Zirconia in Dentistry: Applications and Limitations

How yttrium content distinguishes generations of dental zirconia — from the stronger 3Y to the more translucent 5Y — and why the aging phenomenon has guided the evolution of this material.

Zirconia used in dentistry is stabilized with yttrium oxide (Y2O3), and the molar percentage of this stabilizer directly determines the properties of the final material, following a simple principle: more yttrium, more cubic crystalline phase, more translucency, but less mechanical strength. 3Y-TZP zirconia (3 mol% yttrium), the original formulation, is predominantly tetragonal, with the best overall mechanical strength but lowest translucency, and remains the reference choice for extensive bridges and posterior restorations subject to high chewing loads.

Later generations — 4Y-TZP and 5Y-TZP (or 5Y-PSZ) — progressively increase yttrium content and, consequently, the cubic phase component, reaching up to about 50% in 5Y zirconia, with an increase in light transmission on the order of 43-45% and optical properties much closer to those of a glass-ceramic. This aesthetic gain, however, comes with a real mechanical cost: the flexural strength of 5Y zirconia, while generally still higher than lithium disilicate, is significantly lower than the original 3Y-TZP, which is why 5Y is mainly indicated for single crowns and three-unit restorations in the anterior area, while 4Y-TZP, with an intermediate trade-off, is often preferred for three-unit restorations that include a molar.

A relevant technical aspect that has guided part of zirconia's evolution concerns the aging phenomenon, or low-temperature degradation (LTD): in the presence of moisture, the tetragonal phase of 3Y-TZP zirconia tends over time to progressively transform into monoclinic phase, a process that can alter the material's surface properties over the long term. Higher-yttrium-content formulations, containing a smaller percentage of transformable tetragonal phase, are less prone to this phenomenon — an additional advantage, beyond translucency, that has contributed to their clinical adoption.

On the application side, the use of layered zirconia with a veneering ceramic to achieve superior aesthetics, widespread especially in the material's early adoption phase, has over time shown a non-negligible incidence of chipping of the ceramic veneer — a complication that has progressively shifted clinical practice toward monolithic zirconia use, where the entire restoration is made from a single, homogeneous block of colored and characterized zirconia, often leveraging multilayer blocks with built-in color and translucency gradients, eliminating the veneering interface that was the structural weak point of the layered system.

On Oralzon you'll find monochromatic and multilayer 3Y, 4Y and 5Y zirconia blocks for CAD/CAM milling, along with 10-MDP monomer resin cements indicated for cementation on zirconia.