Den norske tannlegeforenings Tidende
17.01.2019
Tema: Keramer Two decades of zirconia as a dental biomaterial - what have we learned?
Yttrium oxide stabilized tetragonal zirconium-dioxide polycrystal (referred to as yttria-stabilized zirconia, Y-TZP or briefly zirconia) is a durable dental ceramic material that has exceptional biocompatibility. These properties make it an excellent material for use in the oral cavity. Zirconia was first introduced as a framework material (first generation) for tooth-supported single crowns and fixed dental prostheses (FDPs). The survival rates of these constructions are high, and the only draw back has been the exposition to superficial porcelain chipping, so called chip-off fractures. This was leading to learning curve in veneering techniques as well as development of new more translucent zirconia materials that can be used as monolithic structures without veneering porcelain or cutback structures where only the labial facades are veneered.
The purpose of this article is to describe the material properties of different zirconia materials as well as some clinical indications.
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 (1).
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 (2).
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 (3). 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 (4).
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 (5).
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, 2370ºC). During the fabrication process, zirconia reconstructions are sintered at temperatures well above 1170º 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 µm depending on production history (6).
In order to avoid zirconia turning into monoclinic phase during cooling, small amounts (2-3 mol %) of stabilizing oxides, like yttrium oxide (Y2O3, yttria) are added to the material. The yttria-doped material is then stabilized in the tetragonal phase also at room temperature (7).
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 (7). (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 (7). 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 (8-10). 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 m1/2 respectively (7, 11, 12). (Table 1) The fracture toughness is an important property for evaluating the fracture behavior and crack propagation of a ceramic material. The fracture toughness value could help to evaluate the damage tolerance and long-term clinical success of the material.
Phase transformation from t-m has also been seen on the surface of zirconia material in in vitro studies due to environmental stresses like presence of water, body fluids (saliva) and especially hot water vapor (autoclave) (13, 14). Chevalier and co-workers showed that in a humid atmosphere the tetragonal grains on material surface might transform into monoclinic ones. As the monoclinic grains are 3-5 % larger, this sudden volume expansion leads to swelling on the material surface and enables water to penetrate through grain boundaries resulting micro- and macro-cracking of zirconia (14). This phenomenon is called low temperature degradation (LTD) and it was first thought to be detrimental for zirconia constructions in oral cavity. However, in 7 to 10 years clinical follow-up studies almost no signs of low temperature degradation of the frameworks have been seen and the survival rates of zirconia frameworks have been excellent (3, 15). But spontaneous t-m phase transformation can occur on the material surface due to mechanical stress induced by surface treatments like grinding (16).
First-generation zirconia has a regular polycrystalline structure without any amorphous phase (glass). Compared to glass-ceramics this difference in microstructure makes zirconia mechanically more durable, but with optical properties such as high surface reflection, low translucency and an extreme light scattering property that give the material an opaque appearance. It is important, however, to remember that first-generation zirconia is not opaque but have unfavorable optical properties regarding potential for tooth resemblance (17).
The polycrystalline structure of zirconia cannot be etched with hydrofluoric acid (HF) and the bond strength is not as high as the one that can be achieved to HF etched porcelain or glass ceramics (18). In clinical studies loss of retention and secondary caries are typical complications (19, 20). One of the reasons for this could be poor bond strength, especially since it's known that long-term water storage is decreasing the bond strength (21). Other possible reasons for loss of retention might be related to surface properties and precision as a result of milling, or choice of cement. Zirconia can be milled either in pre-sintered stage (soft machining) or fully sintered stage (hard machining, e.g. Hot Isostatic Pressing, HIP zirconia) and the milling of zirconia, especially in fully sintered stage, often results in a glossy surface with low surface roughness. Since the material is highly inert, a chemical reaction with some bonding products and cements is unlike to occur, which is detrimental to micromechanical retention. If production (milling) is done with a 3-axis milling unit, or if the geometry of the preparation doesn't allow for precise milling (if drill compensation is needed), then the ferrule or cement gap might be insufficient, again making the reconstruction susceptible for loss of retention (22). Finally, zinc phosphate cement was previously recommended frequently for zirconia, with properties (brittle, water-soluble, a low retentive cement) that are unsuitable for some cases with respect to the aspects mentioned above, should not be recommend at all. This was confirmed and concluded in a study by Larson et al (23). Kern et al (24) have described a method for bonding to zirconia, and this and other bonding procedures for zirconia will be discussed in anoth
Gå til medietThe purpose of this article is to describe the material properties of different zirconia materials as well as some clinical indications.
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 (1).
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 (2).
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 (3). 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 (4).
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 (5).
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, 2370ºC). During the fabrication process, zirconia reconstructions are sintered at temperatures well above 1170º 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 µm depending on production history (6).
In order to avoid zirconia turning into monoclinic phase during cooling, small amounts (2-3 mol %) of stabilizing oxides, like yttrium oxide (Y2O3, yttria) are added to the material. The yttria-doped material is then stabilized in the tetragonal phase also at room temperature (7).
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 (7). (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 (7). 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 (8-10). 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 m1/2 respectively (7, 11, 12). (Table 1) The fracture toughness is an important property for evaluating the fracture behavior and crack propagation of a ceramic material. The fracture toughness value could help to evaluate the damage tolerance and long-term clinical success of the material.
Phase transformation from t-m has also been seen on the surface of zirconia material in in vitro studies due to environmental stresses like presence of water, body fluids (saliva) and especially hot water vapor (autoclave) (13, 14). Chevalier and co-workers showed that in a humid atmosphere the tetragonal grains on material surface might transform into monoclinic ones. As the monoclinic grains are 3-5 % larger, this sudden volume expansion leads to swelling on the material surface and enables water to penetrate through grain boundaries resulting micro- and macro-cracking of zirconia (14). This phenomenon is called low temperature degradation (LTD) and it was first thought to be detrimental for zirconia constructions in oral cavity. However, in 7 to 10 years clinical follow-up studies almost no signs of low temperature degradation of the frameworks have been seen and the survival rates of zirconia frameworks have been excellent (3, 15). But spontaneous t-m phase transformation can occur on the material surface due to mechanical stress induced by surface treatments like grinding (16).
First-generation zirconia has a regular polycrystalline structure without any amorphous phase (glass). Compared to glass-ceramics this difference in microstructure makes zirconia mechanically more durable, but with optical properties such as high surface reflection, low translucency and an extreme light scattering property that give the material an opaque appearance. It is important, however, to remember that first-generation zirconia is not opaque but have unfavorable optical properties regarding potential for tooth resemblance (17).
The polycrystalline structure of zirconia cannot be etched with hydrofluoric acid (HF) and the bond strength is not as high as the one that can be achieved to HF etched porcelain or glass ceramics (18). In clinical studies loss of retention and secondary caries are typical complications (19, 20). One of the reasons for this could be poor bond strength, especially since it's known that long-term water storage is decreasing the bond strength (21). Other possible reasons for loss of retention might be related to surface properties and precision as a result of milling, or choice of cement. Zirconia can be milled either in pre-sintered stage (soft machining) or fully sintered stage (hard machining, e.g. Hot Isostatic Pressing, HIP zirconia) and the milling of zirconia, especially in fully sintered stage, often results in a glossy surface with low surface roughness. Since the material is highly inert, a chemical reaction with some bonding products and cements is unlike to occur, which is detrimental to micromechanical retention. If production (milling) is done with a 3-axis milling unit, or if the geometry of the preparation doesn't allow for precise milling (if drill compensation is needed), then the ferrule or cement gap might be insufficient, again making the reconstruction susceptible for loss of retention (22). Finally, zinc phosphate cement was previously recommended frequently for zirconia, with properties (brittle, water-soluble, a low retentive cement) that are unsuitable for some cases with respect to the aspects mentioned above, should not be recommend at all. This was confirmed and concluded in a study by Larson et al (23). Kern et al (24) have described a method for bonding to zirconia, and this and other bonding procedures for zirconia will be discussed in anoth
