Graded High-translucency Zirconias: A study of load-bearing capacity and translucency

Objectives: The load-bearing capacity of 3Y-TZP can be significantly increased by glass-infiltration. The glass/zirconia graded surface layer is known to effectively reduce and transfer the maximum tensile stress from the surface into the interior of the material. Thus, in this study we investigate how this glass-infiltration technique affects the load-bearing capacity and translucency of cubic-containing zirconias (3Y-, 4Y-, and 5Y-PSZ).
Methods: For each material, 40 discs were fabricated and assigned to Graded (G) and Homogeneous (non-graded) groups. Graded samples were produced by pre-sintering (1350°C for 1h), followed by the application of an in-house developed glass, and glass-infiltration occured concurrently with sintering. Sintering conditions for both Graded and Homogeneous groups were: 3Y at 1530°C for 2h, and 4Y and 5Y at 1450°C for 2h. Discs (12 × 1 mm) were subjected to a free-standing piston-on-3-balls test (n=10), or to a Hertzian test (n=10) with discs bonded to a dentin-analogue (G10). Composite-beam theory and plate-on-foundation theory, respectively, corroborated the two test scenarios. Optical, compositional, and microstructural characterization were also performed.
Results: The load-bearing results and statistical significances are listed in the table. A significant increase in load-bearing capacity (24 - 37%) is observed for graded zirconias compared to their homogeneous counterparts for both test configurations. Graded high-translucency zirconias (G4Y and G5Y) achieved similar load bearing capacity to that of homogeneous 3Y-PSZ, regardless of whether it was fractured in free-standing or bonded modes. The reflected translucency (TP) of G5Y varied with pre-sintering temperatures, from TP = 23 (0.4) -- pre-sintered at 1350°C, to TP = 27 (0.6) -- 1450°C, while the homogeneous 5Y has TP = 30 (0.7).
Conclusions: Graded zirconias can withstand ~30% more flexural load than their homogeneous counterparts, making graded high-translucency zirconia (G4Y and G5Y) as durable as homogeneous 3Y-PSZ, thus fulfilling the same clinical applications with improved esthetics.