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Background: Promising results of initial clinical trials with yttria-stabilized zirconia have led to more extensive use of zirconia in dental implant superstructures. The applications have extended to abutments and complex individually designed crown-abutment one-piece structures. Little is known about their clinical success and the primary cause of failures., Purpose: The aim of this study was to identify the cause of fracture of retrieved implant-retained one-piece prostheses that failed during clinical use., Methods: Nine fractured restorations were analyzed with fractographic methods and their fracture origins were identified., Results: All but two of the fractures originated in an area of tight contact between the implant or titanium screw and the abutment base. Results of the evaluation showed that zirconia-based implant restorations with very thin walls in the region connecting the prosthesis to the implant are vulnerable to damage from the screw retaining process and fracture from non-axial loads. Two restorations failed due to veneer fractures., Significance: The findings suggest that large crowns on narrow implants or implants with internal fixation should preferably not be made with zirconia abutments, or that a new design approach should be considered., (Copyright © 2018 The Academy of Dental Materials. All rights reserved.)