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When zirconia was introduced as a structural framework for dental restorations, only a few companies provided the computer-aided design and computer-aided manufacturing (CAD/CAM) systems needed for fabrication. A typical system included a scanner, a milling machine, a sintering oven, and the compatible zirconia milling blocks. They were referred to as “closed” systems, meaning that a particular scanner worked only with that manufacturer’s milling machine, which would only accept the same manufacturer’s own zirconia milling block. Eventually, “open” CAD/CAM systems were introduced, which enabled the use of design files from one scanner to be read by different milling units. These open units could use milling blocks from outside sources, making it possible for suppliers around the world to provide zirconia milling blocks. Today, some online suppliers sell “knockoff” zirconia milling materials. Dental crowns and bridges are now considered medical devices and the materials used in their fabrication must obtain a US Food and Drug Administration (FDA) 510(k) number. The FDA 510(k) process allows a manufacturer to acquire premarket approval based on a pronouncement that the given product is “substantially equivalent” to another product that has been previously approved. The device classification name, or product code, used for approval is “EIH,” which is described as “powder, porcelain.” Zirconia is not porcelain but yet is grouped in that category; therefore, the requirements appear incongruous with the product classification, in the author’s opinion.
How much of the available product, however, is legitimate, especially at prices much lower than that of US companies? Zirconia materials from known US and international suppliers have differences. This is easily observed when examining the product labeling. For example, two companies in China and India offer zirconia milling blocks that are compatible with brand-name milling units. These milling blocks differ from most reputable zirconia brands in their chemical concentrations of zirconium dioxide, hafnium dioxide, yttrium dioxide, and aluminum. Subsequently, processing these blocks will not produce the same results.
In the early 2000s, the use of CAD/CAM technology spurred a whole new generation of ceramic substructures consisting of zirconium dioxide. Several manufacturers introduced crown-and-bridge frameworks milled from blocks of presintered yttrium-stabilized zirconium dioxide blocks. The oversized milled frameworks were then sintered for 11 hours at 1500°C, providing excellent fit with 900 MPa to 1300 MPa of flexural strength. Other manufacturers milled fully sintered zirconium dioxide blocks (because the process removed the shrinkage factor), which one study found to have a superior marginal fit.1 Both fabrication methods provide a framework with sufficient flexural strength, allowing them to be used for multi-unit posterior bridges.2 After the completion of the framework, leucite-reinforced material was either layered or pressed on to reach the final contour of the restoration.
This article traces how zircon, found naturally in the earth, is processed and then reaches the dental laboratory and finally the patient’s mouth.
Although most zircon deposits come from Burma, those found in Australia are the oldest mineral deposits on earth: 4.4 billion years old. The name is derived from the Persian word zargun, meaning golden-colored, and the German adoption of zircon. Centuries ago, zircon was thought to have mystical powers that could provide protection from evil spirits, bring wealth and wisdom, and improve sleep. Also known as zirconium silicate (ZrSiO4), zircon is one of the minerals containing the highest concentration of zirconium (Zr); the other is baddeleyite, also known as zirconium oxide (ZrO2). Zirconium is always found mixed together with hafnium (Hf), uranium (Ur), and thorium (Th).3 No mining operation in the world produces only zircon; it is a co-product of heavy-mineral mining.4 Zircon is used in such products as abrasives, refractory materials, dinnerware, electrical porcelain fixtures, and as a source of zirconium metal.
German chemist Martin Klaproth discovered zirconium in 1789, the same year he discovered uranium.5 Zirconium is found not only in the earth’s crust throughout the world, but also in lunar rock samples acquired during Apollo missions to the moon.6 This grayish-white metal is in the titanium family of metals, which also includes hafnium, and is the 19th-most abundant element in the earth’s crust.7 Scientists use it to determine the age of igneous rocks, which are formed through the cooling and solidification of magma or lava.6 The weathering of solidified lava has created large zircon deposits in beach sands throughout the world. Zircon is commonly found in this form mixed with silica, ilmenite, and rutile. Australia and South Africa have the largest commercial deposits, but significant deposits also exist in the United States, India, Thailand, Cambodia, and Russia’s Kola Peninsula.
In the 1950s, researchers began developing applications for this material. Today, an enormous number of products contain some form of zirconium, from antiperspirants, alloys, gemstones, thermal barrier coatings, and products used in kidney dialysis (zirconium binds to urea), to, of course, dental restorative materials and implants.
Raw zirconium is found in two forms: the silicate mineral zircon (zirconium silicate) and the oxide mineral baddeleyite (zirconium oxide).7,8 Zircon contains approximately 46% zirconium, while the richer baddeleyite has an estimated 70% zirconium.8 Zircon was first isolated in 1824 by Swedish chemist Jöns Jacob Berzelius, who accomplished this by mixing potassium and potassium zirconium fluoride.
The major source of zircon is sand and gravel that are collected from coastal waters by floating dredges. A spiral concentrator removes unwanted particles using magnetic separators (eg, ilmenite), electrostatic separators (eg, rutile), and gravitational separators (eg, quartz). Baddeleyite is also obtained from sand and gravel deposits but contains such high concentrations of zirconium oxide that it can be used without refining.
In 1925, Dutch chemists Anton E. van Arkel and Jan Hendrik de Boer invented the thermal iodide process to purify zirconium by thermally decomposing zirconium tetraiodide.9 This method was expensive to operate, so 20 years later, Wilhelm Kroll found a more cost-effective alternative by using magnesium to break down zirconium tetrachloride. In recent years, the Kroll method has been replaced.
Various methods now exist for purifying zircon, depending on the level of purity required, but these methods all have three steps in common. First, the zircon is separated by chemical, thermal, or mechanochemical means, followed by solubility differentiation, and finally the separation of the residual impurities from zirconium compounds.9
Today, purification of zirconium from impure zircon includes several chemical reactions, resulting in a chloride-free “batch.”10 A stabilizer is then added to the solution along with hydroxylamine (NH2OH) to adjust its pH level to 10. The solution is spray dried to 400°C to remove the solvent.11 After drying, the powder is calcined (heated to remove undesirable compositions) at 600°C for 2 hours, forming 98% tetragonal-phase zirconia.12 The calcined powder is then ground down to create uniformly sized zirconia particles.10 Depending on the zirconium’s intended use, further processing of the powder will be necessary.
Fabrication of Milling Blocks and Discs
The largest manufacturer of zirconia powders is Tosoh Corporation in Japan. Tosoh supplies zirconia powder in the form of spray-dried granules to allow for easy processing and the prevention of agglomeration during storage. The powder is composed of granules that are 60 µm. A granule consists of 0.04-µm particles, which are clusters of crystallites measuring 0.027 µm. This powder is supplied to 98% of the various manufacturers that produce milling blocks and discs (pucks).
Once the powder is obtained, the methodology of block/disc fabrication varies by manufacturer. Generally, the zirconia powders are mixed with a wide variety of organic binders, consisting of major and minor binders, a lubricant, and a wetting agent.13 These can include natural products (cellulose or clays) or synthetic products (polyacrylates or polyvinyl alcohol). The zirconia material is then formed into various sizes of block and disc shapes using three basic techniques. After the zirconia mixture is placed into a mold, compaction pressure is used to condense the material. This compaction pressure is applied either in one direction (uniaxial), from opposite sides (biaxial), or evenly from all sides (cold isostatic). A combination of these different directional compaction pressures is also used in the block/disc fabrication process when uniaxial pressure is applied first, followed by isostatic pressure.14
Not all manufacturers follow the same methodology of fabrication. Even though off-brand blocks or discs appear to be the same and are advertised as sharing equal properties and compatibility with name-brand materials, the density (compaction of the particles) may not be consistent throughout the entire block or disc and may result in restorative failures for dentistry.
The zirconia material at this stage is referred to as being in a “green” state.14 These green-state blocks/discs are then partially sintered (presintered) in a special furnace, which furthers the stability and condensation of the block.15 The partially sintered blocks are referred to as “bisque fired” blocks or discs. The presintering temperature has a bearing on the hardness, machinability, and roughness of the blocks.16
The dental laboratory should know the quality and source of the milling block/disc when purchasing zirconia materials. This material is used in a restoration for a patient, who trusts the dentist placing it, who in turn depends on the laboratory using certified materials. If sourcing is not confirmed, neither the laboratory nor the dentist may know until a problem occurs.
Even slightly altering the process can lead to significantly different material properties. For example, starting in 1985, a French company,
St. Gobain Desmarquest, distributed zirconia and alumina ceramic femoral heads worldwide to most of the orthopedic industry. Until 1997, the sintering was accomplished in a batch furnace similar to a porcelain firing oven, where the material was placed at room temperature and then gradually heated, held at a higher temperature, and finally cooled to room temperature. In 1998, due to increasing demand, the company changed the sintering batch furnace to a tunnel furnace to decrease the processing time. This type of furnace operates by using a conveyor belt to move the items through multiple heating chambers. Using the tunnel furnace caused a change in the sintering cycle, which altered the microstructure and strength of the final product. All other steps in the fabrication were kept the same.
By the end of 2000, two major orthopedic companies—DePuy (France) and Smith & Nephew (US)—reported unusual brittle fracture rates had increased from 0.002% to 8.000% in the St. Gobain Desmarquest femoral heads. The French Medical Agency and the FDA issued recall notices and, subsequently, many lawsuits were filed.17 Masonis et al18 found the tunnel furnace and batch furnace exhibited the same temperature profiles on heating to a maximum temperature of 1450°C but differed in their cooling stage, which was three times shorter for the tunnel furnace than for the batch furnace. Examination of the failed femoral heads revealed a significant increase in the level of the monoclinic phase of zirconia. In fact, a 100% transformation from the tetragonal to monoclinic phases was found near the bore surface in a failed head from the tunnel furnace. The failure was associated with accelerated aging. This is an important lesson on the sensitivity in the processing steps required to produce a reliable milling material.
In 2014, the amount of counterfeit automobile parts in the United States increased by 83%.19 The International Chamber of Commerce estimated in 2015 that the global market reached $1.77 trillion in counterfeited goods.20 It is difficult to distinguish authentic and fake milling materials because so many companies that provide zirconia powder as a “raw material” have their own formula for manufacturing.21 This factor makes it challenging to speak about zirconia as a single material. In 2014, The Journal of Dental Technology (JDT) remarked on reports from some of the US zirconia suppliers finding various contaminants in gray market material. One contaminant, talc (not normally present in quality zirconia powder), can affect the material’s composition and purity. JDT also noted counterfeit materials that have not gone through the proper protocols and testing can fail as a restoration in the patient’s mouth.22
If a laboratory is concerned about the source of a particular milling material, it can request testing. The Tosoh Corporation can analyze and confirm whether its powder was used in milling blocks, free of charge.
Other factors that influence the physical properties of the zirconia materials may not be apparent to the prospective buyer or even the manufacturer. The strength of the milling block and subsequent restoration is subject to inherent processing flaws, impurities, and pores that can exist in the block itself.23 The location of these structural defects is not evenly distributed through the stock material. Testing results can produce different values from the same material based on the location of the sample acquisition.24 To counter the variability in testing results and define a probability density distribution, Weibull statistics can be applied.25 Weibull statistics are based on the weakest link theory, which suggests that when an ample number of weak areas exist in a material, the failure of that material originates from the largest and most catastrophic defect.26 This testing method provides an interpretation of strength parameters and the probability of failure and defect data analysis, which is based on a collection of multiple failures, not just a single event.27
Siarampi et al26 evaluated the flexural strength based on survival probability and Weibull statistical analysis of two genuine commercial domestic zirconia cores. The hypothesis was that because both zirconia core materials had similar fractographic and crystallographic properties, the Weibull statistical analysis of the flexural strength (Weibull modulus [m] and characteristic strength [µ0]) would be the same. A low m value indicates more flaws and defects are present in a material and, therefore, would decrease the reliability factor.28,29 They found the Brand A zirconia core material to have a higher flexural strength than the Brand B product. However, the Brand A zirconia core material had a lower m value than the Brand B version. They concluded that even though these two zirconia core materials were similar in composition, the flexural strength and reliability differed. The higher-strength material also had a lower m value, which increased the probability of failure. This study revealed that even high-strength zirconia milling materials may be more prone to failures due to structural defects.
Stabilizing Zirconia Dioxide
At atmospheric pressure, zirconia undergoes polymorphic phase changes from cubic to tetragonal to monoclinic. The cubic phase is stable from 2370°C (4298°F) to its melting point of 2680°C (4856°F). Cubic zirconia has an octahedral molecular configuration in which all eight oxygen atoms are equidistant from each other and tetrahedrally coordinated to four equidistant zirconium atoms. The tetragonal phase is stable from 1170°C (2138°F) to 2370°C (4298°F). Tetragonal zirconia also has an octahedral molecular configuration, except that in this phase, four of the eight oxygen atoms are 15% closer to the zirconium atoms. The crystalline structure appears distorted. The monoclinic phase is stable below 1170°C. Monoclinic zirconia also has a distorted crystalline configuration, with oxygen atoms at varying bond lengths and angles. This is the phase in which the naturally occurring zirconia is found.30
When zirconia cools from the melting point to room temperature, it experiences two phase transformations. At approximately 2370°C, the cubic phase transforms to the tetragonal phase, and when the temperature drops to approximately 1170°C, the tetragonal phase transforms to the monoclinic phase. The transformation energy that occurs is so great that maintaining either the cubic or tetragonal phase at room temperature is impossible.
In 1975, Garvie et al31 proposed that by adding a cubic oxide, the tetragonal phase (the strongest and toughest stage) could be maintained at room temperature. Various cubic oxide additives were found to have this stabilizing effect. Traditional additives (magnesium oxide, calcium oxide, cerium oxide, and yttrium oxide) have lower-valent cations (fewer electrons than protons, resulting in a positive charge) that provide oxygen vacancies to help compensate for defects. These atomically smaller cations relax the matrix, which prevents cracking throughout the tetragonal-to-monoclinic phase transformation during cooling.32 The extent of stabilization (full or partial) and to which phase the zirconium will be retained at room temperature are determined by the amount of additive and its solubility in zirconia dioxide.30 For example, when smaller amounts of additives are used, not all of the zirconia dioxide is stabilized, allowing for two or more phases to exist; thereby, it is referred to as partially stabilized zirconia.
Yaseem et al32 found the quality of the additive is directly related to the sintering and stabilization of zirconia. By using x-ray diffraction, they found the non-uniform particle shape and size of calcium oxide and magnesium oxide tend to create large numbers of agglomerates (clusters) and were poor performers as an additive, whereas the particle shape and size of yttrium oxide made it more suitable as an additive. They also observed—using scanning electron microscopy and x-ray diffraction plots—the particle size of the zirconia powder played an important role. Finer powder resulted in more successful sintering and stabilization.
Translucency in Anterior Zirconia Full-Contour Restoratives
As strong as full-contour zirconia is, the major drawback is its lack of esthetic translucency. By increasing the density and decreasing the alumina, some improvements in the translucency level have been made.33 When incident light is exposed to a zirconia crown, for instance, part of it is directly reflected, a portion is absorbed, and another section of light is transmitted. The transmitted portion experiences scattering, which is interior reflection and refraction. This scattering results from internal pores, impurities, defects, and grain boundaries.34 The tetragonal zirconia crystal is birefringent, having different indices of refraction (anisotropic).35 If adjacent grains do not have the same crystallographic orientation, then a discontinuance of the index of refraction occurs at their borders.36 The presence of pores between the grains also contributes to the discontinuity of the refractions that impedes translucency.33 Alumina, which is added to zirconia for strength, has a different index of refraction, which also contributes to the decrease in translucency by scattering.37
To increase the translucency of zirconia, different methods have been introduced that focus on creating a more isotropic index of refraction that decreases the birefringent effect. One approach is to decrease the amount of alumina from 0.25 wt% to 0.10 wt% or 0.05 wt%38; however, the hydrothermal stability is then lowered, as well.39 Another method of improving the translucency level is increasing the yttria content and adding cubic-phase zirconia40; nonetheless, this approach reduces the flexural strength and fracture toughness because of a decrease in the transformation toughening effect of the tetragonal phase of zirconia.38,40 For example, one company increased the yttria content in its zirconia from 5.2 wt% to 9.32 wt%.38 As a result, however, the bending strength (flexural strength) dropped from 1100 MPa to 600 MPa and the fracture toughness decreased from 5 MPam0.5 to 2.4 MPam0.5.
Additives are not the only factor that can affect the translucency of zirconia. The sintering temperature32 and atmospheric conditions during sintering also impact it.42,43 The heating method and temperature determine the density, porosity, and grain size of zirconia, thereby influencing the scattering of light.44,45 Manufacturers are experimenting with different formulations of additives, sintering methods, temperatures, and grain modification to maintain the strength of zirconia while increasing the translucency without sacrificing resistance to hydrothermal degradation.
The rate at which zirconia milling materials are entering the dental marketplace has soared. The quality of these materials can vary in numerous aspects that include the zirconia powder, the further refining by the manufacturer, and the methodology used for processing the millable blocks and discs. So many steps are needed between extracting raw zircon from the earth and turning it into the zirconium end product used in dental restorations. The origin of the restorative materials you use and how they were processed can be the difference in a successful restoration or a failure.
About the Author
DDS, MAGD, CDT
Associate Professor, General Dentistry
Virginia Commonwealth University School of Dentistry