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Introduction
Esthetic dental materials are used widely in modern restorative dentistry because of their excellent harmony with natural teeth. Among them, resin composites are the most popular restorative materials. The advantage of resin composites for restoration would be a color match to the adjacent teeth with a wide range of shade options as well as agreeable mechanical properties that are high enough to sustain a range of mastication loads. On the other hand, previous studies reported that restorative resin composites are discolored when exposed to the diverse oral environment.123
The discoloration of resin composites might be mediated by water. Chemical degradation can occur if resin composite absorbs water and other colorants because water is an excellent solvent. Chemical degradation can lead to corrosive wear of the resin surface through softening and hydrolytic processes. As the corroded layer is worn out because of mastication or tooth brushing, a fresh surface is exposed and the corrosion cycle continues. Clinically, this can lead to a loss of restoration contour, as well as an increase in surface roughness, and discoloration.24 Generally, the optical and physical properties of resin composites are affected by the length of degradation.
A range of factors affect the speed of the degradation reaction. Among them, the filler, matrix, type of chemical bond, pH, copolymer composition and water uptake are important. In particular, pH is an unfavorable factor for hydrophilic resins because it affects the degradation rates through catalysis.5 In the oral cavity, pH varies according to the oral environment and tooth surface conditions.6 Acids produced by the bacterial metabolism, such as acetic, propionic and lactic acid can change pH.7 Alkaline beverages, such as mineral water, ion beverages, green tea, and herbal tea, are also potential sources of pH variations. Ideally, polymers of resin composites should not be degraded in an oral environment, in which the pH changes dynamically in an aqueous medium.8 According to previous studies, a lower pH had been shown to have an adverse effect on the wear resistance of resin composites.57 In addition, highly alkaline solutions have been shown to accelerate hydrolysis and produce surface microstructural damage.9
On the other hand, the influence of pH on the color stability of resin composites has not been studied sufficiently. Thus far, previous studies focused mainly on the influence of tea, coffee, and soft drinks on the color stability of glass ionomer cements, resin veneers, indirect composites, provisional resin materials, and compomers.3101112 However the limitation of their studies was that they did not explain whether color change was cause by the pH or by the colorant in the drink solutions.
In order to exclude the effect of colorant in the solutions, this study examined the effects of water with different pH on the color stability of restorative resin composites with different shades. For the study, the color difference (ΔE*) and translucency parameter (TP) of one resin composite with four different shades under three pH conditions were evaluated to test the color stability.
Materials and Methods
Specimen preparation
For the study, a resin composite (Filtek Z350XT, 3M ESPE, St. Paul, MN, USA) with four different shades (A2, A3, B1, and B2) was used. An LED light-curing unit (L.E.Demetron, Kerr, Danbury, CT, USA) was used for light curing. To prepare the specimens, a metal ring mold (inner diameter, 8 mm; thickness, 2 mm; n = 30 for each shade) was filled with resin and light cured for 40 seconds under a light intensity of 1,000 mW/cm2. The light cured specimens were then removed from the mold and aged for 24 hours in a 37℃ dark and dry chamber. The specimens (n = 10) were then immersed into three pH solutions (pH 3, 6, and 9), respectively, for 14 days. To produce the pH solutions, distilled water was mixed with diluted acetic acid to prepare the solutions at pH 3 and 6. A solution at pH 9 was prepared by mixing dilute NaOH with distilled water. Mixing with acetic acid or NaOH was performed under slowly stirring condition using a magnetic bar. All the processes were conducted at 23 ± 1℃ under 60 ± 3% relative humidity.
Evaluation of color
A spectrophotometer (CM-3600d, Konica Minolta, Osaka, Japan) was used to measure the color of the specimens. The stored specimens for 24 hours were selected (n = 10) for the first color measurements. The initial color of the light-cured specimens was measured by placing the specimen at the center of the target mask in reflectance (%R) mode with white and black backgrounds, respectively. This target mask had a 6 mm hole at the center. This hole enables consistency in specimen placement during the measurements. After the first color measurement, each specimen was immersed in the designated test solution for 14 days. During immersion, the solution was replaced daily. After 14 days, the specimens were removed from the test solution, rinsed with running water, and spot dried with tissue paper. The second color measurement was performed using the immersed specimens (n = 10) under the same measurement conditions as before. From the measured reflectance data, the CIE L*a*b* color coordinates were evaluated using the internal software of the measurement system. The ΔE* was obtained using the following formula:
where Δ is the difference between the first and second measurements. Here, L* represents the degree of grayness, and corresponds to lightness. The parameter a* represents the red - green axis, whereas b* is a parameter for the blue - yellow axis.
The TP values were determined by calculating the color difference between the readings using the following formula:
where the subscripts 'B' and 'W' refer to the color coordinates over a black and a white background, respectively. The change in the translucency parameter, ΔTP (TP after immersion minus TP before immersion), was calculated for each specimen.
Statistical analysis
SPSS 15.0 software (SPSS Inc., Chicago, IL, USA) was used for statistical analysis. The results of the color change were analyzed using two-way ANOVA for the resin shade and pH of the solution. A post hoc Tukey test was performed followed by a multiple comparison procedure. All tests were analyzed at p < 0.05.
Results
Tables 1 and 2 list the CIE L*a*b* color coordinates and color changes of the specimens before and after immersion in the test solutions. The specimens of A2 and A3 shades have lower L* and higher b* values than those of the B1 and B2 shades. L* values increased after immersion in test solutions except in pH 3. The ΔL*, Δa*, and Δb* values ranged from -0.30 to 1.40, -0.19 to 0.12, and -0.69 to 0.17, respectively, depending on pH. The difference among each coordinate was less than 1.5. In the pH 6 solution, ΔL* had higher tendency than in other pH solutions regardless of shade.
Table 3 shows the ΔE* of the specimens before and after immersion in pH solutions. ΔE* of the tested specimens were significantly different among three pH solutions (p < 0.05). In the pH 6 solution, ΔE* was significantly higher than others, but the color difference was only slight (ΔE*, 0.33 - 1.58). For shades of resin composite, ΔE* of A2 shade was significantly different from B1 shade (p < 0.05). Other shades were not significantly different from others.
Table 4 shows the TP values and TP difference (ΔTP) of the specimens between the values before and after immersion in the pH solutions for 14 days. TP after immersion of pH solutions had a decreasing tendency except for pH 3 solution, but the absolute values of ΔTP in all pH solutions were very low (0.12 - 0.95).
Discussion
Tooth-colored restorative dental materials are attractive because of the harmonious match with the host teeth by satisfying the aesthetic requirements of a range of users. The restored dental materials are influenced by the dynamic changes in the environment in the oral cavity. In particular, foods and beverages have a wide range of pH because of their diverse ingredients and have a significant effect on the restored resin composites in the tooth cavity.
Previous studies reported that acidic conditions can degrade resin composites.81113 According to Poggio et al., cola had the lowest pH (2.55) and might damage the surface integrity of resin composites.14 The probable degradation of resin composite might be due to the acid-related hydrolysis of ester radicals present in the dimethacrylate monomers, such as bisphenol A glycidyl methacrylate (Bis-GMA), ethoxylated bisphenol A dimethacrylate (Bis-EMA), urethane dimethacrylate (UDMA) and triethyleneglycol dimethacrylate (TEGDMA).8 On the other hand, Buchalla et al. reported that storage in acidic solutions had very little effect on resin-based luting cements.15 Eisenburger et al. observed no significant differences between the resin luting cement surface profiles after 7 days immersion in citric acid and saline, respectively.16
Recently, Cilli et al. reported that an alkaline medium appears to be more suitable for accelerating composite hydrolysis and producing microstructural damage than acidic media.9 According to their study, the strong influence of the alkaline medium on the composite properties was attributed to their interactions with OH ions during the hydrolysis process. At a pH of 13, an alkaline medium provides a million times as many hydroxyl ions as are present in solutions at neutral pH or low pH. In addition to the possibility of hydrolytic degradation of silane couplers and fillers, it is also possible to induce hydrolysis of the inorganic particles themselves with an excess of OH ions.
From the result of others studies, high color change in acidic (pH 3) or alkaline (pH 9) solution was expected due to hydrolysis of resin composites.9111314 In contrast, the results of this study were the opposite. In this study, ΔE* of the tested specimens were significantly different among three pH solutions (p < 0.05). In the pH 6 solution, ΔE* was significantly higher than the others due to the higher ΔL* value. On the other hand, the absolute of ΔE* was also low (at best 1.58) because the absolute value of ΔL* was low (at best 1.40).
According to individual ability of the human eye to appreciate differences in color, three different intervals were used to distinguish the changes in color values, that is, ΔE* < 1, imperceptible by the human eye; 1.0 < ΔE* < 3.3, appreciated only by skilled person, both clinically acceptable; ΔE* > 3.3, easily observed, these color changes are not clinically acceptable.1718 Overall, the resulting change of color of the specimens immersed in the pH 6 solution cannot be easily perceptible and is clinically acceptable. In the case of resin shade, there was significant difference between A2 and B1, but the absolute value of difference was also very slight. Clinical importance was also negligible. Likewise, TP after immersion of pH solutions had a tendency to decrease except for pH 3 solution but the absolute values of ΔTP in all pH solutions were very low (0.12 - 0.95). These results indicate a similar but imperceptibly low color change to the specimens by different pH solutions that can occur in the oral cavity.
The limitation of this study is the static effect of the pH solutions. The oral cavity is always under dynamic stresses, such as temperature variations, wear by mastication activity, and pH variations due to the different foods and beverages. Moreover, it is unclear if such minor color changes observed in the present study are relevant. Therefore, further study will be needed to evaluate the color changes under such dynamic environments.
Conclusions
Within the limitation of this study, ΔE* of the tested specimens were significantly different between the pH solutions. However, from the results of this study, the resulting color change was too low to be perceived by the naked eyes. Therefore, such small color changes can be free of the clinical issues arising from the aesthetic requirements.
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