Clinical failures of all-ceramic fixed partial dentures (FPDs) have been a major problem for dentists. Objective: The objective of this study was to identify the fracture origins and critical flaw sizes of clinically failed all-ceramic FPDs. Methods: We retrieved and analyzed nine fractured FPDs made of a lithia-disilicate-based core ceramic and two FPDs with chipped ceramic veneers. In spite of the possibility of additional intraoral damage to fracture surfaces subsequent to fracture, we were able to identify the critical flaw in 81.8% of the FPDs using optical microscopy. The critical flaw size was used to calculate failure stresses. Results: Each of the nine nonveneered experimental FPDs fractured in the vicinity of the connector areas. For all bridges, crack initiation occurred within the outer surface of the prostheses. Critical flaw sizes of the failed FPDs ranged from 70 to 216 mm. Failure stresses of the fractured FPDs, estimated by fractographic techniques and fracture mechanics relationships, ranged from 238 to 310 MPa. These strength values imply that the core ceramic was over-stressed. Estimated failure stresses for two of the FPDs, which had failure initiation sites within the ceramic veneer surfaces, were 39 and 50 MPa. The change in the orientation of fracture features indicated that a multidirectional load was applied to the ceramic surface in 64% of the cases. Conclusions: We conclude that the fracture initiation sites of the all-ceramic FPDs were controlled primarily by contact damage in the veneered units and that the crack propagation patterns were controlled by the loading orientation. This study was supported by NIH-NIDCR Grant DE 06672.

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