Objectives: To investigate mouth-motion step-stress fatigue and damage modes of all-ceramic three-unit fixed-partial-dentures (FPDs) using off-axis loading and microscopic analysis. Methods: 13 standardized all-ceramic CAD/CAM fabricated Procera-AllZircon-FPDs*(Zr2O-framework) veneered with NobelRondo* were cemented to individualized zirconia-abutments* based on Replace-straight-Groovy* implants (premolar: RP-4x13mm; molar: WP-5x13mm) (*NobelBiocare). FPDs were adhesively luted (Panavia F2.0, Kuraray Medical). Specimens were incubated in water at 37°C for at least 14-days. A single load test was performed to estimate initial fracture resistance. Mouth motion (1.5-2Hz) step-stress accelerated testing (ELF-3300, Bose EnduraTEC) was employed with three stress-time-varying profiles. Failure was defined by delamination and/or core fracture. Specimens were inspected (polarized-specular-reflection-light-microscopy) at regular intervals to visualize crack propagation and failure mode. Reliability software (Alta Pro, Reliasoft) was employed. Preliminary results: Off-axis single-load-test at the buccal cusp pontic area demonstrated porcelain delamination (~1340N) followed by core fracture (~1874N). Inner cone/ median radial crack propagation were observed in the porcelain during fatigue resulting in partial delamination. In cross-section two specimens revealed an additional fracture of the core pontic, joining the indentation area and the connector. Weibull-stress-level-probability-curves were calculated and the reliability (2-sided at 90.0 % confidence bounds) for 50K cycles and a 500N load indicated a value of 0.72(0.90/0.37). Conclusion: Based on current data implant-supported zirconium oxide FPDs exhibit appropriate fatigue resistance for posterior clinical application. Supported by Nobel Biocare, Goteborg, Sweden. christian.stappert@nyu.edu

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