Fracture strength tests are conducted to estimate the basic mechanical performance of restorative systems. Such tests have to reflect clinical failure patterns equivalent to those obtained from retrieved clinically failed restorations. Objective: To fractographically examine retrieved clinically failed restorations and compare the fracture patterns observed with those obtained from fracture strength tests preformed on exact replicas of the in vivo failed restorations. Methods: 19 crowns and 17 fixed partial dentures which fractured during in vivo service were collected. All restorations were zirconia veneered restorations. An exact replica of each restoration was made using the same veneer and framework materials and was subjected to fracture strength test by applying axial loading. Both the clinically failed and the laboratory loaded restorations were fractographically examined using SEM and the stress at failure was estimated (p<0.05 was considered significant). Results: Clinically failed restorations fractured due to either delamination of the veneer ceramic (57%), defects at core veneer interface (15%), hoop stresses on the inner surface of the framework (9%), fracture of the connector (10%), or from CAD/CAM defects (9%). For fracture strength test performed on the replicas the observed failure patterns were cone cracks in the veneer ceramic (65%) and fracture of the connector (35%). The average stress at failure for the clinically failed restorations was 65 MPa for defects that originated in the veneer ceramic and 760 MPa for the fractures that originated in the zirconia frameworks. The stresses at failure of the fracture strength tests were significantly 19-25% higher. Conclusion: Fracture strength tests do not resemble the loading environment that occurs intra-orally neither in regards to the stress at failure or in terms of the resultant fracture patterns. These tests should be re-designed to include more parameters that occur intra-orally.

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