The purpose of this article is to describe the material properties of different zirconia materials as well as some clinical indications.
Hovedbudskap
First-generation zirconias are today materials that can be used with great confidence for many clinical situations.
Recently developed translucent and high-translucent zirconia materials are promising, but long-term follow-up studies are still lacking and the gain in esthetic properties could include the loss in mechanical properties.
The choice of material should be done with great care, using materials only that are well known for the clinician.
A strive for aesthetic dental reconstructions led to the development of material combinations with porcelain as the main aesthetic component supported by a strong, tough framework material. The materials often considered to be the gold standard for dental reconstructions were specially developed high-gold alloys used with compatible porcelains, combinations that emerged in dentistry in the late 1950s under the name porcelain fused to metal or later metal ceramics (MC). After a long period when MC was the dominating material combination for fixed dental prosthesis (FDPs) there was a growing demand for even more aesthetic and less expensive metal-free materials ().
That led to the development of densely sintered oxide ceramics, like aluminum oxide (Al2O3, often referred to as alumina) and later yttrium oxide stabilized tetragonal zirconium-dioxide polycrystal (referred to as yttria-stabilized zirconia, Y-TZP or briefly zirconia). Alumina was the dominating oxide ceramic material under the 1990s, either glass infiltrated (as a hybrid ceramic) or densely sintered, but in both cases in combination with veneering porcelain. The clinical outcome of crowns made of those materials was promising, but studies showed that the strength and toughness of alumina was somewhat limited when used for FDPs ().
First-generation zirconia-based materials, on the other hand, have been used since the mid 1990s with results showing that they can be used for almost all types of FDPs, tooth-supported and implant-supported ones, as long as the dimensions are sufficient. The survival rates are high and the only draw back has been that FDPs are prone to superficial porcelain chipping, so called chip-off fractures (). Regarding cores and frameworks, however, zirconia has shown to have superior mechanical properties compared to alumina and is nowadays widely used clinically for routine treatments ().
The focus of the development of zirconia-based reconstructions has been in two main directions during the last decade. One direction was to learn how to veneer the first-generation zirconia without creating detrimental residual stresses in the veneering porcelain, thus reducing the risk for chip-off fractures. The other direction was to develop zirconia materials with optical properties closer to the natural tooth structures, in order to make them feasible for use monolithically (in full anatomy). Then aesthetically acceptable reconstructions might be produced without the need of comparably weak veneering porcelain. This offers a possibility to combine strength with sufficient optical properties in one and the same material. By changing the microstructure of zirconia, it is possible to increase translucency and to decrease the light scattering properties of the material, with the intention to at the same time preserve the unique mechanical properties of zirconia ().
Many different zirconia materials are available today and both mechanical and optical properties differ to such an extent, that it is important for the clinician to be able to distinguish between the different materials when deciding what material to use in a specific clinical situation. The purpose of this article is therefore to enlighten the material properties of different zirconia materials as well as some clinical indications.
Properties of stabilized zirconia
Zirconia is a polymorphic material that occurs in three crystal phases depending on temperature: monoclinic (m, 2370C). During the fabrication process, zirconia reconstructions are sintered at temperatures well above 1170 C, which results in a tetragonal material structure. During cooling, when the temperature passes approximately 1170 C, phase transformation occurs in grains where tetragonal crystals transforms into monoclinic ones. Since the monoclinic grain is 3-5 % larger in volume compared to the tetragonal ones, volumetric expansion occurs which leaves the material with high residual stresses, very brittle and prone to spontaneous crack growth within the material. Zirconia grains are microscopically visible but vary in size from 0.2 to 0.8 m depending on production history ().
In order to avoid zirconia turning into monoclinic phase during cooling, small amounts (2-3 mol %) of stabilizing oxides, like yttrium oxide (Y 2 O 3, yttria) are added to the material. The yttria-doped material is then stabilized in the tetragonal phase also at room temperature ().
Favorable mechanical properties are achieved in the material using stabilizing oxides in a process described as transformation toughening. When a crack is formed in the surface of the material, it tends to grow and expand into the bulk of the material. Local tensile stresses at the crack tip area mediate a t-m transformation of the zirconia grains in the area under stress, leading to a volumetric expansion of 3-5 % in the crack tip area, thus resulting in a local residual compressive stress. For the continuing growth of the crack, loading forces first need to neutralize the residual compressive stress in the crack tip area, before tensile stresses can start to build up. Consequently, higher loads are needed for continuing crack growth, which practically means that the residual compression prevents further crack propagation Figure 1).
Zirconia has been shown to be an excellent material for use in the oral cavity. It is highly chemically stable and the thermal conductivity is extremely low (). In vitro and in vivo studies have shown that zirconia has relatively low tendency to adhesion and colonization of bacteria on the surface of the material and it is chemically very close to titanium-oxide First-generation zirconia has more favorable mechanical properties than all the other dental ceramic materials. Both flexural strength and fracture toughness are high, ranging from 800 to 1500 MPa and 9.4 to 11.5 MPa m 1/2 respectively Table 1) The fracture toughness is an important property for evaluating the fracture behavior and crack propagation of a
