When restoring a tooth, the clinician faces what material should be used for the restoration. The major factors that may influence the final choice are esthetics and strength of prostheses. Metal ceramics have been conventionally used as reliable materials. However, due to the request of esthetic dentistry, all ceramic prostheses are replacing metal based restorations more and more.
Zirconia has been used in prosthetic dentistry for the fabrication of crowns and fixed partial dentures for more than 15 years, in combination with CAD/CAM techniques. Zirconia (ZrO2), also named as “ceramic steel”, has optimum properties for dental use with superior toughness, strength, and fatigue resistance, in addition to excellent wear properties and biocompatibility. The introduction of zirconia based ceramics as restorative dental materials has generated considerable interest in the dental community. The mechanical properties of zirconia are the highest ever reported for any dental ceramic. This may allow the realization of fixed partial dentures and permit a substantial reduction in core thickness. These capabilities are highly attractive in prosthetic dentistry, where strength and esthetics are paramount.
At ambient pressure, pure zirconia has three crystalline phases. At room temperature and upon heating up to 1170 degrees Celsius, the symmetry is monoclinic. The structure is tetragonal between 1170 and 2370 degrees Celsius and cubic above 2370 degrees Celsius. This transformation from the tetragonal (t) phase to monoclinic (m) phase upon cooling is accompanied by a substantial increase in volume about 4.5%, sufficient to lead to catastrophic failure. This transformation is reversible and begins at 950 degrees Celsius on cooling.
Zirconia used in dentistry has some stabilizers such as ceria, yttria, alumina, magnesia and calcia. This stabilizing oxide allows to retention of tetragonal structure at room temperature and the control of the stress induced t-m transformation, efficiently arresting crack propagation and leading to high toughness.
[Fig. 1] Electron micrograph of zirconia after sintering
The aspect resulting from transformation of the tetragonal phase to the monoclinic phase is transformation toughening. t-m transformation result in an increased strength and toughness of the material. This feature is beneficial for biomedical applications, where crack propagation is a crucial issue. Due to the metastability of tetragonal zirconia, stress generating surface treatments such as grinding and sandblasting is liable to trigger the t-m transformation with the associated volume increase leading to the formation of surface compressive stresses, thereby increasing the flexural strength but also altering the phase integrity of the material and increasing the susceptibility to aging. (Fig. 2)
[Fig. 2] Transformation toughening
Fig. 3 shows a Vickers indentation in a commercially available 3Y-TZP for dental applications under 98.1N load. Only one short crack is emanating from one of the corners of the indentation. The absence of cracking from the other corners is indicative of the occurrence of the transformation toughening mechanism.
[Fig. 3] Optical micrograph of a Vickers indentation in a 3Y-TZP for dental applications (98.1N load)
Aging – Low Temperature Degradation (LTD)
The low temperature degradation of zirconia is a well documented phenomenon, exacerbated notably by the presence of water. The consequences of this aging process are multiple and include surface degradation with grain pullout and microcracking as well as strength degradation.
During this aging process, the metastable tetragonal phase converts by a slow transformation into the stable monoclinic phase, starting at the surface in the presence of water at relatively low temperature. Aging starts by transforming a single grain at the surface via a stress induced mechanism. The transformation leads to the typical volume increase that induces stress in the neighboring grains and micro-cracks. This results in a large number of transformations, which increases the transformed zone. The micro crack offer a passage through that water can penetrate into the bulk and aging process continues to progress. Particle size reduction, increasing content of yttria and addition of alumina could be reduced the risk of LTD.
The mechanical properties of zirconia strongly depend on its grain size. Above a critical grain size, zirconia is less stable and more susceptible to spontaneous t-m transformation whereas smaller grain sizes (<1micron) are associated with a lower transformation rate. Moreover, below a certain grain size (~0.2micron), the transformation is not possible, leading to reduced fracture toughness. Consequently, the sintering conditions have a strong impact on both stability and mechanical properties of the final product as they dictate the grain size. Longer sintering time and higher sintering temperature lead to larger grain sizes.
Different Types of Zirconia
Traditional Zirconia has been doped 3mol% yttria to stabilize the tetragonal phase at room temperature. Compared to glass ceramics, zirconia in general has certain optical disadvantages due to its relatively high refractive index, which causes a high grade of total reflection. The refractive index changes depending on the orientation of the tetragonal crystals of the zirconia, which can cause birefringence. The high reflection leads to a mirror-like surface than that of natural teeth resulting in poor esthetics.
The other drawback of 3Y-TZP was its opacity. One source of opacity is the presence of alumina. Alumina is added as a sintering aid to help preventing the formation of pores when green state zirconia is placed in the furnace. Alumina grains lead to an enormous number of interfaces. This interfaces can decreases the light transmission when alumina added to zirconia. The alumina content was decreased from 0.25wt% to 0.05wt%. This 0.05wt% alumina containing 3Y-TZP is more translucent than that of 0.25wt%. It’s more susceptible to LTD because there is less alumina to stabilize the tetragonal phase. In addition, these poor esthetics make additional veneering with suitable porcelain materials. However, layering materials are not as strong as zirconia, and this may lead to chipping.
[Table 1] Comparison of Flexural Strength and Translucency
[Table 2] Material Properties of Different Zirconia Types
To overcome the disadvantages such as opacity and mirror-like surface, dental zirconia has been fabricated with increased yttria content recently. Zirconia doped with 8mol% yttria will completely stabilize the cubic phase, whereas zirconia doped with 5 mol% yttria creates partially stabilized zirconia (PSZ) with approximately 50% cubic phase zirconia. 5Y-TZP is sometimes referred to as 5Y-PSZ because it contains the cubic phase more than 50%. The cubic phase of zirconia is isotropic in different crystallographic directions, which decreases the light scattering that occurs at grain boundaries. As a result, the cubic zirconia appears more translucent. The translucency of 5Y-TZP is slightly less than that of lithium disilicate. In some clinical situations, the opacity of the material my help mask discolored substructures or cement.
Stabilized cubic zirconia does not transform at room temperature and therefore will not undergo low temperature degradation or transformation toughening. This means 5Y-TZP has lower fracture toughness than 3Y-TZP. Furthermore, the coefficient of thermal expansion decreases with increasing yttria content. Therefore, manipulation and crown preparation should be done carefully, avoiding thin walls and sharp edges as much as possible. (Table. 1 and 2)
Chipping between zirconia and porcelain
The interface between zirconia and porcelain may be involved on crazing and chipping during function. Chipping is defined as a typical failure of contact loadings, normally produced when a crack generated or propagated by contact loads deflects due to the presence of a free surface nearby. Tensile stress induces fracture of the brittle material perpendicular to the applied forces.
Thermal coefficient mismatches, processing and inherent material defects will increase the probability of crack propagation under loading.
In the case of metal ceramics, an adherent layer of oxide is essential to achieve a strong bond. This will enhance the wettability and adherence of the ceramic. When the temperature attains a certain level, part of this oxide will be dissolved into the glass.
On the other hand, the zirconia core-veneer bond strength is lower than metal ceramics. This can induce chipping and delaminating under loading. Framework surface treatment, the surface finish, the type and method of application of the veneer ceramic may affect this bonding. If bond failure has been pointed as chipping reason, differences in thermal coefficients, liner material and poor core wetting, veneer firing shrinkage, phase transformation, loading stresses, flaw formation, coloring pigments and surface properties have been reported as potential causes.
Zirconia presents a thermo-conductivity much lower than that of other framework materials. This low thermal conductivity retards the ceramic cooling rate at the interface. This generates thermal residual stress. It may induce thermal cycling delamination of the veneering porcelain. Prolonged cooling phases have been proposed to reduce this stress and veneer chipping. Slow cooling time ameliorated the resistance of the veneer restorations, and enhanced the shear bond strength.
Cohesive and adhesive failures of the veneering are recurrent complications of veneered zirconia framework. To counteract this tendency, the overpressing technique has been introduced. Press over zirconia (POZ) technique has a higher strength compared to the layering technique. Using of the pressed ceramic may reduce the chipping incidence, since the fabrication method would reduce the formation of large flaws and minimize the thermally induced residual stresses.
The pressed ceramic will be joined to the zirconia framework by fusion glass ceramic. Higher tensile strength and the superior quality of interface can prevent porcelain chipping. This material exhibits better fracture strength and fatigue behavior when compared to the layered ceramics. In one recent study, 3-unit posterior of POZ had significantly less fractures and chippings compared with layered one.
Wear of Opposing Enamel
One major concern with the use of monolithic zirconia as a restorative material is the abrasive nature against opposing enamel because of this material’s hardness and surface roughness. Several in vitro and in vivo studies were conducted to determine the wear of zirconia against different antagonists, including enamel, have shown zirconia to be comparable to other restorative materials in terms of wear of opposing enamel.
In vitro studies have shown that polished zirconia produces less wear on enamel antagonists than glazed zirconia and feldspathic porcelain crown. It can be explained by the fact that a polished zirconia surface procedures a quantifiably smother surface than the glazed zirconia, therefore proving to be less abrasive to the opposing enamel. Rougher surfaces have been correlated with increased wear of the opposing dentition. The result of in vivo studies demonstrates that polished monolithic zirconia does not cause accelerated wear of the opposing enamel. The wear of both metal ceramics and monolithic zirconia is comparable and that there are no significant differences between the enamel antagonists wear and control enamel wear of the two materials.
Annealing is the heat treatment procedure of zirconia to transform monoclinic phase to tetragonal phase after grinding because grinding induce t-m phase transformation on the surface of zirconia, especially 3Y-TZP. Monoclinic phase transformed by grinding and sandblasting can be returned to tetragonal phase by heat treating at 1000~1100℃ for 5~10 min. Some studies insisted heat treatment did not affect the flexural strength, and some other studies concluded the heat treatment process was helpful for m-t phase transformation. It seems the further study of regeneration firing is needed to prove the influence to property of zirconia.
Bonding to Zirconia
Glass ionomer cement (GIC) and resin based cements are the primary choices for bonding ceramic restorations to the remaining tooth structure. GIC and resin-modified GIC are often used to cement acid-resistant ceramics, mostly because these cements are very easy to use. However, the most popular and effective cements for all type of ceramic restorations are the resin based composites, including the systems containing the 10-methacryloxydecyl-dihydrogen-phosphate (MDP) monomer. The clinical success of resin bonding procedures for cementing ceramic restorations and repairing fractured ceramic restorations depends on the quality and durability of the bond.
As mentioned above, 5Y-TZP has lower fracture toughness and smaller amounts of tetragonal phase, leading to a reduced possibility of t-m transformation and therefore less transformation toughening compared to 3Y-TZP. Furthermore, the CTE decreases with increasing yttria content. It may lead to problems with veneering materials. But 5Y-TZP has better translucency.
3Y-TZP is suited for zirconia hybrid abutment and multi unit framework more than 3-unit bridges. Due to the reduction of fracture toughness and strength in 5Y-TZP, the indications are limited to full contour 3-unit bridges with higher wall thickness.
In reference, a manufacturer of dentistry launched new zirconia discs used special powder conditioning to combine 3Y-TZP and 5Y-TZP oxide ceramic powders for the ultimate in strength and esthetics. It enables full arch monolithic zirconia bridge can be made with translucency of 5Y-TZP and strength of 3Y-TZP.
[Table 3] Application of Zirconia
Material properties and applications of Zirconia have been looked over in this review. All material has their advantages and disadvantages respectively. When choosing a type of restoration, it’s important that clinicians should carefully decide the material considering its properties and features to obtain the better aesthetic and functional results.