Evaluation of Color-matching Ability of a Structural Colored Resin Composite
The present study evaluated the color-matching ability of a structural colored resin composite to compare it with resin composites employing pigments. A structural colored resin composite (Omnichroma [OMC]), a supranano-filled resin composite (Estelite ∑ Quick [ELQ]), and a nano-filled resin composite (Filtek Supreme Ultra [FSU]) were used. Each resin composite was packed into a Teflon mold and pressed down with a clear strip under a glass slide. The specimens were light irradiated through the slide with a light-emitting diode curing unit. The thickness of the specimens (n=6) was measured with a digital caliper before being transferred to distilled water and stored at 37°C for 24 hours. The measurements of the optical characteristics of the specimens on a black-and-white background were performed using a spectrophotometer. D65 (CIE D65) was used as a light source for the spectrophotometer. Measurements were repeated three times for each specimen under each color-measurement condition, and average values for three same-shade specimens were calculated. One-way analysis of variance and Tukey post hoc tests were used (α=0.05). To determine its ability to match the color of artificial teeth, each shade of resin composite was placed in a cavity before performing color measurements. Using a spectrophotometer (CMS-35F S/C) with a flexible sensor, L*, a*, and b* values were obtained. The spectral reflectance curve of OMC showed that it reflected light wavelengths from 430–700 nm regardless of the background color and thickness of the specimens. The percentage of reflectance of ELQ decreased near wavelengths of 550–580 nm. Regarding the influence of background color on CIE L*, a*, b* values, the L* level showed significantly higher values for all tested materials with white backgrounds, and OMC was most affected by the difference in background color. However, a* values of ELQ and FSU were significantly higher with a black background than with a white background, and OMC showed a significantly higher value with a white background than with a black background. The b* values were higher with a white background than with a black background and were significantly higher for all three products, and these tendencies were much greater for ELQ and FSU. The ability of OMC to match the color of artificial teeth showed acceptable color compatibility, regardless of the shade of the artificial teeth and the depth of the cavity. However, ELQ and FSU showed reduced color compatibility, especially for a cavity depth of 3.0 mm. Excellent color matching ability was confirmed for the structural colored resin composite OMC, resulting in reduced color differences and therefore improving the esthetic appearance of the restoration, simplifying shade matching, and compensating for any color mismatch.SUMMARY
Purpose:
Methods and Materials:
Results:
Conclusions:
INTRODUCTION
Resin composites have been widely used because of their esthetics, good mechanical properties, and ease of handling.1 The color matching and esthetic success of resin composite restorations depends on complex factors, including the translucency, opacity, light scattering and transmission, brightness, fluorescence, and opalescence of the restorative materials.2 When light penetrates resin composites, different phenomena associated with the material may occur, such as specular transmission of the light flux through the material, specular reflection at the surface, diffuse light reflection at the surface, and absorption and scattering of the flux within the material.3 Colored pigments are incorporated into resin composites to produce clinically acceptable shades for restorative materials, and it was reported that pigments could influence the translucency and color of resin composites.4 Thus, care should be taken to consider the pigments incorporated into resin composites as an important factor in achieving an optimal esthetic restorative outcome in clinical situations.
The color development is produced by the natural or artificial pigments with an exchange of chemical energy between the light and the pigments.5 When light of a certain wavelength is absorbed by the pigmented material and the other wavelengths are reflected and transmitted, then the material color can be identified. In contrast to pigmented materials, those utilizing structural color employ a color based on light interacting selectively with nanostructures.6 Structural color is expressed only by the physical properties of light without a change in the light energy, as with pigments. Unlike pigment and fluorescence color expressions in materials, structural color is generated from light reflection, diffuse reflection, diffraction, and interference with spatially ordered nano- or microstructures in photonic materials.7 A structural color is generated when certain structures are active in the visible region. The characteristic wavelengths of the diffracted light can be expressed by the Bragg equation,8
where m is the diffraction series, λ is the diffraction wavelength, n is the average refractive index, d is the lattice constant, and θ is the angle between the incident light and normal crystal plane. According to the theoretical basis provided by the Bragg equation,8 changes in the parameters of the above formula can be employed in principle to design a modulated material having structural color.9
Structural colors can be divided into two classes according to their optical properties as iridescent and noniridescent colors. An iridescent color is produced by periodic structures with regular lengths of the order of the wavelength of visible light, known as photonic crystals.10 Noniridescent structural color has angle-independent characteristics, meaning that the color impression is the same for different illuminations and observation angles.11 Noniridescent structural colors produced by amorphous structures in nature are mostly blue with short wavelengths, since the typical bands of scattered wavelengths are located at the edge of the sensitivity range of the human eye.12 Longer wavelength structural colors toward red are difficult to obtain with photonic structures because of significant scattering at short wavelengths. Short wavelength resonances in the single-particle scattering cross-section near backscattering introduce a blue peak in the spectrum that changes the red structural color to purplish or pink. The use of lithography, liquid crystals, block copolymers, and colloidal particles have all been investigated to create artificial structural color materials.13 Among these methodologies, structural color materials based on the assembly of colloidal particles were thought to be the most interesting, since these can create longer wavelength structural colors easily by various routes and can be used in bottom-up nanotechnology.14
A resin composite with a structural color containing spherical fillers, with an average particle size of 260 nm, with a filler loading of 79 wt% (68 vol%), expressed yellow to red colors.15 Simultaneously, structural colors depend on the refractive index distribution, and differences in the refractive indices of the resin matrix and inorganic filler should also be considered. If there is a mismatch between the refractive indices of the filler and resin matrix, the filler will increase and interfere with light scattering.16 Greater refractive index differences between the inorganic fillers and the matrix phase of the resin composites increased the opacity of the materials due to multiple reflection and refraction events at the filler-matrix interfaces.17 The refractive indices increase from monomer mixtures to polymerized structures by around 0.04,18 so it would be appropriate to make the refractive index of the filler slightly higher than those of the base resin monomer mixtures, so that the refractive index of the resin phase might coincide with that of the filler during curing. The structural color can be modulated by changing the differences in the refractive indices of the monomer mixtures and inorganic fillers. When the refractive index of the uniformly sized spherical filler exceeds that of the matrix resin, a stronger structural color might be expressed due to scattering of incident light.19
The color of a material is influenced by various factors, and differences in the light-transmittance characteristics of resin composites can affect their ability to match the color of the restored tooth. The inherent translucency of resin composites might contribute to their optical properties by allowing the background shade to shine through.20 Although it is well known that the reflected color of resin composites is affected by the background color, little is known about the optical properties of structural colored resin composites. The aim of this study was to determine the color matching ability of a structural colored resin composite to compare it with resin composites employing pigments, with respect to their material colors. The null hypothesis to be tested was that neither the background color nor material thickness influenced the optical characteristics of the structural colored resin composite.
METHODS AND MATERIALS
Study Materials
A structural colored resin composite, Omnichroma (OMC; Tokuyama Dental, Tokyo, Japan), a supranano-filled resin composite, Estelite ∑ Quick (ELQ, Tokuyama Dental), and a nano-filled resin composite, Filtek Supreme Ultra (FSU; 3M Oral Care, St. Paul, MN, USA), as listed in Table 1, were used.
Specimen Preparation for Investigating the Influence of Background Color
Standardized specimens, 8.0 mm in diameter and 1.5 and 3.0 mm in thickness, were made with selected composite shades, as shown in Figure 1 (universal for OMC, A2 for ELQ, and A2B for FSU). Each resin composite was packed into a Teflon mold and pressed down with a clear strip under a glass slide. The specimens were light irradiated through the slide at 1000 mW/cm2, for the time recommended by each manufacturer, with a light-emitting diode (LED) curing unit (Pen Cure, J. Morita, Kyoto, Japan). The light irradiance of the curing unit was confirmed using a dental radiometer (Model 100, SDS Kerr, Danbury, CT, USA). The resin disk was then removed from the mold, and the specimen surfaces were not polished after curing. The thickness of the specimens (n=6) was measured with a digital caliper (Digimatic Caliper 500-151-30; Mitutoyo, Tokyo, Japan) before transferring to distilled water and storing at 37°C for 24 hours.
Citation: Operative Dentistry 46, 3; 10.2341/20-002-L
Spectral Reflectance and Color Value Measurement
The optical characteristics were measured using a spectrophotometer (CMS-35F S/C; Murakami Color Research Laboratory, Tokyo, Japan) with a flexible sensor (FS-3; Murakami Color Research Laboratory). The diameter of the illuminated area was 6 mm and the diameter of the area that received light from the object was 3 mm. Commission Internationale de l'Eclairage D65 (CIE D65) was used as a light source for the spectrophotometer, and the illuminating and viewing configurations were set at CIE 45°/d, and spectral reflectance (%) measurements were performed at 10-nm intervals. The color was measured in reflectance mode over white (Y=90.56, X=92.14, Z=110.90) and black (Y=0.01, X=0.01, Z=0.01) backgrounds. All color measurements were performed at wavelengths ranging from 380–740 nm, at 2-nm intervals, and subsequently converted to CIE L*, a*, and b* values. The L* value determines the psychometric lightness from black to white (achromatic coordinate). The a* value (green– red coordinate) and b* value (blue–yellow coordinate) are the psychometric chroma coordinates and indicate hue and chroma factors. Measurements were repeated thrice for each specimen under each color measurement condition, and average values for three same-shade specimens were calculated.
Statistical Analysis
To determine the appropriate sample size for the color value measurements in different backgrounds, a statistical power analysis was performed. After gathering the data, post hoc power tests were performed using two statistical software systems (G Power calculator, http://www.gpower.hhu.de/; and Sigma Plot version 13.0; Systat Software, Chicago, IL, USA), with an f value of 0.75, α value of 0.05, and power of 0.95. This test indicated that the sample size was adequate.
The data obtained were analyzed using a commercial statistical software package (SPSS Statistics Base; International Business Machines, Armonk, NY, USA). One-way analysis of variance and Tukey post hoc tests were used with a significance level of 0.05.
Ability to Match the Color of Artificial Teeth
Maxillary central incisal artificial teeth (Mold A19, Shade A2, A4; Zenopal, GC, Tokyo, Japan) were used to assess the color matching ability of the resin composites. A standardized cavity preparation, 4 mm in diameter and 1.5 mm or 3.0 mm in depth, was performed in the center of the labial side of each artificial tooth. The prepared cavities in the artificial teeth were air abraded with 50 μm aluminum for 5 sec at 0.2 MPa pressure, then treated with a universal adhesive (Bondmer Lightless [known as Tokuyama Universal Bond outside of Japan], Tokuyama Dental, Tokyo, Japan), according to the manufacturer's instructions.
Each shade of resin composite (Table 1) was placed into the cavity with a single application of paste (Figure 2). A layer of transparent strip was placed on top of the resin paste and then pressed with a cover glass, and this was light irradiated for the time stipulated in each manufacturer's instructions using the LED curing unit Pen Cure. After storage in distilled water at 37°C for 24 hours, these specimens (n=3) were submitted to mechanical polishing in a grinding/polishing machine (Ecomet 4 Grinder Polisher, Buehler, Lake Bluff, IL, USA) with a sequence of 2000-grit SiC papers under running water. A final polish was accomplished with a composite polishing paste (PRG CompoGloss, Shofu, Kyoto, Japan) using a polishing disk with a synthetic velvet polishing cloth on the outer surface (Super-Snap Buff Disk, Shofu), for a total of 108 specimens. Using a spectrophotometer (CMS-35F S/C) with a flexible sensor, L*, a*, and b* values were obtained. The color difference (ΔE*ab) was calculated using the following equation:
where ΔE*ab corresponds to the color difference between single resin composite specimens and the corresponding area of surrounding artificial teeth. Measurements were repeated thrice for each specimen and the obtained data values were averaged. The color adaptability was determined from the color differences among teeth and resin composites through 50:50% perceptibility and 50:50% acceptability thresholds, as described in a previous review paper.21
Citation: Operative Dentistry 46, 3; 10.2341/20-002-L
RESULTS
Influence of Background Color on Spectral Reflectance
The influence of the background color on the spectral reflectance curves of the resin composites is illustrated in Figure 3. The spectral reflectance curve of OMC showed that it reflected wavelengths from 430–700 nm regardless of the background color and thickness of the specimens. The percentage of reflectance of ELQ decreased near wavelengths of 550–580 nm (green color) and presented as sigmoid-like curves except with a black background with a thickness of 1.5 mm. Comparing the background colors showed that the spectral reflectance on the white background increased in comparison with that on the black background for all the materials tested, although the reflectance increased at longer wavelengths for ELQ and FSU regardless of the specimen thickness. The spectral reflectance curves of ELQ and FSU showed decreases at longer wavelengths, and these tendencies were clearer with black backgrounds.
Citation: Operative Dentistry 46, 3; 10.2341/20-002-L
Influence of Background Color on CIE L*, a*, and b* Values
The influence of background color on CIE L*, a*, and b* values is shown in Figure 4. The L* level showed significantly higher values for all tested materials with white backgrounds, and OMC was most affected by the difference in background color. However, a* values of ELQ and FSU were significantly higher with a black background than with a white background, and OMC showed a significantly higher value with a white background than with a black background. The b* values were higher with a white background than with a black background and were significantly higher for all three products, and these tendencies were much greater for ELQ and FSU.
Citation: Operative Dentistry 46, 3; 10.2341/20-002-L
Color-matching Ability
The abilities of resin composites to color match to artificial teeth of shades A2 and A4 are presented in Table 2. The color difference (ΔE*ab) ranged from 1.4–2.4 for OMC, from 2.9–15.4 for ELQ, and from 2.6–13.4 for FSU. The color matching of OMC to the artificial teeth showed excellent and acceptable color compatibilities, regardless of the shade of the artificial teeth and the depth of the cavity. However, ELQ and FSU showed reduced color compatibilities, especially for a cavity depth of 3.0 mm, and these tendencies were remarkable with shade A4 artificial teeth.
DISCUSSION
The color of a resin composite is correlated with the light scattering and absorption characteristics, reflectivity, and translucency of the material.22 Inside the composite, incident light is reflected by the filler, resin matrix, pigments, and background color, then recognized as a certain color. The spectral reflectance curve of OMC showed that it reflected almost all wavelengths from 430–700 nm evenly on the black background regardless of the specimen thickness (Figure 3). However, the spectral reflectance curves for the white background showed slightly increased values compared to those on the black background, and the spectral reflectance increased with the wavelength range. Transmitted light contains optical information regarding the background and may affect the appearance of the resin composite.23 When the background color has higher hue and chroma values, the light coloration from the background is absorbed, leading to structural color expression. However, when the background chromaticity is low, the incident light is scattered, so the structural color is weak. The results of this study indicated that the reflection of light from the white background and from the spherical fillers might overlap, making it difficult to detect the structural color due to increased brightness.
Based on the results of this study, the a* values of ELQ and FSU were significantly higher with the white background than with the black background (Figure 4). This was because the color development of the resin composite was affected by the change in the background color, leading to increased reflectance of incident light. The background color is quite important with respect to reflection of the incident light. When ELQ and FSU were placed on a dark color, the specific reflectance visible spectrum corresponding to the colored light was clearly confirmed. When the resin pastes were placed on the white background, uniform light reflection occurred. In this situation, certain reflections of the visible spectrum did not appear, leading to colorlessness. If the resin composite was placed in a low chromaticity environment, weak colored light was generated due to the pigments or dyes, leading to the effective ability to match the color of various peripheral color environments. However, when the background color was black, which has a much lower value than a white background, the a* values of structural colored resin composite OMC showed different tendencies compared to those of ELQ and FSU.
To determine the color-matching ability of the structural colored resin composite OMC, a standardized cavity was prepared in the artificial teeth, which was filled with the resin pastes. To assess the color-matching ability, ΔE*ab, which is designed to provide a quantitative representation of the color difference, was used. Determining the thresholds of perceptibility and acceptability is important for interpreting and evaluating clinical outcomes.24 The acceptability threshold ranges between 2.0 and 4.0, where as much as one-third of the literature refers to a value of 3.7 as being the threshold at which 50% of observers accepted a color difference. Another paper reported that the CIELAB 50:50% perceptibility threshold in dentistry was found to be ΔE*ab = 1.2, whereas the 50:50% acceptability threshold was found to be ΔE*ab = 2.7, and more than half the studies used a threshold of perceptibility of one ΔE*ab unit under controlled conditions. In this study, ΔE values below 1.2 were considered as excellent matches and values of >1.2 and ≤2.7 were considered as acceptable matches.21 The research hypothesis was rejected because the background color and material thickness influenced the optical characteristics of the resin composites tested.
The results of this study showed that the color difference (ΔE*ab) values of OMC (1.4–2.4) were smaller than those of ELQ (2.9–15.4) and FSU (2.6–13.4). All the ΔE*ab values of OMC were rated as acceptable matches regardless of cavity depth or artificial tooth shade, meaning that OMC exhibited the most significant and pronounced color-matching ability in this study. Regarding the color adaptation properties of the structural colored resin composite OMC, it is important that more than 90% of the individual spherical filler has an average particle size of 260 nm.15 With this material, colored light can be clearly observed due to scattering without using pigments or dyes. The structural color may be unobservable because of increased opaque tones, or the refractive index difference between the filler and organic matrix may increase;25 thus, the size and refractive index of the filler are important in determining the color characteristics of the structural colored resin composite. If the filler size is far below the wavelength of visible light, it will not scatter or absorb the light, making structural color less likely to occur. The reflective index of the filler particles should be in the range of 1.47–1.52 and correspond to that of the polymerized resin matrix. To obtain uniformly sized spherical particle fillers with a defined reflective index, a bottom-up colloidal assembly approach known as the “sol-gel method” has been used.26 The assembly of colloidal particles is thought to be one of the most facile and cost-effective methods for fabricating three-dimensional structural color materials.27
Individual differences exist in the color of natural teeth, and the color varies according to the area for restoration. Resin composites must mimic the color appearance of the restored teeth to efficiently achieve patient satisfaction with desirable esthetics.
CONCLUSION
Within the limitations of this study, excellent color matching ability was confirmed for the structural colored resin composite OMC. However, since color changes over the white background showed decreased color matching even when using OMC, care should be taken to ensure that the appropriate chromatic color conditions for restoration are achieved in the oral environment.
Color measurement of resin composite specimens. Resin composite discs were prepared, and the spectral reflectance and color values of the specimens were measured against black and white backgrounds using a fast spectrophotometer (CMS-35FS/C, Murakami Shikisai).
Evaluation of color-matching ability of resin composites. Color measurements of artificial teeth and resin fillings were performed. Abbreviations: ELQ, Estelite ∑ Quick; FSU, Filtek Supreme Ultra; OMC, Omnichroma.
Influence of background color and specimen thickness on spectral reflectance curves of resin composites (n=6). Abbreviations: ELQ, Estelite ∑ Quick; FSU, Filtek Supreme Ultra; OMC, Omnichroma.
Influence of background color and specimen thickness on L*, a*, and b* values of resin composites (n=6). Values in parenthesis indicate standard deviations. For the same materials, means with the same lowercase letter are not significantly different (p>0.05). Abbreviations: ELQ, Estelite ∑ Quick; FSU, Filtek Supreme Ultra; OMC, Omnichroma.
Contributor Notes
Makoto Saegusa, DDS, Department of Operative Dentistry, Nihon University Graduate School of Dentistry, Tokyo, Japan
*Hiroyasu Kurokawa, DDS, PhD, Department of Operative Dentistry, Nihon University School of Dentistry, Tokyo, Japan
Nao Takahashi, DDS, Department of Operative Dentistry, Nihon University Graduate School of Dentistry, Tokyo, Japan
Toshiki Takamizawa, DDS, PhD, Department of Operative Dentistry, Nihon University School of Dentistry, Tokyo, Japan
Ryo Ishii, DDS, PhD, Department of Operative Dentistry, Nihon University School of Dentistry, Tokyo, Japan
Koji Shiratsuchi, DDS, PhD, Department of Operative Dentistry, Nihon University School of Dentistry, Tokyo, Japan
Masashi Miyazaki, DDS, PhD, Department of Operative Dentistry, Nihon University School of Dentistry, Tokyo, Japan
Clinical Relevance
Although excellent color-matching ability was confirmed for the structural colored resin composite Omnichroma, care should be taken to ensure that the appropriate chromatic color conditions for restoration are achieved in the oral environment.