Over the decades the conventional metal ceramic restorations have been considered the standard for providing acceptable esthetics, but some concerns regarding non-optimal esthetics have been reported recently (3). The main complaint for esthetically unpleasant restorations seems to be the opaque porcelain application to mask a metal substructure that causes undesirable light reflection (4).
Esthetically fabricated all-ceramic restorations should have a color and translucency comparable to those of natural teeth (8). There are three methods for evaluating the translucency of dental ceramics: direct transmission, total transmission and spectral reflectance (9). Translucency parameter (TP) defines the difference between reflected colors of a material with a uniform thickness over a black and a white background and provides a value corresponding directly to human visual perception of translucency (10).
Different all-ceramic systems have different TPs. The zirconia core is less translucent than other dental all-ceramic materials such as glass-infiltrated ceramics, which is known for its good esthetic properties (11, 12). Translucency of zirconia is related to the amount and type of additives, the sintering temperature, the atmospheric conditions during the sintering process and the heating methods (13-16).
Therefore, the aim of this study was to evaluate and compare the translucency of two different all-ceramic systems using Vita Easyshade digital shade matching device in an in vitro model.
The first hypothesis was that the different all-ceramic systems would have different translucency. The second hypothesis tested was that the stages of fabricating would affect the translucency of different ceramic specimens.
Materials and methods
In this study, 2 different all-ceramic systems were tested (IPS e.max Press, Ivoclar Vivadent, Schaan, Liechtenstein; Ceramill ZI, Amann Girrbach, Koblach, Austria). Five square-shaped (11 mm X 11 mm X 0.5 mm) specimens were fabricated for each material following the manufacturers' instructions in A1 shade according to Vitapan Classical shade tab (VITA Zahnfabrik, Bad Säckingen, Germany).
For fabricating glass-ceramic core specimens, square-shaped wax patterns were prepared 11 mm X 11 mm X 1 mm, invested in a phosphate-bonded investment (IPS Press Vest Speed, Ivoclar Vivadent, Schaan, Liechtensetin) and burned out in a furnace (VITA Vacumat 300; VITA Zahnfabrik, Bad Säckingen, Germany) at 850̊C. Low translucency (LT) ingots were used to obtain A1 shade. The specimens were heat-pressed (Ivoclar EP600 Combi, Ivoclar Vivadent, Schaan, Liechtenstein) and left at room temperature for 30 minutes. Investment material was removed and specimens polished using a polishing machine (Polix 905, Silfradent, Viterbo, Italy). The thickness of the specimens was controlled with a digital caliper (Pittsburgh, Camarillo, CA, USA) with an accuracy of 0.01 mm and final thickness was set to 0.5 mm ±0.05 mm.
For fabricating zirconia core specimens, Ceramill ZI presintered blocks of Y-TZP ZrO2 (Amann Girrbach, Koblach, Austria) were milled with CAD/CAM system (Cerec 3, Sirona Dental Systems, Bensheim, Germany). The specimens were colored using Ceramill liquid A1 (Amann Girrbach, Koblach, Austria), left to dry for 45 minutes and subsequently sintered in a sintering furnace according to the manufacturer’s heat and time instructions (Elektron, Banja Koviljača, Srbija). At the end of the process the thickness of the specimens was controlled with the same digital caliper and the dimensions of 11 mm X 11 mm X 0.5 mm were obtained.
Before the measurement, all the specimens were ultrasonically cleaned in distilled water for 10 minutes and dried with compressed air.
Translucency was measured using spectrophotometer Vita Easyshade (VITA Zahnfabrik, Bad Säckingen, Germany). The color of each specimen was measured according to Commission Internationale de l'Eclairage (CIE) system based on three coordinates – L*a*b*. The color of the specimen was measured over a white (CIE L*=99.1, a*=1.8, b*=0.4) and a black (CIE L*=0.3, a*=6.4, b*=-20.3) background in a viewing booth under D65 standard illumination (Figure 1). Before the measurement, the spectrophotometer was calibrated according to the manufacturer's instructions. The measurement was performed by one examiner, well trained in color assessment. The color of the specimen was measured consecutively three times, and the average of the three readings was calculated to give the initial color of the specimen.
The TP was obtained by calculating the color difference between the specimen over the white background and that over the black background as follows (10):
TP = [(Lwhite* – Lblack*)2 + (awhite* – ablack*)2 + (bwhite* – bblack*)2]1/2
Veneered and glazed core stage
After the initial translucency evaluation all the specimens were veneered and glazed with layering technique considering the firing shrinkage of the ceramics. After firing the specimens in a ceramic furnace (IPS e.max Ceram, Programat P300, Ivoclar Vivadent, Schaan, Liechtensetin) the thickness of the specimens was controlled and a thin layer of glazing liquid was applied and glazed in the ceramic furnace following the manufacturers' instructions. The thickness of the specimens was controlled again and set to 1.5 mm ± 0.05 mm.
Translucency measurement was repeated and TP calculated as previously described (Figure 2).
The difference between L*a*b* values dependent on the core stage, type of ceramic material and color of the background was analyzed using independent t-test. TP values in different ceramic systems were analyzed using one-way ANOVA and Bonferoni corrections. The statistical analysis was performed using SPSS statistical program 19.0; (SPSS, Chicago, IL, USA).
On the white background the mean L* and b* values for glass-ceramic core (e.max) were significantly higher and a* values significantly lower than L*a*b* values for glass-ceramic veneered and glazed specimens (Figure 3, Table 1). The mean L* values for zirconia core (ZrO2) were significantly lower and a* and b* values significantly higher than for zirconia veneered and glazed specimens on the same background (Figure 3, Table 1).
On the black background the mean L* values for glass-ceramic core were slightly, but not significantly lower than the same values for veneered and glazed specimens, while a* and b* values had the same values (Figure 4, Table 1). For the zirconia ceramics, the mean L* values were significantly higher for the veneered and glazed specimens, and a* and b* values were significantly lower (Figure 4, Table 1).
On both backgrounds the mean L*values for both zirconia core and veneered and glazed specimens were significantly lower than for glass-ceramic ones, with the exception of L* values for zirconia veneered and glazed specimens on the black background where the mean L* values were higher (Figures 3 and 4, Table 2). The mean a* and b* values on both backgrounds were significantly higher for both zirconia specimens (Figures 3 and 4, Table 2).
The translucencies of zirconia core specimens as well as veneered and glazed ones were lower than the glass-ceramic ones (F=75.682; df=3; p=0.000; Figure 5, Table 4). All the veneered and glazed specimens had lower values of translucency in comparison with their core pair (Figure 5, Table 4).
Digitally measured CIE L*a*b* values on the white and black background in this study served for the mathematical calculation of TP (10). In this study, glass-ceramic specimens in different stages of fabrication (core, veneered and glazed) showed higher translucency compared to the zirconia ones and therefore both our hypotheses that the different all-ceramic systems would have different translucency and that the stages of fabricating would affect the translucency of different ceramic specimens were accepted.
Our results are in agreement with some previous studies. Wang et al. reported TP values of the glass-ceramics that ranged from 2.2 to 25.3 and the zirconia ceramics from 5.5 to 15.1 (17). In our study, TP for glass-ceramic veneered and glazed specimens was 12 ±1.1 and for zirconia veneered and glazed specimens 8 ± 0.7. Significantly higher rate of translucency in lithium disilicate glass ceramic in comparison to the zirconia ceramics was also found in Baldissara et al., Kurtulmus-Yilmaz et al. and many others (18-21). This finding can be attributed to the crystalline content of zirconia in order to achieve a greater strength results, but at the same time the inhomogeneity of crystals causes different refractive indices and therefore poor translucency (22, 23). In order to improve the optical behavior of the zirconia restorations, shaded zirconia cores were developed (24-26).
The second hypothesis in our study was that the stages of fabricating would affect the translucency of different ceramic specimens and it was supported by the results. The data obtained from this study showed that the translucency of veneered and glazed ceramics significantly decreased and the translucency parameters of both ceramic systems changed after veneering (Figure 5, Table 3). The results of Heffernan et al. also showed significant decrease of translucency after veneering process (11, 12). They have explained it by the fact that the structure and size of veneer ceramic, increased specimen thickness, reflectance and the interface between the core and veneer ceramic and the changes that occurred in core material after additional firing might cause the translucency difference (11, 12).
The esthetics, together with the translucency of the dental ceramic materials, changes depending on the properties of the material, manipulative variables and the environment (3). Stevenson and Ibbetson reported that the shades of all-ceramic restorations were also influenced by tooth color, thickness of the ceramic layers and material opacity (27). Heffernan et al. suggested that lithium disilicate cores should be fabricated at a minimum thickness of 0.8 mm and glass infiltrated and yttrium stabilized zirconia cores at 0.5 mm according to the manufacturer’s' recommendations (11, 12).
In the present study, the specimen core thicknesses were determined to be 0.5 mm and the veneered and glazed thicknesses were 1.5 mm which is in accordance with Heffernan's suggestion. We did not use cores with different thicknesses because it was previously reported that the material thickness changes the translucency (4, 28-30).
As a limitation of the study, we used only one measuring device (the first prototype of VITA Easyshade), chose only two all-ceramic systems different in structure and fabrication used in our everyday clinical practice and evaluated only shade A1. Therefore, in our further investigation we have to increase the number of specimens, all-ceramic systems, shades and measuring devices.
The translucency of two different dental ceramics was significantly influenced by both material and stages of preparation. Glass-ceramic system revealed higher translucency and in both systems the translucency decreased after veneering and glazing. Within the limitations of the experiment, these results can be valuable and help the clinician to make appropriate esthetic decisions.