الفهرس 
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Abstract
Core veneered zirconia all ceramic restorations gained a lot of interest for the past few years. This is due to combining the strength of zirconia cores coupled with the high esthetic qualities of the veneering ceramic.
Zirconia is considered the material of choice as a ceramic framework due to its unique qualities including, strength, transformation toughening, white color and biocompatibility. However, due to the opaque nature of the polycrystalline zirconia, a more translucent ceramic material must be veneered on the zirconia framework to combine both the strength of the zirconia core and the esthetic nature of the veneering ceramic.
However, different veneering techniques are available nowadays, each of which uses a material with different composition, translucency and reflectivity. Thus, the final color of zirconia based all ceramic restorations is affected by the veneering technique used.
This study was designed to evaluate and compare the color parameters and translucency of different veneering techniques in comparison to a standard Vitapan classic A2 shade tab. Moreover, the effect of low temperature degradation on color and translucency parameters was also evaluated for the different veneering techniques.
For this study, 40 zirconia discs of dimensions 10mm diameter and 0.6mm thickness were prepared by milling using a CNC milling machine. After milling, the discs were fully submerged in a coloring liquid (shade A2) for 20 minutes according to manufacturer’s instructions. The discs were then sintered in a sintering furnace.
Zirliner was then applied to the zirconia discs (n=40) and vibrated till even thickness is achieved and then fired according to manufacturer’s instructions.
To standardize the thickness of the veneering ceramic, two copper molds were fabricated to ensure standard thickness of all veneering techniques.
The discs were then divided into 4 equal groups (n=10) according to the veneering technique used:
• First group: Veneered by layering technique, (n=10)
• Second group: Veneered by pressing technique, (n=10)
• Third group: Veneered by double veneering technique (n=10)and
• Fourth group: Veneered by CAD-on veneering technique. (n=10)
After veneering, the discs were 1.2mm ± 0.1mm thick for all veneering techniques .
The mean color difference ΔE is measured by comparison of the specimens with a standardVitapan Classic A2 shade tab using the following equation:
ΔE = [ (L∗)2 + (Δa∗)2 + (Δb∗)2]1/2
The Translucency parameter (TP) values were evaluated by measuring the mean color difference of the specimens on black and white backgrounds using the following equation:
TP = [(Lb*-Lw*)2 + (ab*- aw* + (bb*- bw*)2]2
Hydrothermal aging was then performed on the discs and the color parameters as well as translucency were measured as previously mentioned and ∆E was compared before and after aging.
All data were tabulated and statistically analyzed. One-way ANOVA test was used to compare between (ΔL), (Δa), (Δb) as well as (ΔE) of the four veneering techniques and a standard A2 shade tab. Repeated measures Analysis of Variance (ANOVA) was used to study the effect of veneering technique, aging and their interactions on mean (TP) value. Tukey’s post-hoc test was used for pair-wise comparisons when ANOVA test is significant
∆L mean values showed statistically significant difference between groups. Pair-wise comparisons revealed that Pressing technique showed the statistically significantly highest mean ΔL (19.22) value resulting in lighter specimens. This was followed by Layering technique and Double veneering techniques, while CAD-on showed the statistically significantly lowest mean ΔL(14.15).
∆a mean values showed a statistically significant difference between the groups (P-value <0.001). Pair-wise comparisons revealed that CAD-on showed the statistically significantly highest mean Δa value (0.26)resulting in a more reddish specimens, followed by Layering and Double veneering techniques while pressing technique showed statistically the lowest value (-1.69)resulting in more greenish specimens.
∆b mean values showed a statistically significant difference between the groups (P-value <0.001). Pair-wise comparisons revealed that CAD-on showed the statistically significantly highest mean Δb value (6.33) resulting in a yellowish specimens, followed by Layering and Double veneering techniques respectively while Pressing technique showed statistically significantly lowest mean Δb value (1.85).
Results of one-way ANOVA and Tukey’s tests showed that there was a statistically significant difference between groups (P-value <0.001) regarding mean color difference ∆E values when compared to a standard A2 shade tab. In addition, pair-wise comparisons revealed that the Pressing group had the highest mean ∆E value (19.39), followed by the Layering group(18.57) and the Double veneering group (16.41), while the CAD-on veneering group had the lowest mean ∆E value(15.52).
∆L mean values also showed statistically significant difference between groups. All (L*) values showed a decrease after aging resulting in darker specimens. Pair-wise comparisons revealed that Pressing showed the statistically significantly highest mean ΔL value(-2.15). Layering technique showed statistically significantly lower mean ΔL value (-1.86) but with non-statistically significant difference from Double veneering. CAD-on showed the statistically significantly lowest mean ΔL (-1.41) but with non-statistically significant difference from Double veneering group.
Δa showed no statistically significant difference between the groups (P-value = 0.161).
Meanwhile, ∆b mean values showed a statistically significant difference between the groups (P-value <0.001). Pair-wise comparisons revealed that Pressing group showed the statistically significantly highest mean Δb values. There was no statistically significant difference between Layering technique and Double veneering groups; both showed statistically significantly lower mean Δb values. CAD-on showed the statistically significantly lowest mean Δb values.
Results of one-way ANOVA and Tukey’s tests showed that there was a statistically significant difference between groups (P-value <0.001) regarding mean ∆E value after aging. In addition, pair-wise comparisons revealed that the Pressing group had the highest mean ∆E value (2.96), followed by the Layering group(2.56) and Double Veneering technique(1.25), while the CAD-on veneering group had the lowest mean ∆E value(1.92).
Repeated measures ANOVA results showed that the veneering technique had a statistically significant effect on mean (TP) values. The translucency of different veneering techniques was measured and the results stated that the Pressing technique showed the statistically highest TP value (11.81), resulting in more translucent specimens followed by Layering technique(10.55). The Double veneering technique (9.13) and CAD-on veneering technique (8.73) showed the lowest mean TP values with no statistical significance between them.
Moreover, Repeated measures ANOVA results showed that hydrothermal aging resulted in a statistically significant decrease in the translucency parameters of all veneering techniques.