Specimen preparation
Sample size calculation was performed with the G*Power 3.1 software (Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany) and the sample size was calculated as 10 per group with alpha-type error of 0.05, a power (1-beta) of 0.95, an effect size of 0.712 obtained from a previous study. [10] A total of 160 specimens (40 from each composite group) were prepared according to the manufacturer recommendations. Composite resins were placed in stainless steel molds with depths of 2 mm and diameters of 5 mm. Mylar strips and glass slides were placed on each specimen to prevent air bubbles, and pressure was applied. Thus, a smooth surface was obtained by removing excess material. Then, the specimens were polymerized for 20 s using a light emitting diode (LED) source (Elipar Freelight II, 3 M ESPE, AG, Germany, 1150 mW/cm2) and removed from the molds. Before polymerization in every five specimens, the power of the light source was checked; it was ensured that the power was higher than 1000 mW/cm2. Standardization was achieved by measuring the thicknesses of the specimens with a digital caliper (Ultra-Cal V, Fowler Corp., Sylvac, Switzerland). After polymerization, the specimens were polished with aluminum oxide-coated discs (Sof-Lex™ XT; 3 M/ESPE, St. Paul, MN, USA). The polishing and numbering processes of the specimens in each composite group were performed by the same investigator (BC).Table 1
Color measurements
In this study, color values were measured using a Commission International de l’Eclairage (CIE) L*a*b* system with a spectrophotometer device (VitaEasyshade V, Vita Zahnfabrik, Bad Sackingen, Germany) using a neutral gray background (L*=64.1; a*=0.3; b*=−3.4). Measurements were performed under D65 standard lighting conditions, and the spectrophotometer was calibrated according to the manufacturer recommendations before each measurement. After the specimens were dried with tissue paper, three measurements were repeated on each surface; the average L, a and b values were calculated. The principle of the CIELAB system is based on the sensitivities of three types of conical adherent sensing cells to red, blue and green light. On this basis, each color would be represented by the abbreviations L, a, and b. The L* value (Lightness) would indicate the lightness and darkness of the color, ranging from 0 to 100, while a* (red to green) and b* (blue to yellow) would indicate the hue. [11]
Initial color measurements (L0, a0, b0) of the specimens were made after storage in distilled water at 37 °C for 24 h. The specimens were divided into 4 different groups according to the type of composite resin; after the first color measurement, they were randomly divided into 4 different subgroups (n = 10). Ten randomly selected specimens were immersed in tea (Lipton Yellow Label Tea, Unilever, Istanbul, Turkey), 10 specimens were immersed in coffee (Nescafe Classic, Nestle, Switzerland), and 10 specimens were immersed in cola (The Coca-Cola Company, Istanbul, Turkey). As a control group, 10 specimens were immersed in distilled water in an incubator at 37 °C for 14 days. The solutions were prepared and refreshed daily. In our study, tea was prepared by immersing 1 tea bag recommended for a standard cup size in 250 mL of boiling water. Coffee was prepared by dissolving 3.6 g of coffee in 300 mL of boiling water and by mixing for 10 min. A new cola bottle was used every day. The specimens extracted from the solutions were washed in distilled water for 5 min and dried. The color measurements were performed at the end of the 1st and 14th days as previously explained.
Color, translucency and contrast ratio measurements
The color change levels in the specimens were calculated using the following CIEDE2000 formulation: [12]
$$\eqalign{\Delta {E_{00}} = & {\left[ {\left( {{{\Delta {L^{‘}}} \over {{K_L}{S_L}}}} \right)} \right.^{2}} + {\left( {{{\Delta {C^{‘}}} \over {{K_C}{S_C}}}} \right)^{2}} + {\left( {{{\Delta {H^{‘}}} \over {{K_H}{S_H}}}} \right)^{2}} \cr \\&{\left. { + {{\rm{R}}_T}\left( {{{\Delta {C^{‘}}} \over {{K_C}{S_C}}}} \right)\left( {{{\Delta {H^{‘}}} \over {{K_H}{S_H}}}} \right)} \right]^{{1 \over 2}}} \cr}$$
The ΔL’, ΔC’ chroma, and ΔH’ color hue values are included in this formula. The RT rotation function is the general rotation function that explains the interactions between chroma and hue differences in the blue region. The weighting function (SL, SC, and SH) adjusted for variations between the total color differences of pairs at coordinates L*, a* and b*. KL, KC and KH were the accurate terms for the test conditions that were set to 1. [13]
$${\text{RT}}{{\text{P}}_{{\text{TP}}_{00}}} = \sqrt {\begin{aligned}& {\left( {\frac{{{L^{‘}}_B – {L^{‘}}_W}}{{{K_L}{S_L}}}} \right)^2} + {\left( {\frac{{{C^{‘}}_B – {C^{‘}}_W}}{{{K_C}{S_C}}}} \right)^2} + {\left( {\frac{{{H^{‘}}_B – {H^{‘}}_W}}{{{K_H}{S_H}}}} \right)^2} \\ & + {{\text{R}}_T}\left( {\frac{{{C^{‘}}_B – {C^{‘}}_W}}{{{K_C}{S_C}}}} \right)\left( {\frac{{{H^{‘}}_B – {H^{‘}}_W}}{{{K_H}{S_H}}}} \right) \\ \end{aligned}}$$
The ΔE00 value was 0.8, and the threshold values were taken as references for perceptibility (PT); the 1.8 threshold values were taken as references for acceptability (AT).
Translucency parameters (TPs) and contrast ratios (CRs) were used to evaluate changes in translucency. TP was determined by calculating the L*, a*, b* values recorded on white (W) and black (B) backgrounds according to the following formula. [14]
CR was calculated as the light reflection ratio of the specimens on black (YB) and white backgrounds (YW) using the following formula: [15]
$$CR\, = \,\left( {{{{Y_B}} \over {{Y_W}}}} \right)$$
Differences in the TP and CR values were calculated using the following formula:
ΔTP = TPafter staining − TPbaseline.
ΔCR = CRafter staining − CRbaseline.
Statistical analysis
IBM SPSS software (IBM Corp. Released 2019. IBM SPSS Statistics for Windows, Version 26.0. Armonk, NY, USA) was used for statistical analysis. Changes in ΔE00 values at 4 h and 14 days, as well as Contrast Ratio changes (ΔCR) and Translucency changes (ΔTP) values of resin composites and solutions, were compared using one-way ANOVA. Post-hoc Tukey test was used to make multiple comparisons. Changes in ΔE00, ΔCR and ΔTP values of the specimens in the same solution according to time were analyzed by paired-samples T test. A p-value of less than 0.05 was considered statistically significant.
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