45 - Resin-Cement Coating on Machined Glass-Ceramic: Effect on Flexural Fatigue Strength

Abstract

To assess the effect of resin cement coating with high and low viscosities on the flexural fatigue strength of machined vs. polished lithium disilicate glass-ceramic. Disc-shaped samples were prepared (IPS e.max CAD) and divided according to surface condition (polishing P and machining M), resin cement coating (yes or no) and cement viscosity (high and low). Surface roughness was measured on a contact profilometer. Cement-coated specimens received primer application (Monodond Etch & Prime) followed by resin cement (Variolink N high and low viscosities), which was pressed over an insulated glass sheet and photopolymerized. Biaxial flexural fatigue strength was evaluated on a piston-on-three-ball set by the step-test method (n = 15) (initial stress of 60 MPa; incremental steps of 20 MPa; 10,000 cycles per step, at 20 Hz). Weibull statistics was used for analysis of the flexural fatigue strength, and ANOVA/Tukey post hoc test (α= 5%) to compare roughness between M and P conditions. Contact angle analysis on goniometer, topographic and fractographic analysis on scanning electron microscope were also performed. Machining produced higher roughness and lower contact angle than polishing, as well a significant deleterious effect on the characteristic flexural fatigue strength ceramic fatigue behavior (M: 247.2 vs P: 337.4 MPa). However, machined and coated groups presented similar fatigue behavior to the polished and coated ones, irrespective of the cement viscosity. Cement coating was able to revert the impact of machining on the fatigue strength of lithium disilicate glass-ceramic. High and low viscosity cements behave similarly on the improvement of CAD-CAM lithium disilicate fatigue strength.