Aim of the study: The aim of this in vitro study was to investigate the flexural strength of three-unit monolithic zirconia FPD with different size of connectors sintered in the SpeedFire Oven.

Materials & Methods: Thirty-five three-unit zirconia (In-Coris TZI C Medi S A1, Sirona Dental Systems GmbH, Bensheim, Germany) FPDs with different connector sizes were milled, sintered, and glazed. Three groups were formed based on the connector size: Group A: 3x3 mm, Group B: 3x4 mm and Group C: 4x4 mm. Flexural strength was calculated in MPa with the aid of a universal testing machine (Model 5566; Instron Corp, Canton, MA) along with the determination of mode of failure. The force was applied at the center of the bridge at a crosshead speed of 0.5mm/min, until it cracked.

Results: Group A had the highest mean flexural strength (1160.71MPa with a standard deviation of 140.70). One-way ANOVA comparing the groups was statistically significant (p<0.001). Post hoc tests using the Tukey-Kramer test showed significant differences between Groups A and B (p<0.001) and Groups A and C (p<0.001), but not Groups B and C (p=0.698). 74.3% of the fractures occurred in the connector part of FPDs and 25.7% happened in the margin of restorations. Concerning the mode of failure, based on Fisher's exact test, the difference between groups was not statistically significant. (p=0.717).

Conclusions:

1- Flexural strength of three-unit FPDs with connector sizes 3x3mm sintered in the SpeedFire oven was significantly higher than that of three-unit FPDs with connector sizes 3×4 mm and 4×4 mm.

2- Group A, three-unit FPDs with connector sizes 3x3mm, that were sintered with the short sintering protocol had clinically acceptable flexural strength while the other two groups did not have acceptable flexural strength.