Machinable Zirconia
A study on the building blocks of restorative dentistry.
Ceramic materials can successfully replicate the esthetic qualities of natural teeth. Even though we have high-strength ceramics such as zirconia, it is still possible for these restorations to fracture in the mouth. Zirconia, like all ceramics, is brittle and does not flex to any great extent. If there is high stress because of sharp preparations or sharp cuts resulting from the use of diamond wheels at the connectors, or if the ceramic flexes, it tends to crack.1-3 However, one significant advantage to zirconia is that if the composition is correctly adjusted, it may actually act to prevent crack propagation and even close a crack that might start in the restoration. Zirconia-based ceramics have been used in a multitude of industries including automobile, medical, aircraft, and military for high-temperature applications, machined parts, and armor. Computer-aided design/computer-aided manufacturing (CAD/CAM) has enabled the use of materials such as zirconia that ordinarily cannot be fabricated in a conventional manner. One of the most important of these materials is yttria, which is partially stabilized tetragonal zirconia. Zirconia (ZrO2) is the oxidized form of zirconium (Zr), just as alumina (Al2O3) is an oxide of aluminum (Al).
Zirconia Phases
Pure zirconia without any added elements is not very useful as a structural material. Zirconia exists in three major phases: monoclinic, tetragonal, and cubic. The monoclinic phase is the room temperature—stable phase.4 Above 1,170°C, it will transform into the tetragonal intermediate phase and at a temperature of 2,370°C, zirconia transforms into a cubic phase. In pure zirconia ceramics, the cubic to monoclinic phase transformation occurs during cooling with about a 5% volumetric expansion (Figure 1). Therefore, pure zirconia forms fractures due to this large expansion stress. In the pure form, it is useful as a polishing compound or whiteners for paints and makeup, but not as a structural part.
The addition of other ceramic components may alter the presence and stability of these phases at room temperature. Zirconia may exist primarily in the tetragonal phase at room temperature by adding components such as calcia (CaO), magnesia (MgO), yttria (Y2O3), or ceria (CeO2). If the right amount of component is added, then one can produce a fully stabilized cubic phase, of cubic-zirconia megacarat “faux” diamond fame. If smaller amounts are added, 3% to 5% weight, then a partially stabilized zirconia is produced. Although the tetragonal zirconia phase is stabilized at room temperature, under stress the phase may change to monoclinic with a subsequent 3% volumetric size increase.5 This dimensional change takes energy away from the crack as well and can stop it in its tracks. This is called “transformation toughening” (Figure 2).6 This helps resist catastrophic failure. Even though a crack may exist in the material, the phase change prevents it from proceeding throughout the restoration.
When this zirconia is fired to full density, it is extremely difficult to machine, in part due to the transformation-toughening affect. Therefore, most milling machines are built to cut the material in a porous form, either partially fired or as pressed only. Because this is the case, it is absolutely critical that high quality-control standards are in place to make sure the blocks possess uniform density and the shrinkage values are well known. The porous blocks, after milling, are fired and shrink by 25% to 30%. If the density is not carefully measured for each batch, then a given block will not shrink to the expected value and the framework will not fit. Also, if it is not uniformly dense, the fired parts will warp (Figure 3).
Manufacturing Milling Blocks
There are numerous ways to fabricate zirconia-milling blocks—all of which can produce good or bad blocks. Most of the zirconia powder for fabricating the blocks is supplied by Tosoh (Tosoh Corporation, www.tosoh.com). However, some manufacturers may make their own zirconia powder. Blocks may be pressed uniaxially in a mold such that the pressure is applied from one direction only. In biaxial pressing, pressure is applied from opposite sides to form a block. Isostatic pressing tries to condense the powder evenly by applying pressure equally on all sides of the mold.
The powder is generally mixed with an organic binder to help the pressing process so that the block maintains its shape and the powder sticks together. The blocks may then be milled as is (in a “green” state) or partially fired to remove most of the binder. If the powder is not uniformly coated, or if the pressure is not maintained during pressing, cracks may develop in the block or large areas of porosity could be formed because of the clumps of binder. Also, density values could vary throughout the block, causing non-uniform shrinkage and warping. Therefore, even though the starting powder may be the same, it is the processing methodology that is important in determining the final quality of the block. Unfortunately, there is no way to determine if a block is bad just by looking at it. Beware that some bargains are indeed too good to be true.
A properly fabricated block, when tested, will have strength values ranging from about 900 MPa to 1,200 MPa.7 Differences among zirconia ceramics also can occur with the level of zirconia purity, the grain size, trace elements, and stabilizing compounds. These differences can have a major affect on clinical outcomes, such as marginal fit, translucency, strength, and long-term stability.
It is possible that zirconia may degrade over time.8,9 However, accelerated aging tests in steam and boiling water zirconia from two manufacturers—Lava™ (3M ESPE, www.3MESPE.com) and VITA In-Ceram® YZ (Vident, www.vident.com)—found no degradation.10 However, results of recent laboratory experiments to be presented at the AADR this month show a significant degradation after 3 years of real-time aging. It is possible that other zirconia block materials may also show degradation. A batch of hip implants from St. Gobains failed catastrophically after a few years in the body.11 This may have been caused by a problem with the exact composition of the zirconia with respect to the amount of yttria and alumina, which led to uncontrolled phase transformation. It is important that zirconia formulations have a small amount of alumina to help prevent instability of the crystal phases.
Handling Considerations
Firing protocols should be carefully followed. Over-firing or under-firing can affect the final crystal size and mechanical properties of the zirconia. Increased porosity may result in a more opaque zirconia as well as cause accelerated aging and fracture of the material. There are different powders with different particle sizes. Most fall into two firing regimes—some have a peak temperature of 1,350°C and others about 1,520°C. Do not alter firing temperatures and cycles to save time. Some “green” blocks with a lot of remaining binder will turn gray if sintered too fast because the char from the binder is incorporated into the zirconia. If the cycle is too fast, the block might “explode” as the organic binder burns out suddenly.
There have been numerous reports of veneer chipping causing a major concern. In several published studies, veneer chipping was about 15% after 3 to 5 years.12-14 In tests of the zirconia itself, it does not seem to routinely fail. In a clinical trial conducted by Clinical Research Associates (CRA), up to 60% of the hand-veneered restorations demonstrated veneer chipping after 1 year and about 20% of pressed veneers showed chipping.15 It is important to note that low-fusing porcelains were used in this study. A review of a variety of clinical trials on zirconia restorations showed a wide range of chipping at 1 to 5 years, from 5% to 25% chipping for low-fusing porcelains such as Cercon® Ceram (DENTSPLY Prosthetics, www.prosthetics.dentsply.com), Lava™ Ceram (3M ESPE, www.3MESPE.com), and IPS e.max® Ceram (Ivoclar Vivadent, www.ivoclarvivadent.com). Chipping was up to 54% after 1 year for Triceram® (B&D Technologies, www.origincadcam.com) porcelain on dense zirconia.16 However, others report minor amounts of chipping problems.17,18 It is important to note that there appears to be a correlation with the firing cycle and peak firing temperature of the veneer. Low-fusing porcelains appear to be less resistant to cracking than high-fusing porcelains and the density of the porcelain is lower.19-21 Zirconia is a thermal insulator. This property prevents heat transfer to the veneer porcelain and thus prevents it from becoming fully dense if fired fast and at a low temperature. In fact, the porcelain may look like Swiss cheese (Figure 4). Fast cooling creates stress in the porcelain that can lead to cracking. Although slow cooling may prevent stress from building on the surface of the porcelain, if it is not fully dense to start with no amount of slow cooling will fix the problem of insufficient density.
The finishing procedures used for zirconia structures may also affect the stability of the veneer porcelain. Any surface adjustment, such as grinding, sandblasting, and even polishing, can change the phase on the surface of the zirconia and may affect the stability and strength of the zirconia as well as the veneer porcelain.19-21 Veneer porcelain thermal expansion coefficients are matched to tetragonal zirconia—monoclinic zirconia has a much different thermal expansion coefficient. During the first firing cycle, the zirconia may be changing phase—physically changing in size—leading to stress in the porcelain. If the surface is adjusted by grinding or sandblasting before porcelain is applied, then a heat cycle is recommended to reverse the transformation. This typically involves a cycle with a hold of 15 minutes at a peak temperature of 1,000°C.
Porcelain’s adherence to zirconia may also be aided by applying a thin wash layer of dentin fired at about 70°C to 100°C higher than the first firing for the dentin layer. This is recommended in some systems such as VITA VM®9 (Vident). The high firing creates a glass that flows into the zirconia and also may create a chemical bond by acting as a solvent for the zirconia. Upon firing the first dentin layer, an excellent bond is created to the dentin wash and, subsequently, the zirconia framework.
Full-Contour Zirconia
Translucent zirconia materials are being increasingly marketed for full-contour restorations. Many of these materials also have their origin from Tosoh, which sells a line of zirconia powders with lower alumina content and finer crystal sizes. Not all translucent and full-contour zirconia blocks are the same. Again, although the starting powder may be the same, it is the processing that may create differences in the final restoration. The BruxZir® (Glidewell Laboratories, www.glidewelldental.com) material may start with a stock powder, for example, but Glidewell applies a series of sophisticated post-processing procedures to the powder, including refining the particle size to create a finer final crystal structure. This also involves suspending the powder in solution to evenly distribute the particles (colloidal suspension) and fabricating the blocks such that these particles come together in a uniform array.
Are full-contour zirconia restorations safe? Although zirconia does have a fine microstructure, there are varied reports as to the abrasion properties as well as no clinical studies on the longevity of full-contour zirconia and the wear of opposing dentition. Some studies show that polished zirconia is very abrasive to the opposing dentition and that glaze protects the natural tooth for sometime until the glaze wears through.22 If a machined surface is simply glazed, the glaze wears off to expose a rough surface that then accelerates tooth wear. If the zirconia is not polished to a mirror finish, the zirconia may cause excessive wear. Other studies demonstrate that polished zirconia is “wear-kind.”23 Studies using an enamel substitute, steatite, or soapstone, tend to show that highly polished zirconia is wear-kind.24 Full-contour zirconia crowns must be highly polished and remain intact. If the occlusion needs to be adjusted and the dentist cannot achieve a high polish, the restoration should be sent back to the laboratory for finishing. Water should be used to cool zirconia because it can get excessively hot during polishing. Coarse grinding may cause cracks that penetrate into the zirconia substructure, causing transformation toughening, which may initially hold these cracks closed. Over time and with exposure to the oral environment and reversal of the transformational stress, the cracks can begin to propagate. Also, the surface of the zirconia can become raised because of the physical increase in size of the transformed monoclinic crystals. This may also cause enamel wear problems. Clinical trials are required to provide a more definitive answer.
Retention Considerations
While no one knows exactly what strength is required for ceramic restorations throughout the mouth, guidelines have been developed based on clinical trial data on ceramic materials such as In-Ceram, Empress, and machined porcelain blocks.25 For example, VITABLOCKS® MKII (Vident) have been well studied clinically. Success rates for this material are generally about 95% at 5 to 10 years in the mouth for inlays, onlays, and crowns.26 Even though the strength value is only about 130 MPa, when bonded onto the remaining tooth it reinforces the tooth structure and, in most cases, can survive stress on a molar crown just as well as zirconia. This also depends on the thickness of the material. Load-bearing capacity is as important as strength. Even a high-strength material must have a minimum thickness to have sufficient load-bearing capacity to resist stress in the mouth.
Zirconia cannot be acid-etched, and so most believe that it cannot be bonded to the tooth structure. Several primers may be used to create a bond to the zirconia. Ivoclar, Kuraray, and Bisco all have materials that bond to the zirconia in combination with resin cements. Glass ionomers bond weakly or not at all. Bonding might improve retention of zirconia to the tooth structure, and these resin cements have an added benefit in that they are not soluble. There again is one controversy with respect to sandblasting the zirconia. If a glass-ionomer type of cement is used, sandblasting on the interior surface with a low pressure (25 psi to 50 psi, two-thirds bar) and small-particle alumina (25 µm to 50 µm) may help to clean the internal surface and provide added mechanical retention for the cement. Some studies have demonstrated a potential problem in sandblasting the internal surface with respect to crack growth, while other studies have shown an improvement in properties.10,27
References
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