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Color, Thickness, Foundation, Ceramics, Zirconium, Gold Alloys, IntroductionThe main challenge in esthetic dentistry is to optimally match the optical properties of restorative materials with the natural teeth [ 1, - 10, ]. Different ceramic systems are commercially available now [ 2, , 11, ]. Among different types of ceramics, the use of zirconia restorations is considerably increasing. Improved physical, mechanical and chemical properties, high fracture resistance and flexural strength, and excellent biocompatibility are some advantages of this ceramic [ 4, , 12, - 15, ]. Although fracture resistance of these prostheses is high, chipping of porcelain, veneer is a major complication. One solution to overcome this problem, was introduction of translucent full anatomical monolithic zirconia restorations [ 12, - 13, , 16, ].Different factors can influence the final color of a ceramic restoration thus, achieving the desired final color in these restorations is considered as a challenge. These factors include the degree of opalescence [ 17, - 19, ], translucency [ 17, - 20, ], fluorescence [ 17, - 19, ], condensation technique, shape properties, surface texture [ 18, - 19, ], chemical nature of the ceramic [ 17, ], ceramic brand [ 18, , 21, ], batches [ 18, , 22, ], underlying tooth structure color [ 2, - 3, , 18, , 20, , 23, ], ceramic firing temperature [ 20, , 22, ], number of ceramic firing cycles [ 18, , 22, ], surface glaze [ 20, , 22, ], ceramic thickness [ 2, , 18, , 22, ], color of the cement [ 2, , 3, , 17, - 18, , 20, , 23, ], manufacturer, and the type of the substructure [ 3, , 17, , 21, - 24, ].One of the most important features for ceramic selection is the translucency of the material. Unlike high-strength core ceramics, high translucent ceramic systems show better esthetic results because they permit more light to transmit and scatter. Light transmission is not always an advantage this means that with increasing translucency, their masking ability reduces, so they are more prone to showing their underlying structures like discolored tooth, foundation materials or luting agents [ 2, - 3, , 25, - 26, ]. Semi-translucent zirconia structure permits a little light to enter and scatter, so it might be concluded that the underlying tooth or substructure has an influence over the resulting color [ 27, ]. To fabricate the ceramic restorations that are more similar to natural dentition in terms of optical properties, the capability of the restoration to mask color variations present in the underlying substructure should be recognized [ 18, ]. Several studies have evaluated the effect of ceramic thickness on masking the color of different foundation materials [ 2, , 4, - 5, , 11, , 18, , 28, - 29, ]. As shown in a study, lithium disilicate glass-ceramic and leucite-based glass-ceramic with a thickness of 2.5 mm could mask the color of yellow zirconia. Zirconia-reinforced lithium silicate glass ceramic with a thickness of 2.5mm could cover the color of yellow zirconia and titanium [ 4, ]. Another study indicated that a 1 mm lithium disilicate ceramic had the ability to mask the gold foundation [ 5, ], while another study showed that a 1.6 mm leucite-based heat-pressed ceramic could cover the color of gold [ 11, ]. The results of one research demonstrated that lithium disilicate with a thickness of 1.5mm could cover the color of silver-palladium and could mask the color of composite resin with a thickness of 2mm [ 5, ].In general, these studies suggested that by increasing the ceramic thickness, shade matching is improved [ 4, - 5, , 11, ]. Although many studies reported the masking ability of different types of ceramic systems, limited information is on hand regarding the masking ability of monolithic zirconia [ 2, , 4, - 5, , 11, , 18, , 28, - 29, ]. The purpose of this study was to find out the influence of different foundation materials on optical properties of monolithic zirconia at variable thicknesses. The null hypothesis was that the foundation materials and ceramic thickness would not affect the final color of monolithic zirconia restoration.Materials and MethodThirty ceramic disks with shade A2 were cut from high translucent monolithic zirconia block (Kerox dental zirconia). The specimens had thicknesses of 0.6mm, 1.1mm and 1.5mm (n=10) and diameter of 10mm )Table 1,). A CAD/CAM system (IMES-ICORE CORITEC 340 i) milled the monolithic zirconia blanks to fabricate the specimens (Fabricating the specimens was done by dry-milling with four axes). Crystallization of the ceramics was done according to the manufacturer&apos,s recommendation in a furnace (MIHM-VOGT Dental- Ger&,auml tebau, HT speed) in the temperature of 1450&,deg C for 8 hours. The specimens were then polished with 220, 400, 600, 800, 1000 and 1200-grit abrasive silicon carbide paper on a grinder-polisher machine (Phoenix Beta Buehler) at100 rpm for 15 seconds under cooling water until the desired thickness (0.6, 1.1 and 1.5mm) had been achieved with a tolerance of &,plusmn 0.02mm. The thickness of each specimen was controlled by a digital caliper (Mitutoyo Corp. CD-8", CSX 500-197-20). The ceramics were cleaned in an ultrasonic bath (Elmasonic S-30 Dentec, North Shore) containing 98% ethanol for 15 minutes and finally dried with oil-free compressed air (Figure 1,).MaterialManufacturerTypeMonolithic zirconiaKerox dental zirconiaHigh translucent Shade H2Composite resinNexco, Ivoclar VivadentDentin Shade A2Non- precious gold alloyAalbaDentType 2 Cu 80.7% Al 7.8% Ni 4.3% Fe, Zn, MnNickel- chromium alloy4 all, Ivoclar vivadentNi 61.4% Cr 25.7% Mo 11% Si 1.5% Mn&,lt 1% Al&,lt 1% C&,lt 1%ZirconiaDental Direkt GmbHDD Bio ZW iso WhiteTable 1.The materials usedFigure 1.Monolithic zirconia disk specimens (from left to right, ceramic thicknesses of 0.6, 1.1, and 1.5 mm)Four disk shaped foundation specimens were fabricated of nickel-chromium alloy (Ni-Cr) (4 all Ivoclar vivadent), non-precious gold alloy (NPG) (AalbaDent), zirconia (DD Bio ZW iso White, Dental Direkt GmbH), and shade A2 build up resin composite (dentin Nexco, Ivoclar vivadent) (Table 1,). The zirconia and wax patterns of Ni-Cr and NPG specimens were milled (12&,times 3mm) by the same CAD/CAM system described earlier then, casting of metallic specimens was done. Build up composite resin specimen (12&,times 5 mm) was prepared in a mold pattern made from mixed polyvinyl siloxane impression material (Extrude medium body Kerr). Composite resin was applied into the mold and a microscope slide was placed on the top of the mold. Then it was cured by a light polymerizing device (Lummat 100 Ivoclar vivadent) for 90 seconds. Composite resin specimen was considered as the control group and comparison of color differences was made with this group (Figure 2,).Figure 2.Foundation material specimens (from left to right, composite resin, non- precious gold alloy, nickel chromium alloy, and zirconia)Monolithic zirconia disks were placed over each foundation material with a water drop between them to prevent the light refraction. Ceramic-foundation material assemblies were placed on a white background and color measurements were done by a spectrophotometer (VITA Easyshade&,reg V) in a dark room. For each ceramic-foundation material combination, five shade measurements were made and the mean value for each combination was calculated. The CIE L*a*b*parameters of each combination were recorded. In addition, the color difference (&,#8710 E) values between each ceramic-substructure assembly and the control group were calculated, using the following formula [ 2, , 14, , 17, - 18, , 27, - 32, ], &,#8710 E = [ ( L T - L C ) 2 + ( a T - a C ) 2 + ( b T - b C ) 2 ] 1 / 2 Where T represents the test groups (NPG, Ni-Cr and zirconia) and C represents the control group which was the composite resin. In this study, &,#8710 E&,gt 1.3 was set as clinically perceptible and &,#8710 E&,lt 2.25 was considered as clinically acceptable. The ability of monolithic zirconia specimens to mask the underlying structure was defined by the clinically acceptable threshold (&,#8710 E= 2.25) it means with color differences below 2.25, monolithic zirconia ceramic could mask its underlying material [ 3, ].The normality assumption was assessed using Kolmogorov-Smirnov test. The data were statistically analyzed using two-way ANOVA with a= 0.05 as the level of significance and whenever a significant interaction was observed, the post hoc Tukey test was carried out. All the computational work was done using the statistical software (IBM SPSS Statistics v18.0 IBM Corp). ResultsKolmogorov-Smirnov test revealed no violation from normal distribution in the groups. As shown in Table 2,, ceramic thickness, foundation materials, and interaction of these two variables had a significant effect on the mean values of &,#8710 E of monolithic zirconia ceramic assemblies (p= 0.001). The mean values of &,#8710 E are shown in Table 3, and Figure 3,.SourceSSdfMSFp valueFoundation39.60219.8095.74&,lt 0.001Thickness17.8528.9343.17&,lt 0.001Interaction6.2541.567.56&,lt 0.001Error11.17540.21--Total241.6863--- SS, Type III sum of square, MS, Mean square, df, degrees of freedom |