21 Prosthetic Dentistry
layering procedure in order to achieve a sound bond as well as an in depth shade effect and fluorescence. Zirliner has four shades for non shaded zirconia framework or clear for shaded framework.6 With the Lava Zirconia coping, sandblasting the outer surface is not necessary as the milling process results in an adequately rough surface to accept the overlay porcelain. Remarkably, very light sandblasting does not appear to have affected the bonding strength and appears to increase wetting of the porcelain.9 Whereas for the Cercon Degudent, it is advised to sandblast the inside and outside of the framework with alumina (110–125 μm, max 0.3 to 0.4Mpa, 45° angle).10 Applying a liner is indicated for the Cercon framework with the layered porcelain,14 but not required with the pressed porcelain.15 Aboushelib et al. evaluated in 6 papers the microtensile bond strength (MTBS) between the zirconia and the veneering ceramic.11,16,17,18,19,20 (MTBS) was chosen because shear bond strength (SBS) test may lead to undesirable stress distribution, causing cohesive failure and wrong interpretation of the data. When comparing polishing to sandblasting Cercon disc or roughening Vitablocks Mark II, no effect on the core-veneer bond strength was noted. The (MTBS) of the Cercon coreveneer would be weaker if the liner was not applied which is basically used to mask the white color of the zirconia and enhance the bond strength between the core and the veneering porcelain.11 The liner should be used with some layering veneers but not in combination with pressable veneers. The two materials have different structural composition and pressing the veneer ceramic over smooth fired liner material results in poor contact between the two materials. The pressable veneers failed mainly cohesively (within the porcelain) with no liner and interfacially (between the core and veneer) when liner is applied, whereas the layered veneers failed cohesively with liner and interfacially with no liner.16 Using the pressing technique will improve the bond strength while the esthetic outcomes are more difficult to achieve because it depends on the precolored ceramic ingots. Aboushelib et al. introduced a double-veneer technique combining layering porcelain over a previously pressed-on ceramic.17 Hence, the (MTBS) of zirconia and press on ceramic wasn’t affected by the addition of a second layer of veneering esthetic porcelain. When colored zirconia frameworks were introduced to enhance the final esthetics of the layered all-ceramic restoration, Aboushelib et al.18 found that the bond strength of colored zirconia was significantly weaker than the white Dental News, Volume XXII, Number I, 2015
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