Strength of Zirconia Crowns as a Function of Yttria Concentration.

Objectives: To compare four different types (based on yttria concentration) of full coverage zirconia restorations in terms of survival rate and fracture resistance after thermocyclic aging and/or thermo-mechanical loading in a chewing simulator.
Methods: Partially-Stabilized-Zirconia (PSZ) crowns with fiber-reinforced resin die assemblies (n=80) were fabricated using 4 different types of zirconia based on their yttria concentration: multilayer 3Y-PSZ/5Y-PSZ (occlusal third), multilayer 4Y-PSZ/5Y-PSZ (occlusal third), monolithic 4Y-PSZ and monolithic 3Y-PSZ to serve as controls (n = 20). Within each group, ten samples 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 ten samples were subjected to thermocycling alone (10,000 cycles, 5-55 C). The samples were loaded to failure to measure its fracture resistance. The data were analyzed for statistical significance by two-way ANOVA and Tukey’s HSD post-hoc test (α = 0.05).
Results: All specimens survived the two aging protocols. The yttria content had a statistically significant effect on the mean fracture resistance of the crowns (p<0.0001). The mean fracture resistance load, from the highest to the lowest: monolithic 3Y-PSZ, monolithic 4Y-PSZ, followed by the two multilayer systems 3Y-PSZ/5Y-PSZ, 4Y-PSZ/5Y-PSZ. The fracture resistance loads between the two multilayer systems 3Y-PSZ/5Y-PSZ, 4Y-PSZ/5Y-PSZ were not statistically significant (p=0.98). These findings indicated that final fracture of the crowns was originated from the occlusal surface. In addition, the mechanical loading protocol did not affect the mean fracture resistance within each group (p=0.18).
Conclusions: Increasing the yttria concentration at the occlusal third of the crown decreased its fracture resistance. The mechanical loading protocol did not affect the fracture resistance of zirconia.