Demystifying the Bonding of Zirconia

Inside Dentistry (ID): We have heard from dentists that "you can't bond zirconia." What is your current thinking regarding zirconia bonding?

Markus B. Blatz, DMD, PhD (MB): The discussion of cementation versus bonding for full-coverage restorations is dependent on the specific material used and the application. If a full-coverage restoration is made with conventional zirconia, then it should be able to be cemented very easily and still have the fracture strength that is needed. However, if we'd like to improve the fracture strength or if we have a resin-bonded restoration, such as a resin-bonded bridge, then bonding is required. It all depends on the inherent strength of the material.

Gary Alex, DMD (GA): A misconception held by many dentists is that "you cannot bond to zirconia."  The fact is that you can bond very predictably and durably to zirconia surfaces using a combination of sandblasting, a phosphate ester primer such as MDP (10-methacryloyloxydecyl dihydrogen phosphate), and an appropriate resin-based cement. As an example, I have a number of zirconia single- and double-winged resin-bonded bridges, in minimally retentive preparations, with no de-bonds after 10 years of clinical service. Matthias Kern, DMD, PhD, has published extensively on this subject.

Nathaniel Lawson, DMD, PhD (NL): I agree with Markus and Gary that you don't need to bond every zirconia restoration, but when you do need to bond zirconia, it is definitely possible. There seems to be a chasm between the academic world and the scientific world on this topic. There have been three systematic reviews and meta-analyses showing that you can bond to zirconia, but if you ask the private-practicing dentist, you'll hear different opinions. Recently, I conducted a social media poll, and of about 200 responding dentists, 40% said that they "didn't believe in a bond to zirconia." Some clinicians don't believe that you can get the required surface texture, but if you look at the scanning electron microscope images, it becomes apparent that you can get about a 1-μm scale surface texture from sandblasting. As Markus said, you don't need to do it for every restoration, but I think that when you need to do it, it is possible.

(ID): How strong of a bond is strong enough?

(MB): That's a question that we have been wrestling with for a long time in adhesive dentistry. People talk about specific values, such as 35 MPa or 34 MPa; however, we have to understand that bond strength is also dependent on the clinician's familiarity with the material. Different clinicians using the same bonding agent can get different bond strengths. In addition, it's not just about the material properties alone but also on which side of the bonding (ie, the tooth interface or the material interface) do we have the weaker bond? I want to achieve high strength on both sides. But, how many megapascals should these bonds be able to withstand? Well, we can say that for dentin, we're looking at about a minimum of 15 MPa if this is what we're trying to achieve. For enamel, we're talking about more like 30 to 35 MPa. But then, you also have different testing methodologies that can potentially give you different results, and people know that. Sometimes, just by changing the testing method, you can see higher bond strengths with the same materials. Given all of these factors, it's very difficult to set one bar. What we're trying to do and what I always say is, "Let's not focus on the numbers. Let's focus on getting the best bond strength possible."

(GA): I agree with Marcus. Bond strength values can vary significantly depending on the testing methodology. Some techniques generate much higher numbers than others. Many years ago, Munksgaard and Retief (among others) suggested that 17 to 20 MPa (using a shear testing methodology) was the minimum bond strength required to dentin in order to counteract the polymerization shrinkage stress of directly placed composites. It was believed that bond strength values in this range were required to produce gap free restorations. In cases of full-coverage restorations with retentive preparations, high bond strength is certainly not as critical as it would be in cases with non-retentive preparations.

(ID): What can be done to combat phosphate contamination when bonding zirconia?

(NL): There are a couple of methods that have been used to clean phospholipids off of zirconia so that MDP molecules can go and bind to it. One of them is to re-sandblast the crown. Another option is to use a cleaning solution like ZirClean^® (BISCO Dental Products). These solutions contain super-saturated concentrations of zirconia, so all of those phospholipids that like to hang out on zirconia restorations then want to go hang out on the zirconia in these solutions. Then you rinse off the crown to get rid of those contaminants. One additional but less-researched method for cleaning zirconia is just to use sodium hypochlorite or bleach to clean out the inside of the crown. There's some research out now showing that you can clean out zirconia crowns just using bleach, but it's probably not as much as has been done on the use of cleaning solutions or re-sandblasting.

(GA): Zirconia has a remarkable affinity for phosphate ions. This affinity extends not only to the phosphate groups in zirconia primers but also to the phosphate groups and ions that are inherent in saliva. When zirconia restorations are tried-in and the intaglio surface is contaminated by saliva, the phosphate ions from the saliva bind to and occupy the same reactive sites that zirconia primers require for chemical interactions. This competition for reaction sites significantly decreases the efficacy of zirconia primers; therefore, it is necessary to "free up" these sites so zirconia primers can function optimally. This can be accomplished by sandblasting the restoration after saliva contamination and/or using strongly alkaline cleaning solutions, such as ZirClean.

(ID): What new applications for zirconia do you think will come next?

(NL): When we first started researching zirconia, we would do studies pertaining to single-unit crowns and sometimes a 3-unit fixed partial denture. Now, we'll see full-arch prostheses made out of zirconia. Markus and I are working with a colleague at LSU on a study that's looking at what kind of wear the zirconia full-arch prosthesis is going to have on opposing denture teeth. Also, with translucent zirconia, we have esthetic anterior applications. Around 8 years ago, esthetic zirconia would've sounded like an oxymoron, but now, we have zirconia materials that are more translucent. I think that from our end on the research side, we're going to stay busy looking at these kinds of properties of zirconia.

(GA): I think that where we're going to see more zirconia in the future is in the field of dental implants. Many dental implant companies already offer zirconia implants as an alternative to titanium implants. Zirconia has physical and esthetic properties that make it interesting to consider as a replacement for titanium in certain situations. I've also seen a couple of studies that show a superior tissue response around zirconia implants when compared with titanium implants. Having said all of this, zirconia implants clearly do not have the long-term clinical record of titanium implants, and more clinical studies are required before they can or should be unequivocally endorsed.

(MB): Yes, that's actually one of the topics that we're looking at very closely. I believe that, like other things with zirconia, we didn't understand the material properly, so we used it the wrong way. We didn't understand modulus of elasticity, we didn't understand load, we didn't understand the electric load of these materials, and now we understand them better. I think there's a place for zirconia implants, but we have to learn from the past and come up with new designs and different types of zirconia than what they are using right now for zirconia implants.

As for the future, we're heading toward printed zirconia. Printing is very limited at this point in time as far as accuracy is concerned and regarding the material selection that we have available. We are currently working on solutions to print ceramics and build up a restoration from the inside, which will allow us to mimic much more closely the optical properties of the natural tooth and maybe even the physical properties.

Gary Alex, DMD, maintains a  private practicein Huntington, New York.

Markus B. Blatz, DMD, PhD, is a professor of restorative dentistry in the Department of Preventive and Restorative Sciences at the University of Pennsylvania School of Dental Medicine in Philadelphia, Pennsylvania.

Nathaniel Lawson, DMD, PhD, is the director of the Division of Biomaterials at the University of Alabama at Birmingham School of Dentistry in Birmingham, Alabama.

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