Shining a Light on Translucent Zirconia
Continuing developments of popular material require attention and education
Jason Mazda
For many years, dental professionals have had to choose between strength and esthetics when treatment planning for crowns, bridges, and other indirect restorations. Stronger materials were easily distinguishable from the natural dentition in the mouth, and more esthetic materials were prone to failures. Over the past few years, however, one type of material has begun displaying the potential to combine strength with esthetics, and manufacturers have engaged in a race to perfect it. Although room for improvement remains, the current generation of high-translucency zirconia materials warrants an in-depth exploration in the interest of offering patients the best possible treatment. “There is no way you can practice in today’s world of restorative dentistry without having zirconia in your armamentarium, but it is extremely important to know how to use it and when to use it,” says John F. Weston, DDS, owner and director of Scripps Center for Dental Care.
Some say that many dentists and even laboratories have embraced zirconia without learning how and when to use it. According to the National Association of Dental Laboratories’ Materials & Equipment Survey, despite being widely utilized for full-contour restorations for less than a decade the material already accounted for 29% of dental laboratories’ crown-and-bridge sales in 2016, equaling the percentage of porcelain-fused-to-metal (PFM) restorations and just slightly less than all-ceramics (31%).1 The survey also found that zirconia accounted for 59% of restorations fabricated using computer-aided design/computer-aided manufacturing (CAD/CAM).1
“Everyone has moved to zirconia very quickly,” says Lee Culp, CDT, owner of Sculpture Studios. “Dentists like it because it generally does not break, it is ‘esthetic enough,’ and it does not need to be bonded, so they can cement it virtually any way they desire. Laboratory technicians like it because it is cost-efficient and easy—it mills easily with long bur life, and material costs are low. However, most dentists do not know the difference between the original formulation and modified formulations; many technicians do not even understand what the newest high-translucency zirconias do and the challenges associated with them.”
Adding to the confusion is a nomenclature that is somewhat lacking in creativity and clarity—high translucency, super translucency, and ultra translucency can have various meanings for different manufacturers.
“Materials that reached the market 10 years ago were termed ‘translucent’ because they were more so than the previous generation of zirconia,” says Nathanial Lawson, DMD, PhD, Assistant Professor and Director of the Division of Biomaterials at UAB School of Dentistry. “Now the newest generation also is being called translucent zirconia, and as a result, some dental professionals are struggling with even the most basic decisions, because there is a significant difference.”
The new developments do not appear to be slowing down; conversely, zirconia manufacturers are gaining momentum as they continue to meet more and more of dentists’ and laboratories’ demands.
“At least in the United States, zirconia is everywhere at this point in time,” says Rella Christensen, PhD, team leader of the nonprofit Technologies in Restorative and Caries (TRAC) Research. “It has taken over the market; however, we find there is a dearth of knowledge among both clinicians and laboratories.”
Sturdy from the Start
Although zirconia has been used in restorative dentistry for root canal posts since 1989, for abutments since 1995, and for all-ceramic posterior fixed partial dentures since 1998, PFMs and glass-ceramics remained prevalent for crown-and-bridge indications.2
In 2001, 3M introduced Lava Zirconia, which was initially used primarily for layered and pressed restorations. The material was strong, millable, and both closer to natural tooth color and more biocompatible than metal.
“Dentists were really excited to have options beyond PFM, glass, and gold,” says Jim Buchanan, National Account Manager for 3M Oral Care. “Patients were becoming more astute about what they wanted. A need existed for a new material that could help laboratories produce what the dentists and patients wanted. Zirconia was promising because it was basically tooth colored and it was strong.”
In 2009, Glidewell Laboratories introduced BruxZir Solid Zirconia for monolithic restorations, and by the end of the following year, millions of units had been prescribed.3 Other manufacturers eventually followed with their own versions of the product.
“Zirconia was really introduced to dentistry as a substructure, but BruxZir changed everything,” Christensen says.
This early generation of dental zirconia displayed more than 1000 MPa (megapascals) of flexural strength, compared with 460 MPa for Ivoclar Vivadent’s IPS e.max lithium disilicate and 100 MPa or less for the veneering porcelains on PFM restorations. 4,5 It also offered fracture toughness of approximately 5 to10 MPa/m½, more than twice that of most glass-ceramics.6
While flexural strength is cited more frequently in marketing materials, several experts say fracture toughness is just as important, if not more so.
“Fracture toughness is important because it indicates how well a material can prevent crack propagation,” Lawson says.
Buchanan says 3M emphasizes this in testing. “Fracture toughness tests more closely replicate what occurs in the mouth, as test samples contain built-in defects,” he notes.
Interestingly, despite its high flexural strength, the original BruxZir zirconia also has proven to be wear-friendly to opposing tooth structure in TRAC Research studies.
“Our work shows quite clearly that it has been significantly more gentle and tolerable to opposing dentition than many people expected,” Christensen says.
Still, early monolithic zirconia had its limitations. It was mostly opaque, so the esthetic zone was not an option, and it was even less than ideal for the posterior region. Problems occurred with layered zirconia debonding as well, and areas with minimal tooth structure required full coverage.
“Everyone knew zirconia was really strong, but it required significant post-processing work, such as layering porcelain, to improve its esthetics,” says Katie J. Kosty, COO of Sagemax Bioceramics, the largest manufacturer of dental zirconia in the US.7 “We saw a need in the market for more esthetics in addition to that high strength.”
Fresh Formulations
Manufacturers attempted to meet the demand for more esthetic zirconia by experimenting with new formulations. The first version of zirconia employed in dentistry was in the high-strength tetragonal crystalline phase, stabilized with 3 mol% yttria and usually enhanced with 0.25% alumina.8 One early method of increasing translucency involved reducing the alumina content to lower than 0.05%, which, along with an improved processing technique, was found to have minimal effect on the mechanical properties of the material. Unfortunately, translucency increased only slightly with this process.8
A more dramatic increase in translucency was achieved when manufacturers used a higher percentage of yttria to stabilize zirconia containing a significant cubic crystalline phase interspersed with the tetragonal phase, resulting in 10 to 15% cubic-phase zirconia.8 The specific formulations as well as shading technologies to incorporate elements such as fluorescence are proprietary to the manufacturers. Much of this innovation has occurred at the powder stage, before discs or blocks are created. Tosoh Corporation claims to have more than 95% of the world's market share for zirconia powder used in dental applications, while some companies such as Sagemax and Kuraray Noritake source some or all of their own zirconia powder.
“We have innovated three translucent materials since the days of the framework-only business, as the market requested more areas of the mouth to be served,” says Jay Thomas, product manager for Tosoh. “These materials were specially engineered from scratch with characteristics that differentiated them from the powders that were in existence at the time. In 2010, Tosoh Zirconia powder grade Zpex® was introduced which revolutionized zirconia for dental use providing the combination of higher translucency, superior strength, and hydrothermal aging resistance. In 2014, Zpex Smile® was introduced with improved characteristics to serve anterior applications, and in 2015, Zpex® 4 was introduced, providing properties midway between Zpex and Zpex Smile. All have been well received in the market.”
While esthetics measurements are used less frequently than strength numbers in marketing materials, manufacturers use several methods to quantify these properties.
“There is no single universally accepted method or set of test guidelines that will be able to quantify all of these attributes together, but common methods are percent transmission (percent translucency), translucency parameter (TP), contrast ratio, and opacity,” says Akash Akash, Vice President of Research & Development for Glidewell Laboratories. “Esthetics are affected by several factors, such as shade match, value, translucency, gradient in translucency, gradient in chroma/color, etc. The relative importance of these is subjective.”
For translucency, the newer zirconias can rival the 44% provided by lithium disilicate, according to Kuraray Noritake, which tests 0.5-mm samples with a light wavelength of 700 nm. Kuraray Noritake’s Katana line features several levels of translucency: UTML (43%; statistically equivalent to lithium disilicate), STML (38%), and ML and HT (both 31%). The translucency levels of older generations of zirconia are significantly lower.
“Very few dental restorations would ever require more than 43% translucency, so now we are striving for more strength,” says Manabu Suzuki, Manager of Kuraray Noritake’s Overseas Sales Department.
Physical properties are an issue again because cubic zirconia offers lower flexural strength and fracture toughness than tetragonal zirconia. These new generations of zirconia display flexural strength in the general range of 600 to 800 MPa, which is still higher than most other restorative materials, but significantly lower than traditional zirconia. 9
“With the high-translucent zirconia that contains a formulation of the cubic form, the materials are not nearly as strong,” says Gregg Helvey, DDS, CDT, MAGD, Adjunct Associate Professor at Virginia Commonwealth University School of Dentistry. “It does not undergo transformation toughening like that of tetragonal zirconia. When a crack starts, tetragonal zirconia can squeeze together and stop the crack; cubic zirconia does not have that capability, which is why it has lower strength numbers.”
TRAC Research found that 2% of the early high-translucency zirconias were experiencing breakage during cementation.
“When pressure was placed on the material during the cementation procedure, the crown broke. We published this information right away,” says Christensen, who with her husband, Gordon, co-founded and then directed CRA (now called Clinicians’ Report) in 1976. “The study results indicated that enough yttria had been added to compromise the transformation toughening of the material, almost like a contaminant in the product. That has since been corrected, as companies are adding slightly less yttria. The property of transformation toughening is the advantage that zirconia has over any other existing dental ceramic, so we cannot lose that in the race for translucency.”
In other testing, Lawson and colleague Edward A. McLaren, DDS, MDC, found that zirconia containing cubic elements exhibited flexural strength “intermediate to lithium disilicate and traditional zirconia.” They found that sandblasting the inside of bars of this high-translucency zirconia significantly reduced its strength, but that sandblasting the inside of crowns and bonding them to replica teeth did not weaken them.
“As long as you bond it, sandblasting the inside of the crown should not weaken the zirconia,” Lawson says. “However, if the dentist plans to use a regular cement, such as a resin-modified glass ionomer, then we recommend asking the laboratory not to sandblast the inside of the crown with alumina.”
The exact role of resin bonding as well as the minimum thickness requirements are currently under investigation, according to Markus B. Blatz, DMD, PhD, Chair and Professor of Restorative Dentistry at the University of Pennsylvania’s School of Dental Medicine.
“Resin bonding strengthens and reinforces ceramic materials, but is not needed for conventional high-strength zirconia with adequate thickness. For the lower-strength, translucent zirconia, however, resin bonding may be critical to avoid fractures,” says Blatz, who has studied zirconia bonding for almost 2 decades and introduced the APC zirconia-bonding concept, which includes air-particle abrasion, zirconia primer, and adhesive composite resin.10
Using a dedicated zirconia primer can help with bonding, Weston adds.
“The bond strength of zirconia to tooth structure does not match that of porcelain,” he says, “but using a zirconia primer on the intaglio surface of the restoration, followed by a self-etch universal resin-bonding system placed inside the restoration and on the tooth, allows you to seat restorations with a greater reliability of retention.”
The coloring of these materials has evolved as well. Even high-translucency zirconia initially required staining and glazing to meet the esthetic requirements of the anterior region. Recently, however, several manufacturers have developed pre-shaded and multilayered discs.
“Zirconia was white as chalk, and it was necessary to dip it in stains,” Helvey says. “If the dentist grinds a little too far through it, the color reverts to chalk white. Now, with some of these products, the pre-sintered materials are already stained, and CAM software makes it possible to move restorations within the discs or blocks to achieve the desired shade. If it is necessary to make adjustments, that color is uniform throughout the block.”
The esthetic results that can be achieved from the gradient materials have impressed even high-end technicians.
“The pre-shaded and multilayered zirconias are incredibly beautiful,” Culp says. “They give us flexural strength, surface hardness, and esthetics. We are fabricating anterior teeth and full-mouth rehabilitations with these new materials.”
The pre-shaded and multilayered zirconias also make the laboratory process more efficient and streamlined, as the stain-and-glaze step can be eliminated.
“Dentists and dental technicians expect the highest esthetics using CAD/CAM,” Suzuki says. “They also want workflow efficiencies, color consistency, and reliable quality.”
Necessary Knowledge
As simple as it may seem, modern dental zirconia has many complexities that are important to understand.
“We have worked with zirconia quite successfully for almost 20 years, but with all the new brands, products, and formulations, I worry that many people think zirconia is all the same, which is not the case,” Blatz says.
The first consideration for anyone involved in fabricating dental restorations is to be confident in the sources of the material. Impurities in the powder, different compositions, uneven homogeneity in a block or disc, and improper sintering in the laboratory are among the factors that can result in restorative failures or worse.11
“I recommend using reputable manufacturers that have shown in their research and over time that they have superior materials,” Blatz says. “Zirconia has become extremely popular, but few realize that it is a very delicate material. For example, if the process of creating the zirconia discs is faulty or the sintering furnace is not calibrated well, the resulting material will be very different than what you expect, with less strength and possibly other different properties. I suggest avoiding cheap materials and manufacturing equipment. Also, make sure that all components of the manufacturing process—scanners, design software, milling machines, and sintering furnaces—communicate and work flawlessly. Failing to recognize that each of these factors plays a critical role in the quality of the final product may lead to unnecessary fractures, especially for high-translucent zircona.”
Sagemax and Tosoh both say they test every batch of powder for radioactivity. In ADA 69 (ISO 6872:2015),the American Dental Association specifies that the limit for radioactivity in ceramics used in dentistry must be <1.0 Bq.g-1 of U238, according to 3M Oral Care, whose Lava Esthetic meets these requirements.
“Some companies offer lower prices, but ultimately, if you do not know exactly where your products come from, you cannot guarantee a level of quality,” Kosty says. “Everything that comes from the earth has a certain level of radioactivity, so we have always felt the need to test for that and the chemical composition. We need to ensure that we are reducing risk for the patients and subsequently the dentists and technicians as much as possible.”
Proper sintering according to the manufacturer’s instructions and proper furnace maintenance are also necessary.
“Small deviations in firing temperature can affect not only translucency but other physical properties as well,” Lawson says.
Knowing the material’s limitations is important. For example, Blatz is currently studying the impact of cementation when the thickness of the restoration is below the recommended 1 to 1.5 mm.
“I have seen people who think they can use minimal thickness in the range of 0.4 to 0.6 mm, which may be possible for conventional zirconia, but probably is not for lower-strength high-translucency material,” Blatz says. “I am worried about clinicians stretching the indications too far, thinking this material can do anything. It is a great material, but it is still a brittle ceramic with physical limitations.”
While no-prep veneers are currently very popular among patients, as many do not want the dentist to grind their teeth, Weston says, zirconia simply is often not a viable solution for this type of restoration.
“The bond strength of zirconia does not compare well to that of lithium disilicate, so for partial coverage in the anterior region, zirconia will not work very well and the esthetics can be challenging,” says Weston, who adds that pressing porcelain to the internal surface on zirconia Maryland-style bridges provides a much better bond strength. “If the manufacturers can figure out ways to improve the bond strength, then we could utilize zirconia for some partial-coverage situations. Currently, however, we see failures when preparation retention is minimal, and one relies purely on bonding the zirconia. Using this material for veneers just does not work very well.”
Indeed, the same physical properties are not ideal for every indication. The International Standards Organization issues standards for different classes of restorations, and manufacturers generally provide recommendations.
“Following the indications, contraindications, and guidelines when selecting the bridge designs is important for dentists and laboratories,” Akash says.
Still, opinions vary regarding specific properties that are ideal for each situation, so it is important to be educated on the subject. For example, Weston says he does not routinely use 1200-MPa zirconia in the posterior region because the strength and fracture toughness make it difficult to manage if adjustments, removal, or endodontic therapy need to be done.
“When you cut into high-strength zirconia, you can actually see sparks fly in the mouth,” says Weston, who does use traditional high-strength zirconia for implant substructures and bridge frameworks where strength is paramount. “I do not know that you need to go above the 600 to 800 MPa range in the posterior; I have never seen something at that level break, and it is much easier to manage.”
Meanwhile, translucency is not necessarily beneficial in all situations.
“There is still a place for conventional zirconia with higher strength and opacity—for example, to block out a discolored tooth, a titanium abutment, etc,” Blatz says. “And, of course, for implant components and frameworks, strength is all-important.”
In most situations, however, the esthetics of zirconia still can be improved. Culp says he hopes dentists and technicians encourage manufacturers to continue to push the envelope in this area.
“Many dentists and technicians today are really throwing out everything we have achieved esthetically over the past 20 years just to offer a low-cost option to the patient that theoretically should never break,” Culp says. “Economic factors have driven the growth of zirconia, and then afterward we started to make it prettier.”
While manufacturers do utilize measurements such as translucency percentage, esthetics also must pass the eye test. Christensen says TRAC Research utilizes ratings from patients and dentists in its research.
“The patient’s eye will decide if it is esthetic or not, based on the way it blends in the oral cavity with the surrounding dentition,” Christensen says. “We need to be careful because in science we are always looking to quantify rather than rely on opinions, but when you judge the clinical esthetics of a restoration, the patient’s perception is the most important factor.”
Indeed, Weston says looking at fluorescence and translucency alone may not be sufficient.
“We need to get the material in the mouth surrounded by lips and soft tissue,” he says. “Then we can see how it reacts to different light sources and evaluate its use for high-end cosmetic cases.”
Christensen notes that the next major hurdle for manufacturers after translucency and pre-shading/multilayering is mastering color.
“Zirconia does not color easily,” Christensen says. “Internationally, there is no zirconia that we have seen that matches the VITA Master Shade Guide, although several are making a good attempt at matching the VITA Classical Shade Guide. We are still struggling with color, and I do not think dentists understand that they may be altering the physical properties if they alter the color beyond just surface paint, which does not have longevity.”
What’s Next?
Better coloring is not the only area in which zirconia could be improved in the near future. As far as the material has come in the past decade, dentists and laboratories have their eyes on several other developments they would like to see next.
“‘Perfect’ zirconia with top-notch esthetics and 1000-plus MPa will likely be a reality in the future,” Akash says.
Esthetics likely will continue to develop beyond simply achieving optimal translucency. For example, multilayer materials are still being perfected.
“I find pre-shaded, multilayer, high-translucent zirconia materials most interesting for a variety of monolithic restorations,” Blatz says. “There are gradual shade and translucency differences between the dentin and the enamel layers to provide improved esthetics. I am convinced that we will see further improvements in this area.”
Thomas says Tosoh anticipates chairside zirconia becoming increasingly popular as manufacturers continue to develop “speed sintering” and presintered blocks and discs. Glidewell Laboratories and Dentsply Sirona already offer presintered or partially sintered zirconia for chairside milling applications.
Of course, the most significant development almost certainly would be the ability to print zirconia restorations on a 3D printer. Once it is more advanced and affordable, 3D printing technology offers the promise of higher precision and minimal material waste, as well as the potential for printing multiple colors and even materials together.
“Everyone is trying to strive for a material and process that looks very natural with the highest strength and translucency that dentists and patients want along with productivity for the laboratory,” Buchanan says. “Over time, medium- and long-term, it could be that they will be printed rather than milled.”
Printing color could revolutionize shade matching, bringing digital precision to another part of the process.
“We hope to print zirconia in less than 10 years,” Suzuki says. “The dream is to be able to scan shades intraorally and print a colored zirconia that can be sintered very quickly.”
Multiple manufacturers, however, were more skeptical regarding how soon this technology will be ready.
“Demand for Tosoh zirconia to be utilized in 3D printing has not yet presented itself,” Thomas says. “Dense zirconia parts are normally formed by pressing or other forming methods. That is how it begins to densify, and it reaches its ultimate density after sintering. We know companies are testing zirconia for 3D printing; however, commercialization seems far off.”
Whether milled or printed, it is a near certainty that improved formulations will continue to offer more options to dentists and technicians during the next several years.
“The carrot is out in front of all of these manufacturers,” Helvey says. “Nobody will reach a point and say, ‘OK, that is as far as we can go.’ Producing a better aging and crack-resistant material will drive the market.”
Because different indications call for different characteristics, it is unlikely that there ever will be one perfect zirconia, but what about a perfect line of zirconia?
“I think we will attain a near-perfect array of zirconia options,” Christensen says. “I do not know if anything is ever perfect, but there are enough very bright people whose minds are on this to attain what is needed. It is very important to understand that we are on the upward side of the curve. We have not hit perfection yet and leveled out. We are still grinding up the hill. We will solve some of the problems, but it is important to understand that we are still at a stage when a significant amount is unknown. And for researchers, it is a race to learn as quickly as possible.”
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