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 reliability of Y-TZP core/veneer/composite trilayers under 30° off-axis mouth-motion step-stress fatigue.

METHODS:

CAD/CAM zirconia Y-TZP plates (12x12x0.5 mm, IPS e.max ZirCAD*, *Ivoclar Vivadent) were veneered with two types of veneering porcelain (12x12x0.7 mm) before adhesive bonding (Alloy Primer + Panavia 21, Kuraray) to water aged (60 days) resin-based composite blocks (12x12x4 mm, Z-100, 3M/ESPE). Bonded samples were stored in water at least 7 days prior to fatigue testing. In Group Layer (n=24), the Y-TZP plates were veneered using a hand build up technique (IPS e.max Ceram*). In Group Press (n=24), the Y-TZP plates were veneered using a lost-wax press technique (IPS e.max Press*). Samples (n=21 Layer and n=25 Press) were mouth-motion step-stressed using 3 profiles at 1.5-2 Hz (El-3300, Bose/Enduratec), with a tungsten carbide indenter (r=3.18 mm) until cracks reaching the veneering/core interface were observed (polarized reflection light microscopy). All flat-layer specimens were angled 30° in respect to the loading axis, simulating posterior tooth cusp inclination. Step-stress profiles were determined from single-cycle load-to-failure tests (n=3) from Group Layer.

RESULTS:

Single-cycle-load-to-failure-test yielded 825 N for the Group Press and 803 N for the Group Layer. Irrespective of the applied veneering technique Y-TZP trilayers failed from deep cone cracks reaching the veneer/core interface. None of the Y-TZP cores exhibited radial fractures. Weibull stress level probability curves were calculated (Alta Pro, Reliasoft) and reliability (2 sided @ 90% confidence bounds) at 100,000 cycles and 200 N load was determined.

Reliability Layer technique Press technique

Upper 90% CB 0.72 0.80

Value 0.51 0.64

Lower 90% CB 0.25 0.41

CONCLUSION:

After 30° off-axis loading the step-stress fatigue reliability of layered and pressed over Y-TZP ceramics was comparable. The failure mode was identical for both veneering techniques. Supported by NIDCR-P01-DE10976 and Ivoclar Vivadent.