ReviewAn overview of zirconia ceramics: Basic properties and clinical applications
Introduction
Zirconia is a crystalline dioxide of zirconium. Its mechanical properties are very similar to those of metals and its color is similar to tooth color.1 In 1975, Garvie proposed a model to rationalize the good mechanical properties of zirconia, by virtue of which it has been called “ceramic steel”.2 Zirconia crystals can be organized in three different patterns: monoclinic (M), cubic (C), and tetragonal (T). By mixing ZrO2 with other metallic oxides, such as MgO, CaO, or Y2O3, great molecular stability can be obtained.1 Yttrium-stabilized zirconia, also known as tetragonal zirconia polycrystal (TZP), is presently the most studied combination.3 The aforementioned three phases are present in a common ZrO2 crystal. Every transition between the different crystalline reticulations is due to a force on the zirconia surface, and this produces a volumetric change in the crystal where the stress is applied. When a stress occurs on a zirconia surface, cracking energy creates a T−M transition. This crystalline modification is followed by an expansion that seals the crack.4 ZrO2 stabilized with Y2O3 has better mechanical properties than other combinations; although its sintering is much more difficult, this is the principal kind of zirconia considered for current medical use.
The first proposal of the use of zirconium oxide for medical purposes was made in 1969 and concerned orthopedic application. ZrO2 was proposed as a new material for hip head replacement instead of titanium or alumina prostheses.5 They evaluated the reaction upon placing ZrO2 in a monkey femur and reported that no adverse responses arose. Orthopedic research focused on the mechanical behavior of zirconia, on its wear, and on its integration with bone and muscle. Moreover, these first studies were largely carried out in vivo because in vitro technology was not yet sufficiently advanced. Prior to 1990, many other studies were performed, in which zirconia was tested on bone and muscle without any unfavorable results.6, 7, 8, 9, 10, 11 Since 1990, in vitro studies have also been performed in order to obtain information about cellular behavior towards zirconia.12 In vitro evaluation confirmed that ZrO2 is not cytotoxic13, 14, 15 (Fig. 1). Uncertain results were reported in relation to zirconia powders that generated an adverse response.16, 17 This was probably due to zirconium hydroxide, which is no longer present after sintering so that solid samples can always be regarded as safe. Mutagenicity was evaluated by Silva and by Covacci, and both reported that zirconia is not able to generate mutations of the cellular genome18, 19; in particular, mutant fibroblasts found on ZrO2 were fewer than those obtained with the lowest possible oncogenic dose compatible with survival of the cells.19
Moreover, zirconium oxide creates less flogistic reaction in tissue than other restorative materials such as titanium.20 This result was also confirmed by a study about peri-implant soft tissue around zirconia healing caps in comparison with that around titanium ones.21 Inflammatory infiltrate, microvessel density, and vascular endothelial growth factor expression were found to be higher around the titanium caps than around the ZrO2 ones. Also, the level of bacterial products, measured with nitric oxide synthase, was higher on titanium than on zirconium oxide. Zirconia can up- or down-regulate expressions of some genes, so that zirconia can be regarded as a self-regulatory material that can modify turnover of the extracellular matrix.22
Zirconia has mechanical properties similar to those of stainless steel. Its resistance to traction can be as high as 900−1200 MPa and its compression resistance is about 2000 MPa.1 Cyclical stresses are also tolerated well by this material. Applying an intermittent force of 28 kN to zirconia substrates, Cales found that some 50 billion cycles were necessary to break the samples, but with a force in excess of 90 kN structural failure of the samples occurred after just 15 cycles.23 Surface treatments can modify the physical properties of zirconia. Exposure to wetness for an extended period of time can have a detrimental effect on its properties.24 This phenomenon is known as zirconia ageing. Moreover, also surface grinding can reduce toughness.25 Kosmac confirmed this observation and reported a lower mean strength and reliability of zirconium oxide after grinding.26
Section snippets
Clinical aspects
In the search for the ultimate aesthetic restorative material, many all-ceramic systems have been proposed. Dental research is nowadays directed towards metal-free prosthetic restorations in order to improve aesthetical outcome of FPD restorations. Natural look of soft tissue in contact with fixed partial dentures is influenced by two factors: mucosal thickness and typology of restorative material. Metal-free restorations allow to preserve soft tissue color more similar to the natural one than
Conclusion
Although clinical long-term evaluations are a critical requirement to conclude that zirconia has good reliability for dental use, biological, mechanical, and clinical studies published to date seem to indicate that ZrO2 restorations are both well tolerated and sufficiently resistant. Ceramic bonding, luting procedures, ageing and wear of zirconia abutment should be evaluated in order to guide adequate use of zirconia as prosthetic restorative material. Patient selection, coupled with adequate
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