Effects of surrounding and underlying shades on the color adjustment potential of a single-shade composite used in a thin layer
Article information
Abstract
Objectives
This study aimed to evaluate the surrounding and underlying shades’ effect on the color adjustment potential (CAP) of a single-shade composite used in a thin layer.
Materials and Methods
Cylinder specimens (1.0 mm thick) were built with the Vittra APS Unique composite, surrounded (dual specimens) or not (simple specimens) by a control composite (shade A1, A2, or A3). Simple specimens were also built only with the control composites. Each specimen’s color was measured against white and black backgrounds or the simple control specimens with a spectrophotometer (CIELAB system). The whiteness index for dentistry (WID) and translucency parameters (TP00) were calculated for simple specimens. Differences (ΔE00) in color between the simple/dual specimens and the controls were calculated. The CAP was calculated based on the ratios between data from simple and dual specimens.
Results
The Vittra APS Unique composite showed higher WID and TP00 values than the controls. The highest values of ΔE00 were observed among simple specimens. The color measurements of Vittra APS Unique (simple or dual) against the control specimens presented the lowest color differences. Only surrounding the single-shade composite with a shaded composite barely impacted the ΔE00. The highest CAP values were obtained using a shaded composite under simple or dual specimens.
Conclusions
The CAP of Vittra APS Unique was strongly affected by the underlying shade, while surrounding this composite with a shaded one barely affected its color adjustment.
INTRODUCTION
Restoring anterior teeth with direct composites is challenging for clinicians mainly because the color stratification strongly impacts the final esthetic result [12]. In addition to the proper color selection, the stratification of a restoration also relies on the clinician's experience in understanding the relationship between composite translucency and the thickness of the applied layer [3456]. Moreover, the shades of most commercially available composites are based on the Vitapan Classical shade guide, which covers only a small fraction of natural tooth colors [7]. However, the composite tends to adjust its color toward the dental hard tissues when the restoration takes place, compensating for small color differences [891011]. The ability of a composite to change its color toward the surrounding structure is usually called its color adjustment potential (CAP). Moreover, the composite’s translucency may also adjust to the remaining tooth structure [10].
Several restorative materials have been developed based on these concepts to reduce the technical sensitivity and facilitate esthetic restorations. Multi-shade universal composites are provided in a universal opacity and limited options of shades [9]. Therefore, although esthetic restorations can be obtained by using a single increment, the results still depend on the clinician’s ability to adequately select the composite shade. However, single-shade composites have also been developed to eliminate the color selection step since adequate color matching is achieved with materials presenting enhanced CAP [8910111213]. The color of regular composites is mainly based on the presence of pigments, which exchange energy with the incident light to determine the visualized color [14]. In contrast, color adjustment to the surrounding structure requires the composite to present some structural color and increased translucency [101315161718].
Structural color can be defined as that produced by interactions between incident light and structured materials with different refractive indices [19]. In dentistry, structural color can be achieved by adding well-distributed spherical nano-fillers smaller than 380 nm (corresponding to the lower bound of wavelengths of visible light) to the composite [15]. This approach improves the material’s CAP and allows color-matched restoration using single-shade composites. Moreover, color adjustment can also be related to the composite’s translucency. More translucent resin-based materials allow the underlying substrate color to strongly affect the restoration's final color [1017].
Single-shade composites can also be helpful in repairing esthetic restorations considering their improved ability to adjust the color of both surrounding and underlying composite layers. In resin restoration repairs, using single-shade composites with high CAP facilitates the restorative procedure by eliminating the shade selection step and could yield more predictable results. However, most studies evaluating the CAP of composites have used specimens with 2 mm or more of thickness, while thin composite layers are commonly used to repair restorations [11]. Additionally, the combined effect of underlying and surrounding shades on CAP values has barely been evaluated. Therefore, the present study aimed to evaluate the effect of the surrounding and underlying shades on the CAP of a single-shade composite. We hypothesized that both the surrounding and underlying shades would have a similar effect on CAP values.
MATERIALS AND METHODS
Experimental design
The single-shade Vittra APS Unique composite (FGM, Joinville, SC, Brazil) was evaluated in the present study. The independent variables were the composite shade (A1, A2, or A3) and the location where these shades were placed: 1) underlying, 2) surrounding, and 3) both (underlying and surrounding). The dependent variables evaluated were the color difference (ΔE00) from the control composites and the CAP. The ΔE00 between the evaluated single-shade composite without any surrounding or underlying shade was also measured to calculate the CAP.
Production of specimens
Disc-shaped specimens were built-up with the evaluated composite surrounded (dual specimens) or not (simple specimens) by the control Forma composite (Ultradent, Indaiatuba, SP, Brazil). The manufacturers and the monomeric and filler composition of composites studied are presented in Table 1. Control composites (A1D, A2D, and A3D) were used to obtain 3 different surrounded shades. Simple specimens were also built using only each shade of the control composite. These simple specimens were used as underlying and control shades for the ΔE00 calculations.
Monomeric composition and filler content of the evaluated resin composites
Silicon matrixes with a 10-mm diameter and 1.0-mm depth were used to create the simple specimens. The composites were inserted in a single increment and polymerized for 40 seconds using the light-curing unit (LCU) Radii-Cal (SDI, Victoria, Australia). The irradiance reaching the specimens was approximately 800 mW/cm2 since the LCU tip was placed approximately 2 mm from the specimens to allow the light to reach the entire surface. Three simple specimens were built for each composite shade of Forma used (A1D, A2D, and A3D), while 12 simple specimens were built-up for Vittra APS Unique. Of these 12 simple specimens, 9 were used to obtain dual specimens: 3 for each surrounding shade (Forma A1D, A2D, and A3D). For this purpose, the simple specimens were fixed in the center of another silicon matrix with a diameter of 24 mm and a depth of 1.0 mm. The empty area surrounding the cylinder of Vittra APS Unique was filled with the Forma composite at shades A1, A2, or A3. The Forma composite was polymerized with four 40-s photoactivations, with the position of the LCU tip changed between each photoactivation to cover the entire surface of the specimen.
Color readings
Color readings were carried out in triplicate using a spherical spectrophotometer (SP60; X-Rite, Grand Rapids, MI, USA). A spectrophotometer with an 8-mm-diameter reading aperture was used in reflectance mode. The observer angle was defined as 2°, and a D65 illuminant was used during the color measurements. The color of the simple specimens was determined over a black background of a grayscale (ColorChecker Grayscale; X-Rite). For Vittra APS Unique, the simple specimens were also placed over the control specimens before the color readings to define the effect of the underlying shade on the ultimate color. The color of the dual specimens was measured over a black background and control specimens (the same shade of the surrounding shade was used for the underlying composite). Finally, the color of simple specimens was also measured under a white background (ColorChecker Grayscale; X-Rite) to allow calculation of the translucency parameters (TP00) of the materials. No coupling agent was placed between the specimen and the background.
Whiteness index and translucency parameter
The whiteness index for dentistry (WID) of composites was calculated using the color coordinates of simple specimens measured against a black background, since the equation was developed using this background color. The following equation was used [20]:
The difference in color coordinates of simple specimens measured against the black and white backgrounds was used to calculate the TP00 of the specimens [21]. The CIEDE2000 color difference was calculated with the following equation [2223]:
Where ΔL′, ΔC′, and ΔH′ are the changes in luminosity, chroma, and hue, respectively. SL, SC, and SH are the weighted functions for each component. KL, KC, and KH are the weighted factors for lightness, chroma, and hue, respectively (KL = KC = KH = 1). RT is the interactive term between chroma and hue differences.
Actual color difference
The color of simple specimens of the Vittra APS Unique composite and controls (shaded composites) was measured against a black background. The color differences were calculated using the formula CIEDE 2000 according to equation 3 [2223] and called ΔE00_1.
Where ΔL′, ΔC′, and ΔH′ are the changes in luminosity, chroma, and hue, respectively. SL, SC, and SH are the weighted functions for each component. KL, KC, and KH are the weighted factors for lightness, chroma, and hue, respectively (KL = KC = KH = 1). RT is the interactive term between chroma and hue differences.
CAP
Three other color differences (ΔE00_2) were calculated to make it possible to estimate CAP values according to the local where the Forma composite was placed (Figure 1):
Schematic illustration showing the specimens` arrangement to calculate color differences. The actual color difference (yellow arrow: ΔE00_1) among the composites was calculated by comparing the color of simple specimens built with shaded composites and with a single shade over a black background. Comparing the color of dual specimens with the shaded composites over a black background estimated the surrounding shade effect (orange arrow: ΔE00_2*surrounding). The effect of the background (green arrow: ΔE00_2*underlying) was estimated by comparing simple specimens of the shaded composites with those of the single shade. Finally, the combined impact of the background and surrounding shades (red arrow: ΔE00_2*both) was calculated by the color difference between shaded composites and dual specimens measured over the shaded backgrounds.
Surrounding: For each shade (A1, A2, or A3), the color of the central area of the dual specimens, corresponding to the Vittra APS Unique composite, was measured against a black background. The difference between this last color and that of the shaded composites over black background was calculated (ΔE00_2*surrounding).
Underlying: Single specimens of the Vittra APS Unique composite were placed over the specimens of shaded composites (A1, A2, or A3) before the color measurement. Two shaded specimens (same shade) were superposed, and the color of the upper specimen was measured. For an underlying shade, the color difference between the Vittra APS Unique and the shaded composite was calculated (ΔE00_2*underlying).
Both: The color of the central area of the dual specimens was measured over a specimen of the shaded composite. The same surrounding shade was used for the underlying specimen. The difference between this color and that measured for the superposed shaded specimens was calculated (ΔE00_2*both).
For each location where these shades were placed (underlying and/or surrounding), the CAP was calculated using the following equation [12]:
Data analyses
The data were analyzed for a normal distribution (Shapiro-Wilk test) and homogeneity of variance (Levene's test). One-way analysis of variance (ANOVA) was used to analyze the WID and TP data. Two-way repeated-measures ANOVA was used to analyze the ΔE00 and CAP data. The independent variables were shade and location (i.e., where these shades were placed), defined as a repetition factor. Pairwise comparisons were performed using Tukey’s test, and a significance level of 95% was set for all analyses.
RESULTS
Whiteness index and translucency parameter
One-way ANOVA showed that the composite significantly affected both WID (p < 0.001) and TP00 (p < 0.001), and the results are presented in Table 2. The whitest composite was Vittra APS Unique, followed by Forma shade A1D, and the darkest was Forma shade A3D. Regarding the TP00, Vittra APS Unique was the most translucent material, and no significant difference among the shades of the Forma composite was observed.
Means (standard deviations) of the whiteness index and translucency parameters of composites
Color differences
Repeated-measures ANOVA demonstrated that both independent variables—shade (p < 0.001) and location (p < 0.001)—affected the values of ΔE00, but the interaction between the factors was not significant (p = 0.234). The results are presented in Figure 2. Irrespective of the shade, the highest values were observed for ΔE00_1 followed by ΔE00_2*surrounding. The smallest color discrepancies from the controls were observed for ΔE00_2*underlying and ΔE00_2*both, and no significant difference was observed between them. For all delta values, the highest and lowest mismatches were observed for shades A3 and A1, respectively.
Mean values (with standard deviations) for the values of ΔE00 calculated for different comparisons and composite shades. For all shades, ΔE00_1 > ΔE00_2*surrounding > ΔE00_2*underlying = ΔE00_2*both. Irrespective of the calculation arrangement, A3 > A2 > A2. The dashed horizontal line indicates the 50:50% acceptability threshold.
CAP
Location was the only independent variable that significantly (p < 0.001) affected the values of CAP. The p-values calculated by repeated-measures ANOVA for both shade (p = 0.191) and the interaction between shade and location (p = 0.607) were not significant. The results are presented in Figure 3. Regardless of the shade, the lowest CAP values were obtained when the control composite only surrounded the Vittra APS Unique composite. No significant difference was found between the underlying location and the underlying and surrounding location.
Mean values (with standard deviations) of color adjustment potentials calculated for different composite shades and the locations where these shades were placed. For all shades, underlying = both > surrounding. The shade did not affect the color adjustment potential values.
DISCUSSION
Esthetic restorations using direct composites are traditionally built using the stratification technique, which requires composites with different translucency levels (e.g., dentin and enamel) and shades [12]. An additional clinical challenge using this technique is to define the thickness of each increment by combining opaquer and more translucent materials [6]. The development of universal composites facilitates this procedure since these materials present a single translucency [9]. However, the most promising change in the simplification of the restorative procedure was the development of single-shade materials, which eliminated the color selection step. Single-shade composites have been designed to increase their color adjustment ability. Consequently, improved matching between the composite and the surrounding substrate is expected. Then, single-shade composites could also be an interesting approach to repair defective resin restorations in esthetic areas when only a thin composite is sometimes used.
In the present study, the color of simple specimens built with either Vittra APS Unique or shaded composites was compared to determine the “true” color differences among these materials. In this scenario, any possible effect of the surrounding substrate on the ultimate color of specimens is eliminated. Then, using a standard black background permits a fair estimation of any color difference among the materials. The measured color of the Vittra APS Unique composite was whiter than those observed for all shaded composites. The difference between the averages of WID (7.8 units) calculated for Vittra APS Unique and the whitest shaded composite A1D was higher than the difference (5.9 units) defined as clinically unacceptable by a prior study [24]. It is essential to emphasize that the A1 shade is the second lightest tab in the Vita Classical shade guide. Usually, composites darker than that are required to obtain adequate color matching, and adequate esthetic restorations using Vittra APS Unique rely significantly on its CAP.
As expected, placing a shaded composite under or around the Vittra APS Unique specimens reduced their color discrepancy from the control specimens. Regardless of the shade, the reduction on ΔE00 caused by only using underlying shaded composites (−4.6 to −4.1) was more pronounced than that observed when placing the same composite around (−1.8 to −0.7) the Vittra APS Unique. Moreover, combining both approaches (−5.0 to −4.3) barely increased the color adjustment reached using only the underlying shaded composite. The reduction in ΔE00 values indicates improved color matching of the single-shade composite to the adjacent substrate. The CAP value is directly proportional to the color difference reduction, and the color adjustment for Vittra APS Unique relied mainly on the underlying shade. Therefore, the hypothesis of the study was rejected.
Matching the color of the composite to that of an adjacent substrate can be achieved based on the structural-color phenomenon or by increasing the material`s translucency [101117]. In dentistry, structural color in resin composites has been reached by using 260 nm spherical fillers that generate a red-to-yellow color (e.g., Omnichroma from Tokuyama Dental, Tokyo, Japan) [1525]. Interestingly, the relationship between the refractive indexes of the organic matrix and filler contents affects both structural color and translucency [17]. While similar refractive indexes increase a composite`s translucency, the structural color becomes stronger as the refractive index of fillers exceeds that of the organic matrix. The manufacturer of Vittra APS Unique states that its color adjustment ability is due mainly to its translucency increasing after polymerization. The conversion of monomers to polymers tends to increase the refractive index of the organic matrix, reducing its mismatch with that of inorganic filler [2627].
Vittra APS Unique barely adjusted its color to that of the surrounding substrate, since the CAP values as a function of underlying shade ranged from 0.19 (for A1D) to 0.06 (for other shades). The low values are similar to those observed for regular composites in a prior study [12]. However, the CAP values due to changing the background shade ranged from 0.39 (A3D) to 0.50 (A1D). Using shaded composites under and around the Vittra APS Unique resulted in CAP values from 0.42 (A3D) to 0.49 (AD1), showing that the surrounding color barely modified its color adjustment ability. Instead, its color adjustment ability was strongly dependent on the underlying shade. The TP00 measured for Vittra APS Unique was at least 7.0 units higher than the controls, which were developed to have a similar translucency to tooth dentin. This difference is more than 2-fold higher than the 50:50% acceptability threshold (2.62) [28], which demonstrates the high translucency of this single-shade composite.
The present study's findings show that the ultimate color obtained with the single-shade Vittra APS Unique composite used in a thin layer strongly depends on the underlying shade. When repairing a composite restoration, the shade of the remaining composite on the floor of the prepared cavity is usually similar to the required final color restoration. In this scenario, however, a thin layer of Vittra APS Unique resulted in ΔE00 of at least 4.3, which is higher than the 50:50% acceptability threshold [24]. Therefore, these results indicate that using the single-shade Vittra APS Unique to repair some defective restorations in esthetic areas is an unreliable approach. It is important to emphasize that no optical solution was placed between the specimen and the background. Different results could be observed when bonding a single-shade composite to the underlying older resin [29]. Furthermore, only a single material was evaluated in the present study, and the results cannot be extrapolated to other single-shade composites.
CONCLUSIONS
The present study's findings demonstrated that the CAP of a single-shade composite used in a thin layer was more strongly affected by the underlying shade than by the surrounding shade.
Notes
Funding: This study was supported by the Coordination for the Improvement of Higher Educational Personnel -, Brazil (grant CAPES/PROCAD #3001/2014, and Finance Code 001).
Conflict of Interest: No potential conflict of interest relevant to this article was reported.
Author Contributions:
Conceptualization: Faria-e-Silva AL.
Data curation: Barros MS, Silva PFD, Santana MLC, Bragança RMF Formal analysis.
Investigation: Barros MS, Silva PFD, Santana MLC, Bragança RMF Methodology.
Resources: Faria-e-Silva AL.
Supervision: Faria-e-Silva AL.
Validation: Barros MS, Silva PFD, Santana MLC, Bragança RMF, Faria-e-Silva AL.
Writing - original draft: Barros MS, Silva PFD, Santana MLC, Bragança RMF, Faria-e-Silva AL.
Writing - review & editing: Barros MS, Silva PFD, Santana MLC, Bragança RMF, Faria-e-Silva AL.
