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Objective: to study the survival rate and fracture resistance of four different types of monolithic zirconia crowns based on their yttria content, after artificial aging under thermocylcing aging alone and combined thermo-mechanical loading using a chewing simulator. Materials and Methods: full coverage anatomical monolithic zirconia crowns were made using four types of zirconia materials, Gradient 3Y-PSZ and 5Y-PSZ, Gradient 4Y-PSZ and 5Y-PSZ, Non-Gradient 4Y-PSZ, and Non-Gradient 3Y-PSZ as control (n=20 each). The crowns were bonded to fiber reinforced resin dies. Half of the crowns were subjected to thermo-mechanical loading under 110 N at a frequency of 1.4 Hz for 1.2 million cycles with simultaneous thermocycling (10,000 cycles, 5-55˚C). The other half were subjected to thermocycling alone (10,000 cycles, 5-55˚C). The survival rate for each material was observed. Each crown was loaded to failure to calculate its fracture resistance after aging. Two-way ANOVA was used to test the effect of zirconia type and ageing method on the fracture resistance of the crowns.Results: All four types of zirconia survived the artificial aging with no failures. The artificial aging method did not affect the fracture resistance of zirconia crowns p=0.18. The concentration of yttria at the occlusal part of the restoration affected the fracture resistance p=0.0001. Conclusion: Increasing the yttria concentration at the occlusal third of the crown decreased the fracture resistance. The mechanical loading protocol did not affect the fracture resistance of zirconia. |
