Definition/Introduction

The modern era is known as "Ceramic Age" due to the increasing assortment of ceramic for industrial and medical use. Ceramic materials developed for medical and dental use are termed "bioceramics." The word “zirconium” comes from the Arabic word “zargon” which means “golden in color.” This meaning, in turn, came from the two Persian words "zar," meaning "gold," and "gun," referring to color. German chemist Martin Heinrich Klaproth accidentally discovered zirconium dioxide (ZrO2) in 1789. It was later isolated by the Swedish chemist Jöns Jakob Berzelius in 1824.

ZrO2 is a white crystalline oxide of zirconium found in the minerals baddeleyite and zircon. Zirconium oxide crystals can exist in three phases: (1) the cubic phase (C) which is a square sided straight prism; (2) the tetragonal phase (T) in the form of a rectangular sided straight prism, and (3) the monoclinic phase (M) with a deformed prism with parallelepiped sides. The cubic phase is stable above 2370 degrees Cand with moderate mechanical properties. The tetragonal phase is stable between 1170°C to 2370 °C with improved mechanical properties. The monoclinic phase is stable at room temperature up to 1170°C, with lower mechanical properties, and may contribute to a reduction of the ceramic particles cohesion.

The tetragonal phase can be retained at room temperature by the addition of 3 mol% yttria. Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) provides mechanical properties superior to other all-ceramic systems due to the tetragonal-to-monoclinic phase transformation. Thus, Y-TZP has been increasingly used as a core material for all-ceramic restorations.[1][2][3]

The advances in CAD/CAM technology made the fabrication of stabilized ZrO2 possible, which could not have been processed by traditional laboratory methods. Thus, over the last decade, zirconia technology has resulted in rapid development of metal-free dentistry that may provide high biocompatibility, enhanced aesthetics, and improved material strength.[4][5][6] 

Issues of Concern

In the late sixties the progress of research, the use of zirconium as biomaterials was refined. The first use of zirconium oxide for medical purposes was made in 1969 in orthopedic as a new material for hip head replacement instead of titanium or alumina prostheses.

In dentistry, with the development of computer-aided design (CAD)/computeraided manufacturing (CAM) systems, high strength zirconia can be used for the fabrication of crowns, veneers, fixed partial dentures, posts and cores, implants, and implant abutments. In addition, different auxiliary components such as cutting burs and surgical drills, extra-coronal attachments, and orthodontic brackets manufactured using zirconia are available commercially.[7][8]

Zirconia powder contains small amounts of radioactive impurities from the uranium-radium (226Ra) and thorium (228Th) actinide series. However, zirconia powders with low radioactivity (less than 100 Gyh-1) can be achieved by purifying following appropriate standards.

Heating zirconia at a low temperature (200 to 300 C) can result in the progressive spontaneous transformation of the metastable tetragonal phase into the monoclinic phase in the presence of water, a phenomenon known as low-temperature degradation (LTD). It is a slow transformation that starts in isolated grains on the zirconia surface, leading to an increase in volume. This stresses the adjacent grains and a microcrack appears, allowing water to penetrate and the process to ultimately progress and resulting in a remarkable decrease in strength. This strength degradation is different for various zirconia ceramics, and the variation is related to factors such as stabilizer concentration and distribution, grain size, and the presence of residual stresses. A recent study showed that ceria-stabilized zirconia (12Ce-TZP) was resistant to simulated hydrothermal aging; its flexural strength remained unaffected at a low level of 500 MPa Y-TZP and suffered from low-temperature aging degradation (LTAD) caused by phase transformation.

Aging decreases the physical properties of the material and increases the risk of failure in zirconia restorations. The presence of mechanical stresses and moisture accelerates zirconia aging. Aging leads to changes in the behavior of the material, weakening it, and subsequently, degrading the material, with microcracks decreasing strength properties.

The grain size influences the mechanical behavior of zirconia, with higher temperatures and longer sintering times producing larger grain sizes. The critical crystal size is approximately 1 micrometer, and zirconia with larger crystals are more prone to spontaneous PTT due to lower stability, whereas a smaller grain size generates favorable properties. It is reported that with grain sizes below 0.2 micrometers, PTT does not increase, and fracture toughness decreases.

 The discrepancy of the crown margin adjoined to the pontic was increased by the sintering shrinkage of the bulky pontic in the case of 3-unit and 4-unit frameworks. Therefore, we must beware of the distortion of zirconia-based FDPs with long span units when using partially-sintered blocks or green blocks.

Zirconia does not contain silica and thus hydrofluoric acid etching cannot be used. Therefore, other methods of surface treatments are used for zirconia such as grinding/polishing, surface coating, sandblasting, primer treatment, and laser treatment. Sandblasting with aluminum oxide particles results in irregularities and shallow pits in the surface of zirconia, which improves the micromechanical retention. In addition, coating with a 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-containing silane primer after sandblasting can enhance chemical and micro-mechanical bond strength. 

The selective infiltration etching (SIE) technique also can be used to increase the bond strength of zirconia and resin cement. A glass infiltration agent is heated and is allowed to penetrate the void space between the zirconia grains to re-arrange them. Then the glass infiltration agent is removed using hydrofluoric acid, resulting in the development of inter-grain nano-porosity and generating a relatively rough and reactive surface that exhibits enhanced micro-mechanical bond strength with resin cement. 

One of the technical complications reported in the YTZ-P reconstructions is chipping or delamination of the veneering porcelain. The core-veneer interface is a critical factor in the success of layered restorations. The contributing factors for the fracture may include weakness of the veneer material or the core/veneer bond, stresses and distortion due to the veneering process, unsupported veneering porcelain and configuration of the core and veneer, residual stresses arising from a coefficient of thermal expansion mismatch, and rapid cooling rates after heat treatment.

The use of high-speed polishing rotary instruments is contraindicated for clinical use because of nickel contamination, phase changes in zirconia, and the resultant surface roughness.

Clinical Significance

The restorations are processed either by soft machining of pre-sintered blanks or by hard machining of fully sintered blocks. The mechanical properties of 3Y-TZP are strongly affected by its grain size. The mechanical properties of zirconia have been reported to be higher than other ceramics for dental applications. Fracture resistance of 6–10 MPa/m1/2, a flexural strength of 900–1200 MPa, and a compression resistance of 2000 MPa have been reported. Zirconia restorations bear an average load of 755 N. Zirconia yields higher fracture loads than alumina or lithium disilicate.

Zirconia ceramics yield superior wear behavior and lower antagonistic wear compared to porcelain.

The increased opacity of zirconia ceramics could be useful in esthetically demanding clinical situations, such as in cases of masking dichromatic abutment teeth or metal post and cores, but low light transmission may cause inadequate polymerization of resin cement under zirconia ceramics. When comparing different ceramic restorative materials, zirconia ceramics have the highest relative translucency compared to metals.[9]

Bonded all-ceramic restorations provide superior aesthetics because ceramics allow diffuse transmission as well as the diffuse and specular reflectance of light, reproducing a depth of translucency and color that mimics natural teeth. Nevertheless, considering the optical properties, it could be said that yttrium-stabilized zirconia has a high refractive index, low absorption coefficient, and high opacity in the visible and infrared spectra. 

This material is a non-cytotoxic metal oxide, is insoluble in water, and has no potential for bacterial adhesion. In addition, it has radio-opacity properties and exhibits low corrosion. These are inert material and exhibit biocompatibility. Zirconia implants osseointegrate as well as titanium implants.

Y-TZP has a high fracture toughness and flexural strength. When a crack initiates on the surface of Y-TZP, the stress concentration at the top of the crack causes the tetragonal crystal to transform into a monoclinic crystal, with associated volumetric expansion. In the vicinity of a propagating crack, the stress-induced transformation leads to compressive stress that shields the crack tip from the applied stress and enhances the fracture toughness.

Dimensional changes from the sintering process of Y-TZP can lead to non-uniform cement space in single copings. The manufacturing process of Y-TZP blanks may influence the sintering shrinkage pattern and, consequently, on the prostheses fit.

The production process of these blocks consists of compacting zirconia powder in the presence of a binder through a cold, isostatic pressing process; this leads to the homogeneous distribution of the components inside the block. The pre-sintering temperature of zirconia can influence the hardness, machinability, and roughness of the blocks.

Purification processes, new processing methods (CAD-CAM, hot isotactic pressing), and identification allow wide use of zirconia in technical and biological applications. As many new trends and applications for zirconia are being discovered, the future of this biomaterial appears to be very promising.