Statement of problem
Information regarding the influence of cements and material thickness on the final color of monolithic zirconia restorations is lacking.
The purpose of this in vitro study was to examine the effect of varying resin cement colors and material thicknesses on the color and translucency of a high-translucency monolithic zirconia and to compare these effects with those reported in similar studies that examined other dental zirconia materials.
Material and methods
Katana High Translucent (Kuraray) was used as a monolithic zirconia material. A total of 80 disk specimens (10 mm in diameter) were made in 4 different thicknesses of 0.5 mm, 1 mm, 1.5 mm, and 2 mm (n=20 per thickness). The color of the specimens (Commission Internationale de l’Eclairage [CIE] L*, a*, b* values) before cementation was measured using a spectrophotometer. Specimens within each thickness were further divided into 2 groups: transparent (n=10) and opaque (n=10). A transparent or opaque self-etch adhesive resin cement (Panavia V5) was then applied to each specimen. After cementation, the color was measured again. The translucency parameter (TP) and ΔE were calculated and evaluated with the color measurements by using descriptive statistics, correlation analysis, single specimen t test, 2-way ANOVA, and the Tukey Honestly Significant Difference (HSD) test.
Statistically significant ( P <.001) changes were found with the increasing thicknesses of the high-translucency zirconia specimens. The TP, L*, and b* values decreased, whereas the a* values increased. In both the transparent and opaque groups, statistically significant ( P <.001) increases in L*, a*, and b* values and a significant decrease in TP were found with cementation. The lowest ΔE value (1.19 for 2 mm) was observed for monolithic zirconia-clear cement. The highest ΔE value (8.05 for the 0.5 mm) was observed for the monolithic zirconia and opaque cement combination.
Material thickness and cement color affected the color and translucency of high-translucent monolithic zirconia, with effects similar to those observed with other monolithic zirconia materials.
Successful prosthetic restoration requires attention to optimal esthetic outcomes. Given the findings of this study and its limitations, consideration of cement color (opaque versus translucent) and restoration thickness will affect the final color and transparency of restorations made with high-translucency monolithic zirconia.
For patient satisfaction, restoration of missing teeth and tooth tissues with biocompatible materials that are resistant to occlusal forces also require attention to esthetics. Esthetic restorations should be able to mimic the natural color depth of teeth, luminous transmittance, and anatomic structures.
In recent years, monolithic zirconia crowns have been developed. Monolithic zirconias, which are produced with computer-aided design and computer-aided manufacturing (CAD-CAM) systems, consist of atoms interpenetrating without any organic binder and also have high dimensional stability. Because they have a natural appearance, they do not need to be veneered with feldspathic ceramics, and natural tooth color can be obtained by characterization. Therefore, chipping of the veneer porcelain is not a factor. Monolithic zirconias have high biocompatibility and a nonporous structure, and their abrasion resistance is close to that of natural teeth. They cause significantly lower abrasion in antagonist teeth than veneered zirconia crowns. In addition, they can be used in situations with insufficient interocclusal clearance, even at an occlusal thickness of 0.5 mm because of their high resistance to fracture.
Resin cements are favorable luting agents for indirect esthetic restorations because of their high retentive strength, low solubility, and resistance to wear. The content of resin cement plays an important role in the connection of cement to zirconia. Resin cement systems containing 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) have increased long-term bond strength to zirconia. Hydroxyl groups on the zirconia surface react with the phosphate groups in 10-MDP, and the Z-O-P chemical bond is formed between zirconia and 10-MDP. Primers containing MDP are bonded to the zirconia surface with the covalent bond, copolymerized with methacrylate groups in the content of resin cements, and provide long-term hydrolytic stability.
Dual-polymerizing systems have been widely adopted to ensure optimal polymerization of resin cement in deep areas. Most dual-polymerizing resin cements contain benzoyl peroxide/tertiary amines as initiators of autopolymerization. However, they have disadvantages. The slow polymerization in the autopolymerization mode causes the resin to diffuse into the cement, and water droplets enter the primer-cement interface. Therefore, this type of resin cement exhibits low adhesion to tooth structure when it is only autopolymerized. Furthermore, discoloration occurs as a result of the oxidation of the amine used for autopolymerization. The oxidation of the aromatic amines required to initiate the polymerization of composite resins is considered responsible for the color change in dual-polymerizing resin cements.
The performance of dual-polymerizing resin cement has been reported to be impaired under insufficient light. In an attempt to overcome these problems, a new autopolymerizing system containing a redox initiator without an amine catalyst has been developed. This system includes chemically stabilized 10-MDP in the primer and exhibits high binding values and long-term color stability. As a result of the interaction between the coinitiators in the primer and the initiators in the resin cement, the conversion of monomers is initiated without the need for light. This mechanism accelerates the polymerization process of the cement contacting the primer. This system ensures that adhesive failures between the tooth and cement do not occur in the autopolymerization mode. Therefore, in the present study, resin cement containing 10-MDP was selected for its high binding values to zirconia, its long-term color stability, and the advantages of the special polymerization mechanism.
Color and translucency can be measured by using spectrophotometers. These instruments provide numerical expressions of color in a 3D space. According to Commission Internationale de l’Eclairage (CIE) Lab color coordinates, L* represents the lightness of the object, and a* and b* represent the location of the object on the blue/green to red/purple axis and the purple/blue to yellow axis, respectively.
The color difference (ΔE) can be determined by comparing the differences between respective coordinate values for each object. Visual assessments can represent detectable color differences (perceptibility) or unacceptable color differences (acceptability) that have clinical significance. O’Brien et al interpreted color differences clinically by classifying acceptable values.
Factors such as form, size, surface structure, color, and translucency are important for esthetic restorations. The final color of a translucent material can be affected by the cement shade, underlying tooth color, and thickness of the restorative material. The authors are unaware of previous studies on the effect of thickness reduction on the color and translucency of high-translucency monolithic zirconia materials. However, in a previous study that evaluated the influence of type of cement on the color and translucency of monolithic zirconia, the color coordinates of each type of cement were not determined appropriately. Information regarding the influence of different cement shades and material thickness on the final color of high-translucency monolithic zirconia restorations is lacking.
The purpose of this in vitro study was to examine the effect of varying resin cement colors and material thicknesses on the color and translucency of a high-translucency monolithic zirconia. A secondary purpose was to compare these effects with those reported in similar studies that examined other dental zirconia materials. The null hypothesis was that resin cement shade and material thickness would not affect the color and translucency of high-translucency monolithic zirconia.
Material and methods
Eighty zirconia specimens were produced with the CAD-CAM system from high-translucency monolithic zirconia (Katana High Translucent; Kuraray Noritake) blocks. The specimens were prepared 25% larger to account for sintering shrinkage and sintered in a furnace (Protherm Furnaces; Alser Technic Seramic) for 2 hours at 1550 °C.
After the sintering procedure, the specimens were allowed to cool to room temperature. The surface of the specimens was finished with abrasive paper #180 with water-cooling. The specimens were divided into 4 groups according to their thickness: 0.5 mm, 1 mm, 1.5 mm, and 2 mm.
Transparent and opaque self-etch adhesive resin cement (Panavia V5; Kuraray Noritake) was used to cement a 0.1-mm thickness as used in previous studies. The cement thickness was standardized with metal plates that were prepared to be 0.1 mm thicker than the thickness of the specimens. Each metal plate (thicknesses of 0.6 mm, 1.1 mm, 1.6 mm, and 2.1 mm) had ten 10-mm diameter holes.
The specimens were placed in the holes in the plates. The adhesive resin cement was placed on the unglazed surfaces of the specimens with the help of special tips in accordance with manufacturer’s instructions. A transparent tape (Transparent Strips, Tor VM) was placed on the specimens to provide separation. The cemented glass was placed on the transparent tape to provide a uniform cement thickness. The specimens cemented with transparent color were irradiated for 2 to 3 seconds with a light-polymerization device (Woodpecker Light Cure LED-D; Guilin Woodpecker Medical Instrument), and residual cement was cleaned. The cement was then irradiated for 10 seconds in accordance with the manufacturer’s instructions. Because the opaque cement was polymerized only chemically (autopolymerizing), it was not irradiated and was allowed to polymerize in the mold for 3 minutes in accordance with the manufacturer’s instructions. After polymerization of the adhesive resin cement had been completed, the disk-shaped specimens were removed from the molds.
The reflectance spectrophotometer device (SpectroShade; MHT Optic Research AG) was calibrated in accordance with the manufacturer’s instructions before each measurement. The CIELab values were measured using a spectrophotometer on each of the 80 specimens prepared. For each specimen, measurements were made at 3 different points on the black (b), white (w), and gray backgrounds, and the mean value of these measurements was obtained. The translucency parameter (TP) was calculated by placing the Lw, aw, bw and Lb, ab, bb values (as obtained by the spectrophotometer) of the specimens placed on the white (w) and black (b) background into the following formula: TP=([Lb−Lw] 2 +[ab−aw] 2 +[bb−bw] 2 ) 1/2 .
The ΔE formula was used to evaluate the effect of different thicknesses of the same specimen and different colors of the cement applied on color: ΔE=([ΔL*] 2 +[Δa*] 2 +[Δb*] 2 ) 1/2 . The data were evaluated using a statistical software program (IBM SPSS Statistics, v20.0; IBM Corp). The TP and color difference (ΔE) were calculated and evaluated with the color measurements by using descriptive statistics, correlation analysis, single specimen t test, 2-way ANOVA, and the Tukey Honestly Significant Difference (HSD) test.
Specimens 0.5 mm in thickness had the highest TP, L*, and b* values and the lowest a* values. Specimens 2 mm in thickness had the lowest TP, L*, and b* values and the highest a* values ( P <.001) ( Table 1 ). The Pearson correlation test results revealed a negative correlation between material thickness and TP, L*, a*, and b* values and also a positive correlation between material thickness and b* values. As the material thickness increased, the TP, L*, and b* values decreased, whereas the a* values increased ( P <.001). The TP, L*, a*, and b* values and standard deviations of the specimens after cementation are presented in Table 2 .
|0.5 mm||20||18.7 ±0.5||76.1 ±0.5||-1.0 ±0.03||22.0 ±0.1|
|1 mm||20||15.0 ±0.5||75.2 ±0.2||-0.6 ±0.05||21.1 ±0.1|
|1.5 mm||20||10.9 ±0.2||74.0 ±0.2||0.1 ±0.02||20.5 ±0.2|
|2 mm||20||8.8 ±0.2||73.2 ±0.1||0.6 ±0.02||20.3 ±0.3|
|Cement Shades||Translucency Parameter||L*||a*||b*|
|0.5 mm||16.11 ±0.20 a||79.07 ±0.23 a||-0.30 ±0.02 a||24.04 ±0.10 a|
|1 mm||13.58 ±0.25 b||77.26 ±0.34 b||0.11 ±0.02 b||22.51 ±0.09 b|
|1.5 mm||10.47 ±0.34 c||75.55 ±0.25 c||0.64 ±0.02 c||21.54 ±0.11 c|
|2 mm||8.74 ±0.17 c||74.05 ±0.21 d||0.92 ±0.04 d||21.05 ±0.15 d|
|0.5 mm||10.50 ±0.32 a||83.57 ±0.24 a||0.47 ±0.05 a||24.52 ±0.06 a|
|1 mm||8.56 ±0.28 b||80.74 ±0.38 b||0.71 ±0.05 b||23.06 ±0.13 b|
|1.5 mm||6.67 ±0.26 c||78.61 ±0.32 c||1.24 ±0.04 c||22.06 ±0.19 c|
|2 mm||5.83 ±0.21 c||76.80 ±0.26 d||1.52 ±0.02 d||21.52 ±0.10 d|