Bridging the gap between clinical failure and laboratory fracture strength tests using a fractographic approach

Objective: The aim of this study was to analyze and to compare the fracture type and the stress at failure of clinically fractured zirconia-based all ceramic restorations with that of morphologically similar replicas tested in a laboratory setup. Methods: Replicas of the same shape and dimensions were made for 19 crowns and 17 fixed partial dentures, all made of veneered zirconia frameworks, which fractured during intra-oral service. The replicas were statically loaded by applying axial load in a universal testing machine. The principles of fractography were used to identify the location and the dimensions of the critical crack and to estimate the stress at failure. Failure was classified according to origin and type (P < 0.05 was considered significant). Results: Clinically fractured restorations failed due to either: delamination of the veneer ceramic (28.2 ± 9 MPa), defects at core veneer interface (27.7 ± 6 MPa), the generation of Hoop stresses (884.3 ± 266 MPa), radial cracking (831 MPa), or fracture of the connector (971 ± 343 MPa). The replicas failed by mainly by cone cracking of the veneer ceramic (52.4 ± 34.8 MPa) or by fracture of the connector (1098.9 ± 259 MPa). The estimated stress at failure was significantly higher for the replicas compared to the clinically fractured restorations (F = 6.8, P < 0.01). Significance: Within limitations of this study, careful design of fracture strength test would lead to more clinically relevant data. The performance of zirconia veneered restorations could be further improved with careful design considerations