Abstract
The aim of this study was to characterize a conventional and a pressed porcelain for zirconia core as to biaxial flexural strength (BFS), apparent fracture toughness (FT) and microstructure composition, and to investigate the effect of glaze cooling rate on the BFS of the zirconia/porcelain bilayers. Monolayers of conventional porcelain Vita VM9 and pressed porcelain Vita PM9 (n=15) (12 mm diameter x 1.2 mm thick) were prepared for the BFS test (MPa). Apparent fracture toughness (MPa.m1/2) was measured by indentation technique (n=15). t-Student test was performed for statistical analysis. Scanning electron microscopy and x-ray diffraction were used to analyze the porcelain's microstructure. For the BFS of bilayers, zirconia discs (12 mm diameter x 1 mm thick) (Vita In-Ceram YZ) were veneered with the two porcelains (1 mm thick). After the glaze firing simulation, the specimens were submitted to fast or slow cooling (n=15). Apparent fracture toughness (MPa.m1/2) was measured on the porcelain surface of bilayers (n=15) and residual stress was calculated. Two-way ANOVA (porcelain and cooling method) was used for the bilayer analysis (a=0.05). Vita PM9 monolayer exhibited significantly higher BFS (p<0.01), but there was no significant difference (p=0.41) in the FT between the porcelains. For bilayer specimens, the two-way ANOVA for BFS was significant for the porcelain variable only (p<0.01) better for Vita PM9/zirconia. Two-way ANOVA for the FT for the bilayers was not significant for any variable. All groups showed compressive residual stresses. The pressed porcelain seems to be mechanically more effective for zirconia veneering.
Highlights
Besides the suitable mechanical properties of Y-TZP (Yttria Tetragonal Zirconia Polycristal) ceramics and its high survival rates after 5 years (1), clinical failures such as chippings have been reported (2-4) showing rates of 15.2% in 5 years (1) and 14.5% in 5 years (5)
Two recent systematic reviews involving SCs (5) and FPPs (9) in Y-TZP and other ceramic materials suggested that SCs and FPPs with zirconia core should be not considered as the first treatment option, due to higher chipping rates of porcelain and lower survival rates compared to metal-ceramics (5,9)
Besides the similarity in composition between the studied porcelains, as revealed in the XRD analysis and crystalline phase concentration, the higher values of biaxial flexural strength for the monolayer pressed porcelain PM9 compared to the conventional layered VM9 porcelain may be due to the processing technique
Summary
Besides the suitable mechanical properties of Y-TZP (Yttria Tetragonal Zirconia Polycristal) ceramics and its high survival rates after 5 years (1), clinical failures such as chippings (cohesive fracture of porcelain) have been reported (2-4) showing rates of 15.2% in 5 years (1) and 14.5% in 5 years (5). Those failures may be due to an inadequate bond strength between core and veneer (2), poor mechanical properties of the porcelain veneers for zirconia (1), insufficient support by the framework due to inadequate non-anatomical design (7), increased thickness of the veneer porcelain (8) and distribution of residual stress during porcelain cooling (4) Despite these factors, two recent systematic reviews involving SCs (single crowns) (5) and FPPs (fixed partial prostheses) (9) in Y-TZP and other ceramic materials suggested that SCs and FPPs with zirconia core should be not considered as the first treatment option, due to higher chipping rates of porcelain and lower survival rates compared to metal-ceramics (5,9). Such condition increases the possibility of immediate or late veneer porcelain chipping during chewing (12)
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