specimen preparation
The nine flowable composites tested in this study are described in Table 1.The shades were A2 and Universal (U).Sixty specimens for each composite were produced using t wo different silicone molds (Contrast, VOCO, Cuxhaven, Germany; LOT: 1118534): first, color change specimens (n = 30) 10 ± 1 mm in diameter × 2 ± 0.1 mm thick, yielding the Color Change group; and second, water sorption specimens (n = 30) 15 ± 1 mm in diameter × 1 ± 0.1 mm thick, yielding the Water Sorption/Solubility group.The impression material was not known to contain any leachable plasticizer or other material that would potentially contaminate the surfaces of the specimens.
After filling each mold with the material and covering it with a sheet of polyester film, the top of the material was compressed using a glass plate.The specimens were light-cured on both sides for 60 s (Demi; SDS Kerr Corp., Middleton WI, USA; 1167 mW/cm 2 ).A calibrated radiometer (Bluephase; Ivoclar Vivadent AG, Schaan, Liechtenstein) was used to verify the intensity of the light-curing unit.
The periphery of all specimens was polished with 1200-grit silicon carbide paper to remove flash and irregularities and the surfaces were left unpolished following the ISO 4049:2009 recommendation. 26he specimens were stored in the dark at room temperature until testing.

Color change Immersion of samples in solution
To evaluate the color change, the Color Change group (n = 30) for each composite was divided into six subgroups of five specimens, which were immersed in six different solutions: (distilled water (DW) (pH = 5.84 ± 0.45; Quicesa, Spain), coffee (CF) (pH = 4.58 ± 0.09; Nescafé Classic, Nestlé SA, Vevey, Switzerland), Coca-Cola (CC) (pH = 2.50 ± 0.17; The Coca-Cola Company, Atlanta, GA, USA), red wine (RW) (pH = 3.55 ± 0.14; Cariñena, Spain), tea (TE) (pH = 4.69; English Breakfast, Twinings™, London, England), and orange juice (OJ) (pH = 3.28 ± 0.17; Don Simón, Spain).The distilled water served as a control.To insure that all surfaces were fully exposed to the test solutions, the specimens of each composite were individually immersed in vials containing 2 mL of each solution and stored at 37°C.The solutions were renewed every 2 days and pH measurements were recorded (pH Meter Basic 20; Crison Instruments SA, Alella, Spain) before re-immersing the specimens.

Assessment of color change
The baseline color measurements were recorded for all specimens according to the Commission International de l'Eclairage (CIE) L*a*b* color scale relative to the CIE standard illuminant D65 over a black background, using a reflection spectrophotometer (SpectroShade MHT; MHT S.p.A., Arbizzano di Negrar, Italy).The CIE L*a*b* color system is a three-dimensional color measurement method, where L* refers to the lightness coordinate, which ranges in value from 0 (black)-100 (white), and a* and b* are chromaticity coordinates on the green-red (-a* = green; + a* = red) and blue-yellow (-b* = blue; + b*= yellow) axes, respectively. 1,4  specimens were rinsed with distilled water and blotted dry with absorbent paper before measurement.Three measurements were obtained from each specimen by a single operator and were recorded at baseline (T0), 1 week (T1), 2 weeks (T2), 3 weeks (T3) and 4 weeks (T4), 3 months (T5), and 6 months (T6).Before each color measurement, the spectrophotometer was calibrated according to the manufacturer's instructions.The overall color change values (ΔE) were calculated as follows: Equation where ΔL* is lightness and Δa* and Δb* are the differences in the green-red and blue-yellow axes, respectively.The staining was considered clinically unacceptable when ∆E values were ≥ 3.3. 2,24 er sorption/solubility The water sorption/solubility testing was performed on the Water Sorption/Solubility group (n = 30) for each composite according to ISO 4049:2009. 26ach specimen was weighed, transferred to a desiccator containing dehydrated silica gel (Químics Dalmau SL, Barcelona, Spain) and maintained at 37 ± 1°C for 22 h then 23 ± 1°C for 2 h.The specimens were reweighed and the conditioning cycle was repeated until the decrease in mass of each specimen (m 1 ) was constant.After the conditioning cycle, the diameter and thickness of each specimen was measured three times with a caliper to calculate the specimen volume (V) in mm 3 .To ensure complete exposure, each specimen was suspended in an individual vial containing 20 mL of distilled water and maintained at 37 ± 1°C for the same period as that described for color measurement.
Following the immersion period, the specimens were removed from the vial, excess water was removed with absorbent paper, and the specimen was reweighed (m 2 ).The immersed specimens were then subjected to the aforementioned conditioning cycle until the decrease in the mass of each specimen (m 3 ) was constant.
The water sorption (W S ) and water solubility (W L ) of each specimen were calculated in accordance with ISO4049:2009 26

Statistical analysis
The ∆E observed for the different resin composites (all measurements were performed in triplicate), beverages and times were subjected to 2-way repeated measures analysis of variance (ANOVA) and Fisher' post-hoc test was used for multiple comparisons between groups.These two tests were also used to analyze the water sorption and solubility of resin composites kept in distilled water for 6 months.All tests were performed with a significance level of 95% using StatGraphics Centurion XV (StatPoint Technologies, Inc., Warrenton, VA, USA).Pearson's correlation test was performed to determine possible correlations between water sorption and solubility, water sorption and color change, and solubility and color change.

Color change
The 2-way repeated measures ANOVA was statistically significant (p-value < 0.001) in the following interactions: composite-staining solution; composite-time, and time-staining solution.The data for ΔE were non-parametric and were calculated as the median (Minimum-Maximum); however, a logarithmic transformation of the ΔE values was conducted, the Kolmogorov-Smirnov test showed normality of the transformed data and the ANOVA test maintained the same levels of significance.

Water sorption and solubility
Table 3 shows the water sorption, solubility (µg/mm 3 ) and results of statistical analysis after 6 months' immersion in distilled water at 37°C.The results for water sorption, from lowest to h ig he st, were S on icFi l l™ (SF; Ker r Cor p.) < Smart Dentine Replacement® (SDR; Denstply DeTrey GmbH, Konstanz, Germany) < Venus® Bulk Fill (VBF; Heraeus Kulzer GmbH, Hanau, Germany) = Venus® Diamond Flow (VDF; Heraeus Kulzer GmbH) = FBF = FSXT < Premise Flowable (PF; Kerr Corp.) < TEF < VF.The results for solubilit y, from lowest to highest, were VBF < FBF = PF < VDF < FSXT < SDR < TEF = SF < VF.The lowest water sorption values were obtained with SF at all time points evaluated.Only VF showed values of water sorption and solubility higher than those stipulated by ISO 4049:2009. 26The Pearson analysis showed a positive statistically significant correlation between water sorption and solubility (r = 0.569, p < 0.005).A positive correlation was also observed between water sorption and ∆E (r = 0.059, p = 0.338) and between solubility and ∆E (r = 0.039, p = 0.528) but these values were not statistically significant.

Discussion
Composite resin materials are inevitably exposed to saliva, food and beverages in the oral environment; these factors affect color change as well as oral hygiene 3,27,28 and the surface smoothness of the restoration. 3,21,28Natural saliva has a protective effect because it forms a surface barrier that limits staining 27 and dilutes staining solutions. 23Because there is no effective way to simulate the mouth with fresh saliva, the present study used distilled water, although saliva would be expected to present a much better protective effect.Finishing/polishing procedures may also affect the composite surface quality; therefore, they are linked to the early discoloration of resin composites. 18o standardize and achieve the smoothest surface possible, a polyester strip was used to create a surface rich in resin, which is representative of the clinical situation when matrices are used. 3,21,24The effect of finishing/polishing techniques on discoloration should be considered in future long-term in vitro studies.
Red wine, coffee, tea, orange juice and Coca-Cola are common beverages in the modern diet, and some have a staining potential for restorative materials. 3,23,24he results of the present study demonstrated this staining potential, as in other studies. 1,2,3,4,18,24,27,28xtrinsic discoloration can be removed by daily brushing, a good finishing/polishing technique, and bleaching agents.Some discoloration is easier to remove than others. 2,3,18,20,28Although extrinsic factors cause the most discoloration, intrinsic factors involved in the staining process must be considered because they are irreversible and cannot be removed nor bleached, although teeth can be successfully bleached.
The perceived color match of the material to the tooth might be acceptable even though the material is changing color because the tooth could be changing color in a similar pattern with aging.The superior color matching of VF (48 vol%), SF (66.8 vol%), PF (70 vol%) and FSXT (46 vol%) may be related to their filler content.These composites contain ≥ 70% wt filler; some studies 2,22 have shown that composites with a high filler content exhibit superior color matching.Ytterbium trifluoride, particles that contribute to fluoride release, is a water-soluble component that leaches from composites after immersion in a solution. 2,22This may have affected the discoloration of TEF, FBF and VDF in this study.
VDF had worse color matching than TEF, PF and FSXT but exhibited similar color change to some bulk-fill materials in the universal shade and to VDF in the A2 shade.This color change may be attributable to the silane agent coupling the filler particles and resin matrix because VDF and some bulk-fill composites (SDR and VBF) contain a mixture of urethane dimethacrylate (UDMA) and ethoxylated bisphenol A dimethacrylate (EBPADMA) in their matrix.These two monomers are described in the literature as hydrophobic 2,3,29 and UDMA reportedly exhibits a low ΔE. 2,19Some studies 19,23 have suggested that the silanization of filler particles contributes to discoloration as a result of silane's high propensity for water sorption.This could be related to the varied and complex reactions of silane.The shade of the composite is an additional factor because darker shades exhibit better color matching due to the presence of pigments.Conceivably, universal shades undergo a higher degree of color change 15,16 because of the absence of pigments.In addition, Uchida et al. 16 reported that the greater discoloration associated with lighter shades might result from an environmental breakdown of the polymer, leading to the release of monomers and color change, or from an environmental effect on the retention of pigments and other additives.
In general, the composites included in this study had a ∆E ≤ 3.3, considered clinically acceptable, in distilled water and Coca-Cola. 1,2,18,19,24Only VBF showed a ∆E ≥ 3.3 in these solutions.These values could be related to their universal shade, lower levels of filler content or the degradation of silane.In contrast, orange juice, coffee, tea and red wine resulted in a ∆E ≥ 3.3 in all composites studied. 3,18,23,27ccording to the estimation of Ertaş et al., 18 the 6-month immersion period chosen for this study is using the following formula: Equation 2: Water sorption W S = (m 2 -m 3 )/V Equation 3: Water solubility W L = (m 1 -m 3 )/V ISO4049:2009 26 was considered acceptable when W S was ≤ 40 µg/mm 3 and W L was ≤ 7.5 µg/mm 3 .

Figure .
Figure.Color change progression of each composite and staining solution at the different time points.