Effect of Veneering Techniques on Damage and Reliability of Y-TZP Trilayers

Objective: To test the effect of different veneering techniques on failure modes and the reliability of veneered yttrium oxide partially stabilized tetragonal zirconia polycrystal (Y-TZP) in a veneer/core/composite trilayer configuration subjected to 30- degree off-axis mouth-motion step-stress fatigue. Methods: CAD/CAM Y-TZP zirconia plates were veneered using a lost-wax press technique (IPS e.max ZirPress, Ivoclar Vivadent, test group press, n = 24) and a hand build-up technique (IPS e.max Ceram, Ivoclar Vivadent, control group layer, n = 24) (12x12x0.7 mm). After adhesive bonding (Alloy Primer and Panavia 21, Kuraray) to resin-based composite blocks (12x12x4 mm, Z-100), samples were stored in water for 7 days prior to fatigue testing. Trilayer specimens were mouthmotion step-stressed using three profiles (EL-3300, Bose/Enduratec), with a spherical tungsten carbide indenter (r = 3.18 mm) until cracks reached the veneercore interface. All flat-layer specimens were angled 30 degrees with respect to the loading axis, simulating posterior tooth cusp inclination. Step-stress profiles were determined from single-cycle load-to-failure tests (n = 3). Results: Single-cycle load-to-failure tests yielded 825 N for the test group press and 803 N for the control group layer. Irrespective of the applied veneering technique, Y-TZP trilayers failed from surface deep cone cracks reaching the veneercore interface. Radial fractures of the YTZP cores were not present. Weibull stress level probability curves were calculated (AltaPro, ReliaSoft). Similar reliabilities (0.54 test group press, 0.51 control group layer) were determined at 100,000 cycles and a 200 N load. Conclusion: After occlusion-like loading the step-stress fatigue reliability of pressed and layered veneering ceramics over Y-TZP cores was comparable. Fracture was limited to surface damage in the veneer layer. Failure modes were identical for both veneering techniques. Bulk fracture of the YTZP core material was not observed.