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Objective To compare four different types of monolithic zirconia crowns in terms of survival rate and fracture resistance after thermocycling 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: multi‐yttria‐layered 5Y‐PSZ/3Y‐PSZ, multi‐yttria‐layered 5Y‐PSZ/4Y‐PSZ, monolithic 4Y‐PSZ, and monolithic 3Y‐PSZ as control (n = 20). Half of the samples in each group were subjected to thermo‐mechanical loading under 110 N, 1.4 Hz, 1.2 million cycles with simultaneous thermocycling (10,000 cycles, 5–55°C). The other half were subjected to thermocycling alone. The samples were loaded to failure to measure their fracture resistance. The data were analyzed using by two‐way ANOVA and Tukey's HSD post‐hoc test (α = 0.05). Results All specimens survived the aging protocols. The yttria content significantly affected the fracture resistance of the crowns (p < 0.0001). The mean fracture resistance, from highest to lowest: 3Y‐PSZ, 4Y‐PSZ, followed by the two multi‐yttria‐layered systems. The mean difference between the two multi‐yttria‐layered systems were not statistically significant (p = 0.98). The mechanical loading protocol did not affect the mean fracture resistance within each group (p = 0.18). Conclusions Within each group, there was no difference in fracture resistance after thermocycling alone and thermo‐mechanical loading. However, increasing the yttria concentration at the occlusal third of the crown decreased its fracture resistance. Clinical Significance The term “monolithic zirconia” alone without specifying the actual yttria content is misleading. This term represents different materials with different mechanical properties. The yttria content has an inverse relationship with the fracture resistance of zirconia crowns. The fracture resistance of multi‐layer zirconia crowns is determined by the amount of the weaker zirconia phase at the occlusal part of the restoration rather than enforced by the stronger zirconia at the cervical part of the crown.

Overall, 8-mol% yttria-stabilized zirconia (8YSZ), unlike 3YSZ, is optically transparent and stable against low-temperature degradation but has insufficient mechanical properties due to its large grain size. The influence of the grain size of 8YSZ on mechanical properties was investigated to develop an 8YSZ suitable for dental restoration. Modulation of the grain size and relative density was achieved via a two-step sintering (TSS) process, and the corresponding kinetic window was established. The conditions of TSS employed herein yielded a relative density of more than 99% while maintaining a small grain size of 0.75 µm. On the other hand, the highest biaxial strength and the highest total transmittance attained were 833 MPa and 34.6% (1-mm-thick, 39.1% for a 0.5-mm thick sample) in the TSS 8YSZ with a grain size of 1.25 µm. These results suggest that strength has improved only when grain size reduction and increased relative density are achieved at the same time. The results demonstrate that the ceramic processing method has a significant effect on the mechanical and optical properties of 8YSZ needed for dental restoration and provide a new insight that contrasts previous studies focused on the starting material.

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