Purpose: This study evaluated the effect of thermocycling on the microtensile bond strength of four adhesive luting agents to GN-I CAD-CAM ceramic. The hypothesis tested was that thermocycling did not affect bonding effectiveness, irrespective of the luting agents used.
Materials and methods: Ceramic specimens of two different sizes (6x8x3 mm; 13x8x4 nm) were fabricated from GN-I CAD-CAM ceramic blocks (GC) using a low-speed diamond saw. Two different sized porcelain discs were bonded with one of the four composite luting agents (Linkmax [LM], Panavia [PN], RelyX Unicem [UN] and Variolink II [VL]) according to the manufacturer's instructions. The specimens were stored for 24 hours in distilled water at 37 degrees C and subjected to 0; 10,000; 20,000 and 40,000 thermocycles prior to microTBS testing. Two-way analysis of variance was used to test the influence of luting cement, thermocycling and interaction between both (p < 0.05). The Tukey HSD test determined statistical differences in microTBS for each luting composite between the different thermocycling conditions (p < 0.05). The mode of failure was determined at a magnification of 50x using a stereomicroscope (Wild M5A).
Results: Two-way ANOVA revealed that microtensile bond strength was affected by the luting cement, thermocycling and a combination of both. No difference in bond strength between Linkmax, Panavia F and Variolink II was noticed after 24 hours of water storage (LM: 47.6 MPa; PN: 41 MPa; VL: 36 MPa). RelyX Unicem scored significantly lower than Linkmax and Panavia F (UN: 24.2 MPa). The influence of thermocycling on bond strength was different for the four luting cements. Using Variolink II, the bond strength remained stable after 40,000 thermocycles (43.6 MPa). Linkmax showed a significant decrease in bond strength after 10,000 (26 MPa) and 40,000 thermocycles (14.8 MPa). Panavia F and RelyX Unicem were the most negatively influenced, as all specimens failed prior to testing (pre-testing failures) when the specimens were thermocycled 10,000 and 20,000 times or longer, respectively. Regarding the failure mode, there was a correlation between bond strength and type of failure. Initially, a combination of adhesive and mixed adhesive-cohesive failures was noticed. The percentage of adhesive failures increased, together with a decrease in bond strength.
Conclusion: It was concluded that there were significant differences among the four resin composite cements in terms of their bonding effectiveness to CAD-CAM ceramic after thermocycling. The varying degrees of bonding effectiveness of these adhesive luting agents highlight the need for material specifications.