Effect of repeated firings on flexural strength of veneered zirconia

Objective: Chipping and/or delamination represent a clinical failure of porcelain fused to zirconia (PFZ) prostheses. Causes and solutions have not been completely clarified. The present study was aimed at evaluating the effects of number of firings on the flexural strength of PFZ specimen. 

Methods: Forty-five zirconia specimens in shape of bars were cut, sintered and divided in 3 groups (n = 15). Group 1: veneering ceramic was layered "in bulk" and fired. Group 2: veneering ceramic was layered in three layers, individually fired. Group 3: veneering ceramic was layered in five layers, individually fired. Each layer thickness was controlled by the use of calibrated molds. The total veneering ceramic thickness for all the specimens was 1.2 mm, and the total thickness of the specimen of 2.0 mm. Three-point bending test was performed. Fracture load was recorded in Newton and MPa value was calculated taking into account the bi-layered nature of the specimen. Data were statistically analyzed. 

Results: Specimens obtained with on single firing cycle obtained a statistically significant (p < 0.001) lower flexural strength (54.61 ± 8.98 MPa) than specimens veneered with 3 or 5 firing cycles. The last two obtained very similar results (77.63 ± 13.17 MPa and 73.62 ± 12.38 MPa respectively) and the differences was not statistically significant. 

Significance: In bi-layered PFZ specimen, three to five layers and firings determine higher flexural resistance when compared to a single firing. Thus, a 3-layers veneering procedure is recommended to increase flexural resistance. If a 5-layer procedure is necessary to improve esthetics, it does not decrease flexural resistance.