The effect of liner application on color and masking opacities of zirconia restorations prepared with different veneering techniques

Partially stabilized yttrium polycrystalline zirconia (Y-TZP), has become a preferred core material for all ceramic restorations with superior mechanical properties. However, its opacity due to the polycrystalline content limits its optical characteristics. Therefore, it is aimed to develop more esthetic restorations by applying liner material over zirconia cores and to increase translucency. The aim of this study was to evaluate the effect of liner application on the color and masking the opacities of zirconia restorations fabricated using different veneering techniques. Optical properties of zirconia groups used in this study were compared with a lithium disilicate based all ceramic system (IPS e.max Press, control group). Totally 8 specimens of A2 colored square-shaped specimens (0.4 mm in height) were prepared using Lava (LV), Lava Plus (LVP) and IPS e.max Press (IP), (n = 8). Liner material compatible with the veneer ceramic was applied on half of the zirconia cores. Lava Ceram veneering ceramic (LC) was applied with layering technique and IPS e.max ZirPress veneer ceramic (IPZP) was applied with pressing technique so that the total specimen thickness was 1 mm. Specimens in the control group were also veneered with IPS e.max Ceram veneering ceramic. Specimens were then fired to achieve an auto-glaze. Color and translucency measurements were carried out with a spectroradiometer. Translucency parameter method was used to evaluate the translucency of all specimens. CIE L? a? b? parameters which were obtained from color measurements were recorded; the color difference (?E) values between the IPS e.max Press group and zirconia groups were determined for the evaluation of success in obtaining the targeted color. Clinical perceptibility and clinical acceptability values were based on respectively ?E=2.6 and ?E=5.5. Data were analyzed statistically performing variance analysis and t-tests for pairwise comparisons, the color differences were classified according to the values of the clinical perceptibility and clinical acceptability. Between the assessed zirconia groups and control group the lowest color difference value was measured from IPZP/LVP/L group, while the highest color difference value was measured from LC/LV/LS group. For all values which were ?E>2.6, color differences between the zirconia and control group were considered as clinically detectable. Color difference between the LC/LV/LS and control group was determined to be ?E=5.5. As the color difference values obtained by the rest of the zirconia groups were ?E<5.5, they were considered as clinically acceptable. As for translucency, IPZP/LVP/L group showed the highest translucency (p <0.05). Translucency values of IPZP/LV/L and IPZP/LVP/LS groups were higher than the control group, but these differences were not statistically significant (p> 0.05). Groups prepared using IPZP veneer ceramic showed higher translucency values than groups prepared using LC veneer ceramic (p <0.05). Groups prepared using LC veneer ceramic showed lower translucency than the control group (p<0.05). In evaluating color difference of between groups applied liner or no-liner, color differences among groups were ?E<2.6, so the liner application had no effect on clinical perceptibility. Two-way analysis was performed in this assessment considering two factors, including veneer and core materials. According to the statistical analysis, color difference among core materials and color difference among veneer/core assemblies were not found statistically significant (p> 0.05), color difference between two veneer materials was found to be statistically significant (p <0.05). Within the limitations of this in vitro study, it was determined that liner application had no detectable effect on color and translucency of zirconia cores. In evaluating the difference between the resulting color and the target color, colors of all groups had limits as clinically acceptable. Keywords: Spectroradiometer, color, translucency, liner, zirconia