Background and surrounding colors affect the color blending of a single-shade composite

Abstract This study evaluated the background and effect of surrounding colors on the color blending of a single-shade composite used in a thin layer. Disc-shaped specimens (1.0 mm thickness) were built with the Vittra APS Unique composite surrounded (dual specimens) or not surrounded (simple specimens) by a control composite (shade A1, A2, or A3). Simple specimens were also built with only control composites. The specimen color was measured against white and black backgrounds with a spectrophotometer (CIELAB system). The whiteness index for dentistry (WID) was calculated for simple specimens. Differences (ΔE00) in color and translucency parameters (ΔTP00) between the simple/dual specimens and the controls were calculated. The translucency adjustment potential (TAP) and color adjustment potential (CAP) were estimated based on the ratios between data from simple and dual specimens. The Vittra APS Unique composite showed higher WID values than the controls. No differences between ΔTP00_SIMPLE and ΔTP00_DUAL were observed for any of the shades. The composite shade did not affect TAP values. The lowest values of ΔE00_SIMPLE and ΔE00_DUAL were observed for shade A1 regardless of the background color. For the white background, ΔE00_SIMPLE values did not differ from those of ΔE00_DUAL for all shades. Only A1 showed ΔE00_DUAL values lower than ΔE00_SIMPLE when the black background was used. The highest modulus of CAP (negative values for the white background) was observed when shade A1 surrounded the Vittra APS Unique composite. The color blending ability of the single-shade resin composite used in a thin layer was affected by both the surrounding shade and background color.


Introduction
The stratification of direct composite restorations involving esthetic demands is a challenge for clinicians, and the color match between the restoration and remaining dental structure is sometimes unpredictable. 1,24][5] Additionally, the relationship between translucency and thickness of the composite strongly affects the ultimate color of the restoration. 6,7sing more translucent composites or reducing the thickness of the composite layer increases the visualization of the underlying substrate and its effect on the restoration color. 8][11] These single-shade composites permit enhanced color adjustment potential (CAP) compared with regular composite systems based on multiple shades.The color blending ability of these innovative materials is based on both color shifting and enhanced translucency. 12For instance, the addition of welldistributed round filler particles with an average diameter of approximately 260 nm generates redto-yellow colors (the so-called "structural color"). 13hese colors can be slightly modified according to the light angle of the material.The structural color is well known in some animal colors (e.g., peacocks), but this optical phenomenon is not fully elucidated in dentistry. 14deally, a single shade composite must have a high CAP, which indicates that its shade shifts toward surrounding colors.However, the colorshifting ability of these composites is limited, and the color match with the surrounding substrate might be higher to some determined shades.6][17] Thin composite layers are commonly used to restore esthetic areas such as incisal edges of class IV cavities.In these scenarios, by restoring fractured teeth without the palatal wall, the background blackness can compromise the color match ability of the composite.Furthermore, for CAP, composites can change their translucency when inserted into cavities, which is referred to as the so-called translucency adjustment potential (TAP). 18Therefore, this study assessed the effect of background and surrounding colors on the CAP and TAP values of a single shade composite.

Methodology
The single-shade Vittra APS Unique composite (FGM, Joinville, SC, Brazil) was evaluated in this study.Specimens were built with single (simple) and two (dual) composites.The Forma composite (Ultradent, Indaiatuba, Brazil) was used in the outer area of dual specimens and as the control (simple specimen) to calculate the CAP.The shades A1D, A2D, and A3D of this last composite were used to obtain three different surrounding colors.
Disc-shaped simple specimens were confectioned by inserting composites into a silicon matrix (10 mm diameter, 1.0 mm depth) and covered with a polyester strip on both sides.The composites were light-cured for 40 s using the light-curing unit Radii-Cal (SDI, Victoria, Australia) with an irradiance of 1,200 Mw/cm².The tip of the lightcuring unit was placed far from the composite to allow the light to reach its entire surface.Three simple specimens were confectioned for each shade of the Forma composite.Twelve simple specimens were confectioned for the composite Vittra APS Unique, while nine were used to obtain dual specimens.For dual specimens, the single specimens were fixed in the center of another silicon matrix (24 mm diameter, 1.0 mm depth).The empty area surrounding the cylinder of Vittra APS Unique was filled with the Forma composite with three specimens per shade.The composite was covered with a polyester strip and light-cured with four 40 s photoactivations.The position of the light-curing unit tip was changed between each photoactivation to cover the entire surface of the specimen.Specimens were stored in dry conditions for at least one week before color readings.
Color readings were conducted (triplicate) using a spherical spectrophotometer (SP60, X-Rite, Grand Rapids, USA) with specimens placed against white and black backgrounds (ColorChecker grayscale, X-Rite, Grand Rapids, USA).No coupling agent was placed between the specimen and backgrounds.A spectrophotometer with a reading aperture of 8 mm in diameter was used in reflectance mode.The observer angle was defined as 2°, and a D65 illuminant was used during color measurements.T he CIELAB system f rom t he Com m ission Internationale de L'Eclairage (CIE) was used, and the color coordinates L* (lightness), a* (red-green axis), and b* (yellow-blue axis) were recorded.For dual specimens, the color was measured only at the specimen center corresponding to the composite Vittra APS Unique.
The whiteness Index for dentistry (WI D ) of composites was calculated using the color coordinates of simple specimens measured against the black background since the equation was developed using this background color. 19The following equation was used: The difference in color coordinates of simple specimens measured against the black and white backgrounds was used to calculate the translucency parameters (TP 00 ) of the specimens.The CIEDE2000 color difference was calculated using the following equation: 20,21 Equation 2: where ΔL', ΔC', and ΔH' are the changes in luminosity, chroma, and hue, respectively.S L , S C, and S H are the weighted functions for each component.K L , K C, and K H are the weighted factors for Lightness, Chroma, and Hue, respectively (K L = K C = K H = 1).R T is the interactive term between chroma and hue differences.
Translucency differences (ΔTP 00 ) among controls (A1, A2, and A3) and the simple (ΔTP 00 _ SIMPLE ) and dual (ΔTP 00 _ DUAL ) specimens of the tested composite were calculated.The TP of the tested composite was calculated for each control shade using the following equation: 18 Equation 3: TAP = 1 -(ΔTP 00_SIMPLE /ΔTP 00_DUAL ) Using only simple specimens, ΔE 00_SIMPLE was calculated using the color difference between Forma (shades A1D, A2D, and A3D) and Vittra APS Unique based on Equation 4. Equation 4: The values of ΔE 00_DUAL were calculated by comparing the color measured in simple specimens of composite Forma with those assessed in the center of dual specimens (Equation 4). Figure 1 schematically illustrates the calculation of ΔTP 00 and ΔE 00 .
For each experimental condition (background color vs. composite shade), the CAP was calculated using the following equation: 18 Equation 5: T he dat a were a n a lyzed for t he nor ma l distribution (Shapiro-Wilk test) and homogeneity of variance (Levene's test).WI D data were analyzed by one-way ANOVA and compared to the composite shades.A two-way repeated-measures ANOVA was used to analyze the ΔTP00 and ΔE00 data (one analysis per background color).The independent variables were 'composite shade' vs. 'specimens design' (simple or dual), which was a repetition factor.A two-way repeated-measures ANOVA was also used to analyze TAP and CAP data.For these last analyses, the independent variables were 'composite shade' and 'background color' (repetition factor).Pairwise comparisons were performed using Tukey's test, and a significance level of 95% was set for all analyses.

Results
WI D results are presented in Table 1.The one-way ANOVA (p < 0.001) showed a significant difference among composites regarding WI D .The Vittra APS Unique composite showed the whitest color, and the lowest WI D values were observed for Forma A3D.A two-way repeated-measures ANOVA showed that neither 'composite shade' (p = 0.218) nor 'specimen design' (p = 0.801) affected the values of ΔTP 00 (for the interaction, p = 0.256) (Table 2).The 'composite shade' significantly affected the TAP values (Figure 2).The highest TAP values were observed for shade A1 and the lowest for shade A2.
Table 3 presents the results for ΔE 00 measured against white and black backgrounds.A two-way repeated-measures ANOVA showed that ΔE 00 values were affected only by the factor 'composite shade' (p < 0.001).The factor 'specimen design' (p = 0.059; interaction, p = 0.425) did not affect ΔE 00 .For both ΔE 00_SIMPLE and ΔE 00_DUAL , the highest color difference was observed for shade A3, while shade A1 had the closest color to Vittra APS Unique.For the black background, both the 'composite shade' (p < 0.001) and 'specimen design' (p < 0.001) affected the ΔE 00 values, and the interaction between the factors was also significant (p = 0.047).For both ΔE 00_SIMPLE and ΔE 00_DUAL , the lowest values were observed for A1.Shade A3 resulted in the highest ΔE 00 values, but there was no difference from shade A2 for ΔE 00_SIMPLE .Differences between the specimen designs were only observed for shade A1 (ΔE 00_SIMPLE > ΔE 00_DUAL ).
A t wo -way r ep eate d-mea su r e s A NOVA showed that both 'background color' (p < 0.001) and 'composite shade' (p = 0.007) affected CAP values, and the interaction between the factors was not significant (p = 0.061) (Figure 3).The lowest and the highest CAP values were observed for the surrounding shade A1 for white and black backgrounds, respectively, and no difference was observed between shade A2 and shade A3.Regardless of the composite shade, higher CAP  values were observed for the black background compared to the white background.

Discussion
Esthetic direct restorations that mimic the remaining tooth structure are a challenge for most clinicians.Using composites that change their color based on those of the adjacent substrate would make the restorative procedure more predictable.When simple specimens were evaluated, the singleshade composite was whiter than the others used as controls in this study.Among the controls, the wither composite (shade A1D) presented a mean WI D of 33.0, while the average value for the Vittra APS Unique was 40.8.The difference in WI D (7.8  units) between these two materials is 3-fold higher than the 50:50% acceptability threshold (2.6 units) calculated in a prior study. 22This same study found a difference of 5.9 WI D units as clinically unacceptable.Shade A1 is the second lightest tab (darker only than B1) in the Vita Classical shade guide.Obtaining esthetic restoration using a light single-shade composite (whiter than A1) significantly relies on its color-shifting ability.
Even though thin composite layers are used to restore anterior teeth, specimens thicker than 2 mm are commonly used in studies evaluating the CAP of resin composites. 23,24However, certain areas of the incisal border of an upper incisor, are thinner, and the effect of the dark background (oral cavity) is more pronounced as the translucency of the composite increases.The manufacturer of Vittra APS Unique states that its color blending ability is mainly due  3. Means (and standard deviations) for ΔE00 values for simple or dual specimens of the Vittra APS Unique composite compared with control specimens.

Variable
Background color

White Black
Color difference For each background color, distinct letters (uppercase comparing composite shades and lowercase comparing backgrounds) indicate significant differences using Tukey`s test (p < 0.05).*Composite shade of control simple specimens and the surrounding composite for dual specimens.
Background and surrounding colors affect the color blending of a single-shade composite to its translucency increasing after polymerization.Indeed, the TP 00 measured for Vittra APS Unique in this study was at least 7.0 units higher than the controls, which were developed to have a similar translucency to tooth dentin.Therefore, all TP 00 differences were more than 2-fold higher than the 50:50% acceptability threshold (2.62) estimated in a prior study, 25 which indicates a significant effect of the background color on the final color of the composite.
Regardless of the composite shade, surrounding the tested composite with the controls did not significantly modify the ΔTP 00 values.Moreover, a tendency of ΔTP 00 reduction (translucency shifting) was observed only for A1.Consequently, a positive TAP was observed only for this last shade, and a TAP modulus lower than 0.1 was observed for all other shades.However, it is important to emphasize that Cohen's d effect size up to 0.2 is considered small. 26Then, despite the significant differences, it is essential to emphasize that no clinically relevant color difference is expected among the composite shades due to their TAP.Ideally, the composite color should match that of the adjacent substrate.In the present study, ΔE 00_SIMPLE quantifies the discrepancy in the composite color without any effect of the surrounding substrate.As expected, the lowest ΔE 00_SIMPLE values were observed for shade A1 measured against white (7.3) and black (8.7) backgrounds.Considering that the composites have some color shifting ability, the color difference is expected to be lower for dual specimens (ΔE 00_DUAL ) than for the simples (ΔE 00_SIMPLE ).However, no significant difference was observed for the surrounding shades between ΔE 00_DUAL and ΔE 00_SIMPLE when a white background was used.Otherwise, a tendency of higher ΔE 00_DUAL values (no significant difference) than ΔE 00_SIMPLE was observed for all shades.This increase in ΔE 00 caused by the surrounding color resulted in negative CAP values, which indicates that the composite color did not shift with the surrounding shade.The opposite tendency was observed for the black background, and ΔE 00_DUAL was lower than ΔE 00_SIMPLE (a significant difference for A1).Consequently, higher CAP values were observed against the black background compared to when a white background was used.
The highest modulus of differences between ΔE 00_DUAL and ΔE 00_SIMPLE were observed for shade A1 (1.0 and 1.7 for white and black backgrounds, respectively).These values are between the  50:50% perceptibility (0.81) and acceptability (1.77) thresholds. 27This indicates that color shifting can be perceptible for most observers but it is not clinically irrelevant.However, the modulus of differences below the 50:50% perceptibility threshold was observed for the other shades (from 0.3 to 0.8). 27As a result, the highest CAP values (negative against the white background) were found surrounding the Vittra APS Unique with composite A1.However, irrespective of the surrounding shade and even for shade A1 (-0.13 and 0.20), the CAP values were considered small based on observed Cohen's d effect size. 26onsidering the high translucency of Vittra APS Unique, its manufacturer recommends an additional underlying layer of a more opaque composite to cover very dark substrates.This recommendation suggests that the color blending of this material is mainly due to visualization of the underlying substrate that should have a similar color compared to the surrounding substrate.However, this study's findings showed that placing a thin layer of this single-shade composite over a white background (like some resin opacifiers) reduced the CAP value compared to those observed for the black background.Moreover, ΔE 00_DUAL values higher than eight were observed against the white background for all surrounding shades.These results indicate that simply covering the dark substrate with an opaque resin does not assure an esthetic restoration.In addition, an opaque resin with a similar color compared to the adjacent substrate can be needed.In this scenario, the advantages of using a singleshade composite are lost.Finally, it is essential to emphasize that only a single material was evaluated in this study and results cannot be extrapolated to other single-shade composites.

Conclusions
This study's findings demonstrate that the color of both the surroundings and background affects the color adjustment potential of a single-shade composite used in a thin layer.However, these factors did not interfere with the translucency adjustment potential.

Figure 1 .
Figure 1.A schematic showing the specimen arrangement used to calculate color differences.

Figure 2 .
Figure 2. Means and standard deviations of the translucency adjustment potential (TAP) according to the composite shade.Distinct letters indicate significant differences according to Tukey`s test (p < 0.05).

Figure 3 .
Figure 3. Means and standard deviations of the color adjustment potential (CAP) according to the composite shade and background color.

Table 1 .
Means (and standard deviations) of whiteness indices evaluated in simple specimens.

Table 2 .
Means (and standard deviations) for ΔTP00 values for simple or dual specimens of the composite Vittra APS Unique compared to control specimens.