Statement of problem The effects of cement color and thickness on the color of machinable lithium disilicate (MLD) ceramic luted on metal foundation restorations is unknown. Purpose The purpose of this study was to evaluate the effects of cement color and thickness on the shade matching of MLD restorations luted on silver-palladium (Ag-Pd) foundations. Material and methods Fifteen 1.5-mm-thick ceramic specimens were made from shade A1 LT lithium disilicate blocks. Five resin cements with different colors and opacities (Multilink Automix white opaque, Multilink Automix yellow, Nexus3 white opaque, Nexus3 white, Nexus3 yellow) of 3 thicknesses (300 μm, 100 μm, 50 μm) were sequentially luted to a roughened Ag-Pd alloy foundation restoration block. Five shade measurements were made with a portable spectrophotometer after optically connecting the ceramic specimen to each cement foundation block. The color differences (ΔE) between each specimen assembly and the target block (a 14×14×12-mm shade A1 LT crystalized e.max block) were recorded with the CIE (Commission internationale de l'éclairage) L*a* b* system. Clinical significance was determined by comparing color differences to perceptibility and acceptability thresholds with the 1-sample t test (α=.05). Results Both cement color and cement thickness significantly affected the mean values of the color difference (ΔE) of lithium disilicate ceramic assemblies to the target block (P<.001). Among the 5 cements tested, the lowest mean ΔE was observed for Nexus3 white opaque cement. Overall, the combination of Nexus3 white opaque cement and 100-μm thickness led to the lowest mean ΔE. The means of ΔE were below the clinically perceptible level (ΔE<2.6) for combinations of Nexus3 white opaque of 100-μm and 300-μm cement thicknesses (P<.001 and P=.022, respectively). The means of ΔE were below the clinically acceptable level (ΔE<5.5) for the following combinations: Nexus3 white opaque of 50-μm thickness, Nexus3 white, and Nexus3 yellow for all tested cement thicknesses and Multilink white opaque or Multilink yellow of either 50-μm or 100-μm cement thickness (P<.001). The means of ΔE were above the clinically acceptable level (ΔE>5.5) for Multilink white opaque and Multilink yellow of 300-μm cement thickness. Conclusions The colors of machinable lithium disilicate ceramic restorations placed on Ag-Pd foundation were affected by both the color and thickness of cements. Among the 5 cements tested in this study, Nexus3 white opaque cement of 100 μm or 300 μm yielded shade matches below the clinical perceptible threshold (ΔE<2.6) relative to the target block.
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