Zirconia from A to Z

What do Superman and zirconia have in common? If you said strength (both are unquestionably strong), esthetics (they both look great—even disguised as Clark Kent, Superman is still a dreamboat), and affordability (sure, this one is a little bit of a stretch, but Superman provided his services free of charge), then you are right.

Of course, claiming either as “the best” in their respective fields is somewhat subjective, but it’s getting harder and harder to argue that opinion, especially where zirconia is concerned (Batman fans would likely disagree as to whether Superman is the best superhero).

There’s a lot to know about zirconia, and dental lab professionals need to understand how this material helps them, doctors, and patients.

History

Zirconia is a fairly new material used in dentistry (only in the last 20 years or so) and is usable only in the CAD/CAM workflow, supplanting porcelain-fused-to-metal (PFM) restorations.

“In the early 1990s, the Nobel Company started producing the world's first sinterable dental ceramic produced by CAD/CAM, a material called alumina,” says Ben Godfrey, application engineer at Axsys Dental Solutions. “But, due to difficulty of use and lack of longevity, this material sprung up and then died just as quickly, and the market needed a new material. On the heels of those new cylindrical dental implants, [Per-Ingvar] Brånemarklicensed and partnered with Nobel to produce Procera brand zirconia. The entry price tag was quite high, as the new digital process required investment in a probe-based scanner that touched the die as the die spun—leading to many margin breaks as the probe rode along any undercut and tended to easily break thin edges on stone dies.

“The system was intended to scan only stone dies and produced only thimble copings until software updates in the early 2000s,” he continues. “It wasn’t until the mid-2000s that the full-contour zirconia market got its legs, owing to failed porcelain-to-zirc in years previous. Around about ’08 to ’09, when proper liner agent was developed for layered zirc, the market for monolithic zirconia claimed a lot of ground from PFM and gold.”

Nathaniel Lawson, DMD, director of the Division of Biomaterials at the University of Alabama at Birmingham School of Dentistry, remembers when zirconia made it big. “When it first started, it was just the framework as zirconia,” he says. “So, it was something that they were using just for its strength. And then they had to put porcelain over it to impart enough translucency to make it look natural in the mouth. But when the zirconia revolution started was when they started making zirconia that could be monolithic, so it didn’t have to have any veneering porcelain over it.”

Claudia Fernandez, a lab technician for several decades, and currently a territory sales manager at Smart Dentistry Solutions has been working with zirconia for many years and recalls the early struggles with debonding issues, and the lack of esthetics relegating the material to posterior use. But with better complementary materials and workflows, those challenges are issues of the past, and today zirconia is well suited to play a role in almost any type of restoration.

“Now it has evolved to being more aesthetic and more translucent,” she says. That's thanks to all the technology that now used in manufacturing zirconia.”

Comparison

Zirconia gained traction in dental restorations because of its strength, wear-resistance, and biocompatibility. When compared with older analog materials, like PFMs, or even newer materials, like Ivoclar Vivadent’s IPS e.max, zirconia has clear advantages.

Lawson looks to a study conducted by the Journal of the American Dental Association revealing 277 American Dental Association Clinical Evaluators’ attitudes and opinions towards zirconia.

“The biggest thing was that 57% of people said that zirconia’s biggest advantage was its strength,” Lawson says. “And then 14% of people said its [advantage is] versatility. What we meant by that was, ‘Can [zirconia] be used for a whole bunch?’ It can be used as a crown; you could use it for a hybrid prosthesis, meaning you could replace an entire arch with it; you can use it for an implant abutment; you can use it for a lot of different applications.

“The third was wear resistance because it doesn’t really wear away over time,” he continues. “And then the fourth was cost—and I’m surprised that wasn’t higher—because zirconia really has reduced laboratory costs for crowns, just because you can manufacture them relatively inexpensively.”

According to David Sipperly, sales training manager at Dentsply Sirona, a lack of metal components means that esthetics are improved, and biocompatibility is better.

“All-ceramic restorations typically have better esthetics than PFMs, because you’ve got a metal substructure and gray metal substructure base, particularly when you have tissue recession, you can see that gray line or a metal halo in a patient’s mouth,” he says. “Or you can see a gingiva tattoo that some people would have because of the metal. And some people have sensitivity to metal, so all-ceramic or metal-free is the way to go from a biocompatibility and esthetics perspective. Now that being said, PFMs were the norm 15 years ago. It was 80% of the business, [with] 20% all-ceramic. Today, it’s completely reversed. All-ceramic restorations are 70% to 80% of the and PFMs are 20% to 30%.”

Zirconia’s composition makes it not only esthetic but also strong.

“One of the benefits zirconia versus PFM is you have no metal showing,” says Raffi Hatzakortzian, co-owner of Alien Milling Technologies. “If the patient’s gums do reduce over time, you don’t have to worry about that metal showing. The second advantage would be that it’s actually stronger than PFM, so you do have less chance of breaking.”

But it isn’t just the patient who benefits from a zirconia restoration. Dental lab professionals realize the advantages of using the material.

“The first big one is labor-saving,” Godfrey says. “With the advent of zirconia produced via CNC, you now need only click the mouse to do your design digitally, and as the project is on the screen, you can click buttons and boxes to make things appear, disappear, or adapt. For example, with a click, you can adapt your crown to the contacts perfectly. Secondly, in terms of material selection, zirconia today has dozens of varieties allowing for unprecedented flexibility in adapting to shades. There are hundreds of brands of zirconia on the market, and each can come in several varieties of strength and translucency characteristics. We’ve come a million miles from dead white copings 30 years ago. It can be a daunting task finding a zirconia that works best for your case, but as many people are finding out, once you know what you need from your workflow, it’s a pretty simple process.”

Esthetics

According to Elke Kopp, new procedure marketer at 3M, form and function are the most critical qualities of dental restorations. On the form front, zirconia has an edge.

“The big advantages of zirconia are that they are highly biocompatible,” she says. “They are metal-free; they have a toothlike color; they have a lower minimal wall thickness than e.max, and minimally invasive restorations are possible. They can be dry milled, which is an effective manufacturing process; they have a flexible use depending on needs and are easy to cement. There is no discoloration in the mouth; and low wear. Although veneered zirconia is the golden standard in esthetics, monolithic zirconia is reaching high levels of esthetics.”

A lot of this is, of course, subjective, and it seems to vary depending on who you talk to and their role.

“This is a topic that always stirs debate,” Godfrey observes. “Esthetics are one way in the eyes of the lab [professional], another in the eyes of the dentist, and another in the eyes of the patient. If you want to have an objective discussion, you have to resort to talking about zirconia in an ‘on-paper’ sense. Zirconia can be produced as white or preshaded, and it’s often that lab professionals go for preshaded zirconia because it accelerates the workflow and eliminates the white spots that you’d see as a clinician making small adjustments to the crown at time of seating. “Therefore, if you grind into a preshaded A3 crown, you will never see white spots—the color goes through the whole material,” he continues. “There are a great many methods of achieving finer detail of stain, from pre-sinter liquid application, to glaze kits that transform the look of any zirconia according to the skill of the artist. The ceramists don’t need to fear losing their jobs to automation…yet. Many technicians today don’t put the same effort into zirconia as e.max by virtue of the fact that the 2 materials start from a different esthetic point—e.max has qualities in it that make it easier to achieve the perfect shade without much effort, and almost all technicians prefer to put porcelain on a crown to achieve a quality shade.”

A more recent advance in the esthetics possible with zirconia restorations is the development of multilayered zirconia discs that combine the strength of zirconia traditionally used in the posterior region with the enhanced translucency of zirconia designed for use in the anterior. Fernandez says these materials allow labs to use zirconia without compromising strength for translucency or vise versa, and the manufacturers of these materials are getting better and better at creating smooth transitions between the layers.

“It means that it goes from a veneer as thin as a veneer can be to inlays, onlays, and all the way to a 14-unit, implant supported structure or custom implant abutment structure,” she says.

Ease of Use

Ultimately, the goal is the best-looking, best-performing restoration possible. However, zirconia affords lab professionals the opportunity to easily and consistently provide restorations. That is due largely to the benefits of CAD/CAM and the digital workflow.

“Zirconia is very easy to use,” Hatzakortzian says. “Then the greatest advantage you’re going to have with zirconia is that everything is done digitally, so you don’t have any more casting or waxing or anything like that. If a patient breaks the restoration or it gets lost, it can be remade because milling centers usually keep the files for up to 10 years. So anytime between that, if something does happen, we could just go in and remill it instead of the doctor bringing the patient back in and taking an impression.”

“Zirconia is easy to work with when you control the workflow and know what you want to see as a result or product of your labors,” Godfrey adds. “Every lab has its own workflow, and 2 labs produce different results, even [when] given the same zirconia. It’s the technicians in the labs that make the result work, not the zirconia itself. It should be said that every zirconia will have a different way of absorbing stains, blocking out dark stumps, or generally allowing light penetration, based on its composition. Scientifically, the majority of zirconia out there is stabilized with yttria, but different things happen when you get one that is stabilized at 3% versus 5%, and quite another thing when stabilized with calcium or erbium. Those are not quite as common but are out there still, and they produce a different result and require a different amount of work to get to the correct value/chroma. A good technician should be able to see and compensate for these things, but many dismiss these zirconia as not being worth the effort because of the amount of time it would take to correct the workflow.”

Further, according to Kopp, precolored zirconia makes the process faster and easier.

“Restoration making is established and easy, and a very good fit can be reached due to dry milling,” she says. “With the introduction of precolored zirconia material, it is easier to get restorations with good color match.”

Using a shaded zirconia not only makes it easier to achieve the desired esthetics, but it can also help create a more durable restoration, Fernandez says. While some workflows call for applying stains to the zirconia before sintering the material, Fernandez says any moisture, including that from stains and glazes can, have negative impacts on unsintered zirconia which is why zirconia is milled dry.

“We see that a lot of zirconias out there, especially monolithics, they have to use liquid stains between milling and sintering,” she says. “We don't recommend it. There is no need for it (with pre-shaded multilayer zirconia). We know it is easily stained after sintering without having to add any moisture to the zirconia. We've seen that that is something that could bring some issues.”

Disadvantages

But, zirconia isn’t perfect. According to professionals, for as good as it is, there is still room for improvement including necessary inventory efforts and high cost.

“Zirconia requires high-temperature furnaces (~1500 °C), and attention needs to be paid to maintaining the furnace and using the correct sintering temperature,” Kopp says. “Saliva contaminations can reduce cementations’ strength if not properly cleaned after try-in. Sandblasting is recommended. It has a lower translucency level than glass ceramics, and properly cemented restorations stick very strongly to the tooth. In case of a clinical need to remove the restoration, some effort is required.”

Zirconia is very strong, but only after it is sintered. Before it spends time in the sintering oven, it is very fragile.

Godfrey sees the fragility and cost of zirconia as a bit of a drawback.

“The material is quite brittle, and the supply cost can be rather high when you are in the business of producing All-on-X (AOX) solutions,” he says. “You must keep inventory in that circumstance, because if it should so happen that your AOX chips or breaks before—or during, heaven forbid—sintering, you need backup discs to keep producing. Lab [professionals who] focus on crowns and bridges generally don’t need to throw multiple discs at a problem. [It] can remill a single unit with relative speed and ease. [This is] not so for an AOX. Crack one of those, and you’ve got to not only add the cost of a new disc but also 1 day of lab time, owing to sintering, milling, and postprocessing. AOX is rightly a costly affair.”

Keeping Costs Low (and Consistent)

Zirconia is relatively low in cost and seems to be getting less expensive every day. This is a benefit to everyone involved—labs have cheaper operating costs, dentists can make more money, and patients don’t have to spend as much.

“It’s actually cheaper now than in the beginning,” Hatzakortzian observes. “It was more expensive, but now, because it’s all done digitally, you actually have the prices going down. With a PFM, it’s very hard to find good lab technicians to bring into the laboratory. So you have a scarce resource when it comes to lab technicians. It’s actually cheaper to fabricate as a zirconia restoration than traditional PFM.”

According to Lawson, Insurance companies are paying more for zirconia restorations than PFMs.

“Reimbursement rates are typically higher for zirconia restorations than they are for metal-based restorations,” he says. “But the laboratory bill is much lower for as the zirconia crown than it is for a PFM crown in most large dental laboratories, just because there’s no metal cost associated with zirconia. I think lab [professionals] have gotten so efficient with making zirconia crowns that [it] can offer them at sometimes outrageously low prices.”

According to Sipperly, there is another benefit on the cost-saving front because zirconia is not at the whim of the metals market; it is a fixed cost.

“Metal varies daily due to the precious metal market,” he adds. “You have nonprecious metals, semiprecious metals, and precious metals. They all have different alloys and gold content. They change on price—with the exception of nonprecious metal—daily. With zirconia, you have a fixed cost. Let’s say that a 98-millimeter disc of good quality zirconia, today, sells for about $120. You get about 20 crowns, on average, out of that. So that’s $6 per unit material cost per crown. Just a couple of weeks ago, gold was $1900 an ounce.”

What’s the Best For the Lab

Zirconia’s properties can be confusing, especially because of how it’s manufactured.

“That’s less well understood,” Lawson says. “Zirconia, essentially, is fabricated from a powder that looks like chalk dust. And then they put it into a mold and compact it into something that looks it’s like an oversized hockey puck, but it has the consistency of sidewalk chalk. The way that they compact it into that hockey puck can affect its properties. If it is compacted only from one direction, called uniaxial pressing, the zirconia may not be as homogenous as if it is compacted from all directions, called isostatic pressing.

“The composition of the powder is usually relatively standardized because there are only so many companies in the world that make powder,” he continues. “And a lot of dental manufacturing companies aren’t making their own powder; they’re buying the powder. But they will typically put some of their own additives in there. For example, many manufacturers will add their unique color additives. They don’t really specify, so it’s hard to know what else they could be adding to the powders, but you wouldn’t expect them to be too compositionally different from each other.”

According to Godfrey, there isn’t just one type of zirconia. Manufacturers offer different formulations based on desired traits, strength and esthetics, and how the material is produced.

“Today, manufacturers of zirconia differ tremendously in product offerings, primarily due to production processes and powders used,” he says. “Zirconia can be hot isostatically pressed and can offer yttria stabilizers or erbium-praseodymium stabilizers for a different effect, in terms of color and translucency. These don’t all sinter the same and vary significantly from offering to offering, [but] these things must be taken into consideration when [lab professionals] choose new products. You’ve got to give it an honest effort. It’s a science, but there is art to this science with variables all over the place. You can increase sintering temps slightly for better translucencies, and you can switch to a microlayering technique to produce a depth and warmth of color.”

Per Hatzakortzian, the restoration’s location has an impact on what type of material to use.

“If you’re looking for anterior zirconia, you got to make sure that the Yttria is pretty much 4 and higher,” he says. “The reason for that is you’re going to get much more translucency, but you’re going to lose the strength. And then anything for posterior the yttria level should be around the 3s, which is pretty much giving you much more strength, but less translucency. Again, in the back, you’re not worried about the translucency or the esthetics too much. You’re actually looking more for functionality than the esthetic.”

How do the lab professionals know the best type, and how much, of zirconia to stock? With so many shades and thickness available, inventory costs can add up quickly. Sipperly advises that most labs only need a couple of different types on hand.

“Obviously, they want to use only 2 or 3 because that’s less inventory dollars,” he says. “There are about 16 VITA shades and 1 or 2 bleach shades that they might need, and they come in all different thicknesses. So, even though there’s 1 diameter, there are 3 or 4 different thicknesses that [labs] may need to carry. The goal is that we want to try to use the least the number of zirconias that we can to do our work, but in reality, they’re going to need 2 or 3, hence the anterior zirconia and the high strength zirconia, there’s even a zirconia for abutments, even higher strength, so it’s all over the board.”

While it’s a good idea to keep multiple zirconia shades on hand, Fernandez says multilayered zirconia options allow dental labs to reduce the types of zirconia they need to keep in stock. By adopting multilayered discs of zirconia, a lab can use the same type of zirconia for any area of the mouth and any types of restoration from thin veneers to full-arch, implant-retained bridges.

“So we have the simplicity of inventory right there,” she adds. “We are not having to choose different levels of translucency nor strength. So at the end it is a type of an artistry without compromise.”

Zirconia Is the Real Deal

For all the things that Superman and zirconia share in common, there is 1 important difference—Although Superman exists in comic book pages, toy aisles, and in the movies, zirconia is real and it makes lives better for lab professionals, dentists, and patients every day.