Gliding Contact Fatigue Damage Maps of Veneered Zirconia Structures

Objectives: Dental restorations often fail from the seemingly innocuous chipping of porcelain veneers resulting from occlusal contact damage and wear, making contact fatigue studies relevant. However, most of the fatigue studies use uniaxial loading without any gliding action. This study investigates the effect of fatigue loading with a load-glide-unload action, emulating a masticatory cycle, on the longevity of ceramic restoratives. Methods: A mouth-motion fatigue machine (Elf 3300, Bose/EnduraTEC) applied a controlled stroke profile in water: sphere (WC, r = 1.5 mm) contacting the specimen; specimen translating 0.7 mm with load applied; and sphere lifting off after each stroke. The maximum fatigue load range was 50 - 500 N and the number of cycles ranged from 1 - 1000,000. Flat porcelain veneered zirconia (LAVA) cores were tested along with a monolithic glass and a glass “veneer” bonded to zirconia core which served as a transparent control. Crack evolution is followed in-situ using a video camera system. Results: Failure was defined as any occlusal surface damage which penetrates an entire veneer layer to the veneer/core interface or a flexural radial fracture originating at the internal cementation surface. Previous work demonstrated that traditional R-ratio and uniaxial mouth-motion fatigue damage maps (load-cycles-type of failure) are similar. For load-glide fatigue, the map of surface cone cracking shifted to lower loads but not flexural failure. The accelerated crack growth rate in load-glide-unload fatigue appears similar to that observed in pin-on-disc studies and likely occurs as a result of enhanced compressive and tensile stresses ahead and behind the indenter. Conclusion: Fatigue loading with an occlusion-like gliding action is more severe in creating contact damage than the traditional R-ratio and uniaxial mouth-motion fatigue. Supported by the NYU Research Challenge Fund.