A dental crown usually consists of two parts: the core and the veneer. Cores provide strength for dental restorations, and veneers restore the appearance and color of the teeth. Composite resins and ceramics are common veneer materials. There are several advantages to ceramics, such as wear resistance, favorable visual properties, less plaque accumulation but a major disadvantage is their fragility. Alternatively, composite resins can serve as a substitute for ceramic veneers. In addition to absorbing occlusal forces, composite materials reduce the weight and cost of expanded prosthetics. When compared to ceramics, this category has a lower thermal expansion coefficient, less dimensional change, and less wear on the opposite teeth. Their disadvantages include plaque accumulation and low wear resistance. Additionally, the bond between composite veneers and different cores, such as zirconia, metal alloys, and peak, has been questioned and investigated several studies. As a result of these limitations, ceramic remains a viable veneering option for core materials. In the light of the widespread acceptance and clinical efficacy of composites and ceramics as veneer materials, the present study examined the effect of the different core materials (chrome cobalt-zirconia-PEEK) on the final color of composite and heat-pressed ceramic veneers.
In order to eliminate core thickness effects and make veneer thickness uniform, all samples had the same core thickness. In CAD/CAM systems, 0.5 mm is the minimum thickness for cores and is suitable for all of these restorations. According to Fazi et al., veneer thickness had a more significant effect on final color than core thickness and ceramic color.(11) In this research, thicknesses were based on clinically applicable tooth preparation; 1.5 mm reduction of the buccal surface was followed by 0.5 mm of core to create strength, and a 1 mm veneer to create a toothlike appearance.
Clinically acceptable thresholds for color difference are difficult to determine. Numbers between 0–1 indicate the color difference that is invisible to the naked eye; Numbers between 1-2.5 indicate a slight color difference that can only be seen by trained eyes. Numbers between 2.5–3.7 indicate an amount of color difference that can be seen by human eyes, and the clinically acceptable threshold is 3.7. Several studies have reported the same number. (12)
Based on the statistical analysis in this research, the research hypothesis was confirmed because there is a significant difference between the color changes (ΔΕ) of different groups (P value = 0.00). Cr-Co-ceramic samples did not present acceptable clinical performances (ΔΕ = 6.46). Metal backgrounds are opaque and highly reflective due to their atomic nature, which absorbs and reflects light. The metallic color of the background can be reflected when translucent materials are used on a metallic background. When it comes to non-metallic materials, the passing of light through their bulk is unavoidable, which can have a significant impact on their visual characteristics. On the other hand, the average of color difference for zirconia-ceramic samples was ΔΕ = 1.97, which was not above the clinical acceptable level.
In composite group, despite a minimum veneer thickness of 1 mm, the color covering ability was adequate and there was no significant difference between those samples veneered with composite (P value = 0.186). Color differences for PEEK, zirconia, and Cr-Co cores were 2.75, 3.24, and 2.91, respectively, all of which fell within the clinically acceptable range (E∆ = 3.7); however, their color differences were higher than the threshold visible to the human eye (E∆>2.5). These results can associate with using Crea.lign opaquers of the same color which improved the final color of composite samples, removed opaque shadows from beneath cores, and increased bond strength when combined with primer.
To the best of the authors' knowledge, this is the first study to examine the effect of core type on color parameters of veneer composites made with CAD/CAM systems. Compared with other studies, this study uses a different method and type of veneer, making comparison and appropriate conclusions difficult. Zeighami et. al. found that shadow of the PEEK cores (white and dentin) had significant effects on the final color of veneered samples with indirect composite veneer. Based on 0.5 mm of core thickness and 1 mm for veneer thickness in that study, ∆E for white core was 4 and ∆E for dentin core was 2.75. In the present study, a white core with a thickness of 0.5 mm was veneered with 1 mm CAD/CAM composite and ∆E was calculated as 2.75, which is clinically different from the study of Zeighami et. al. In addition, the effect of core type in composite veneered colores was not statistically significant in the present study, unlike Zeighami's study. The different results could be attributed to the use of CAD/CAM composites in this study, which have better color characteristics and color coverage than the manually placed composites in the Zeighami's study. Also, the spectrophotometer device and the observer's viewing angle were different between the two studies.(12)
With the exception of Cr-Co-Ceramic specimens, all study groups showed color differences within clinically acceptable level (ΔΕ < 3.7). A study by Kourtis et al. found that two factors, the ceramic type and the alloy in metal-ceramic restorations, influenced the final color. In their study, the metal alloy thickness was 1 mm and the ceramic thickness was 1.2 mm. Hues were greater in gold alloys and Cr-Co alloys than in the Ni-Cr and the Pd alloys.(19)
Nakamura et al. similarly found that various metallic and nonmetallic backgrounds did have a significantly different effect on the final color of four types of composite veneer and one heat-pressed ceramic samples. The color differences between all specimens when backed with silver-palladium backgrounds in comparison with A3 color backgrounds (control groups) could be detectable by human eyes (2/26 − 4/67). They concluded that when silver-palladium used as a background it is difficult to achieve an adequate color match. However, color difference was not significant in all type of specimens when A1 color ceramic and gold alloy backgrounds were used. In the present study, composite samples did not show significantly different ∆E in three core groups.(13) This difference in results may due to that Nakamura used a composite with a high filler content to improve strength and wear resistance. As a result, this composite has a translucency similar to ceramics, which may be due to the similar reflection coefficients of the matrix and the filler. The use of materials with higher translucency causes more light to pass through and scatter light from the background. The composite used in the current study (High impact polymer composite) has a high percentage of filler along with micro ceramics in the matrix. Because of the laboratory process, the residual monomer percentage is very low, and the smooth surface shows only minor plaque accumulation and excellent color stability.
According to Chaiyabutr and his colleagues, dental abutment color, cement color, and ceramic thickness influence the color characteristics of lithium disilicate ceramic veneers. The samples used in this study were anatomical CAD/CAM veneers. Dark colored abutment teeth showed the greatest color difference, and in ceramics with thicknesses of 1 mm (no matter whether the cement is opaque or transparent) or 1.5 mm (if translucent cement is used), color differences were higher than the clinically acceptable threshold (∆E > 3.7).(14) According to the current study, the 1 mm thickness of the ceramic veneer was affected by the background color, and in the Cr-Co group, the results were not acceptable from a clinical point of view. Chaiyabut's study indicates the glass matrix and lithium disilicate crystal phase are responsible for the effect of background color on the ceramic veneer color. According to their study, light scattering inside the glass matrix is reduced and therefore the translucency of the veneer increases.
In Koutaya’s study, 0.6 mm densely sintered alumina ceramic was veneered using 2 mm feldspathic porcelain. The effect of the underlying core (high-precious gold alloy, aluminum-oxide ceramic material, titanium metal alloy, yttrium-stabilized zirconium dioxide ceramic material, and glass-ceramic material) color on the final color of the restorations veneered with dense alumina ceramic was statistically significant, however, could not be detected by the human eye (ΔΕ < 2). This result can be attributed to the use of a dense alumina disc veneered with feldspathic porcelain along with opaque cements, which acted as color insulators for the background. In Koutaya’s study, zirconia cores showed more color difference than samples with more translucency such as lithium disilicates, which was due to the white and opaque appearance of zirconia. In the present study, also the average color difference of zirconia cores was reported to be higher in the group of zirconia-composite than other composite groups.(15)
In Suputtamongkol's zirconia-based cores and a lithium disilicate-based veneers were cemented to metal posts and cores or prefabricated posts and composite buildups. According to the results of this research, the background color can affect the overall color of premolar and molar all-ceramic zirconia crowns with 1.5 mm thickness (ΔΕ = 1.2–3.1) and the cement layer had very little effect on the final color. In this study, although color changes can be detected by colorimetric instruments, it was still in the clinically acceptable range regardless of whether the backgrounds are metal posts and cores or prefabricated posts and composite buildups as cores.(16)
Shimada found that the masking ability of ceramic materials are affected by the difference in reflection coefficients between the particles and the matrix, as well as the color pigments. This factor affects the passage and scattering of light from the surface of the material. A higher refractive index results in a higher refraction of light, causing ceramic materials to appear opaque. Depending on the thickness of ceramics, this coefficient changes and determines its color characteristics and its covering properties.(17)
Stawarczyk et al. examined standard samples of PEEK material, zirconia, chromium-cobalt-molybdenum alloy, and titanium oxide along with ceramic veneers of varying thicknesses. The research results indicated that core type, veneer material, and veneer thickness had an impact on the CIE Lab color parameters. In addition, PEEK cores showed the same results as chromium cobalt molybdenum alloys and zirconia cores. Compared to chrome-cobalt-molybdenum, PEEK cores have a similar relative frequency of VITA easy shade parameters. In this study, however, samples of core and veneer were placed on top of each other without cement that could scatter light between them.(18)
In the present study, the core and veneer thickness, as well as the color of the veneer and cement, were standardized to eliminate the effects of variables. However, effects of these variables on the final color of restorations need to be investigated in future studies. Moreover, these materials must be tested in anatomical forms and under conditions more similar to those in clinical practice.