Bonding to Zirconia

Markus Blatz, DMD, PhD (MB): In my world of restorative dentistry, prosthodontics, and adhesive dentistry, this is one of the biggest misconceptions that I have ever come across, and I struggle to understand why it exists. I started studying bonding to zirconia in the early 2000s, and although we continue to learn how to optimize this bond as new materials come to the market, the phrase, "You cannot bond to zirconia" is simply not true. Of course, there is a big difference between bonding to glass-based ceramics and bonding to high-strength ceramics because glass-based ceramics are etchable, which provides a very high bond strength. There are several indications for which I prefer to use a glass-based ceramic material, such as for laminate veneers and partial-coverage inlays and onlays. Nonetheless, that does not mean that high-strength ceramics are not bondable at all. Remember, with the proper protocol, we can even bond to metal; Maryland bridges have been utilized for nearly 50 years.¹

ID: Is there clinical evidence that bonding to zirconia provides sufficient strength in the long term?

MB: I frequently refer people to the many systematic reviews and clinical studies that are available on resin-bonded zirconia bridges. Many involve Matthias Kern^2-4 or me,^5-7 but there are a number of others as well.^8-10 The 15-year success rate for single-wing resin-bonded zirconia bridges is higher than 90%.² These restorations rely exclusively on bonding to tooth structure because the preparations do not have any retentive elements. Therefore, if you could not bond to zirconia, they would all fall out. Bonding to zirconia works, but only if it is done correctly.

ID: Beyond bridges, what other indications are there for bonding zirconia?

MB: The more recent high-translucency zirconia material options have significantly widened the range of indications, which now includes resin-bonded laminate veneers as well as inlays and onlays. This provides the clinician and dental technician with additional options, but it should be noted that long-term clinical studies on these are still sparse. A great advantage of zirconia is that full-coverage restorations with adequate thickness that placed on preparations that offer good retention do not require bonding; conventional cementation is absolutely sufficient. However, keep in mind that resin bonding also increases the fracture strength of ceramic materials and may be indicated for thinner restorations.

ID: What are your clinical recommendations to facilitate or improve bonding to zirconia?

MB: We have researched this topic for more than 20 years, and I have examined tens of thousands of specimens with regard to a number of variables. Currently, the best way to achieve durable long-term bond strength is the APC concept, which we published in 2016¹¹; however, new developments could change this. The acronym represents the three steps necessary for a successful bond to zirconia: airborne-particle abrasion, priming with a zirconia primer, and composite resin adhesive. As with any type of indirect restoration, this protocol should be preceded by 2 to 5 minutes of ultrasonic cleaning in alcohol following intraoral try-in. After the restoration has been ultrasonically cleaned, the first step of the APC concept involves performing airborne-particle abrasion on the intaglio surface. That is followed by the application of a zirconia primer with the ability to chemically bond to non-silica-based, high-strength ceramics. Silane does not help with bonding to zirconia; however, some primers available today have both silane and a special phosphate monomer that can chemically bond to metal oxides, of which zirconia is one. The most well-known of these monomers is MDP. According to a recent study, applying the primer as soon after air abrasion as possible is best, because although air abrasion decontaminates the surface, it can become recontaminated.¹² We have also found that, during air abrasion, some of the alumina particles become embedded into the zirconia surface and some are spread over the surface, which creates additional bonding partners for the MDP or a similar phosphate monomer. Therefore, the sooner that the primer can be applied, the better. The final step of the APC concept is the addition of a composite resin luting agent. New self-adhesive resin cements may soon eliminate the need for the separate primer step. Regarding the bond on the tooth side of the restoration, the strategy used should depend on the specific case. If the restoration is bonded to enamel, as it should be for resin-bonded bridges, we prefer a total-etch technique; however, for a bonding agent selected for a specific situation, it is important to follow the manufacturer's instructions.

ID: How technique-sensitive is this process?

MB: Well, it does require multiple steps, but it is probably less—or at least no more—technique-sensitive than bonding to silica-based ceramics. Technique sensitivity should not be a deterrent to the performance of proper adhesive dentistry or dentistry in general. The easiest method for us is not always the best method for the patient. Dedicating the extra time and effort could allow our patients to keep their teeth longer, which is our core duty, so we owe it to them to educate ourselves and practice new protocols that enhance clinical outcomes.

ID: How useful are zirconia cleaning agents?

MB: They can work quite well. They are designed to clean contaminated surfaces. Therefore, if you use an ultrasonic cleaner and the APC concept, they should not be necessary. However, if you have no ultrasonic cleaner or airborne-particle abrasion capabilities, they can work very well to decontaminate restorations. Some of them can even be used to clean the abutment teeth before cementation or bonding.

ID: How does zirconia compare with other materials regarding the risks of failure?

MB: Full-coverage zirconia restorations have success rates similar to other materials.⁵ Resin-bonded zirconia bridges are typically utilized when implants are not ideal, such as in younger patients who are still growing. If a minimally invasive approach is utilized, the worst-case scenario is that the restoration falls out and needs to be bonded in again. That risk is minor when compared with the risk of an implant or conventional bridge failing. If a resin-bonded zirconia bridge does fail, you can always resort to a more invasive treatment option if necessary. However, once you become more invasive, you cannot go back and do partial coverage. That is a mindset that I hope more dentists consider in their treatment planning.

ID: What factors need to be considered when selecting products to bond to zirconia?

MB: When selecting products, it is important to ensure that the manufacturers have identified them as the proper materials for the given indication. Read the manufacturers' instructions; it is very simple. Of course, we know that some special phosphate monomers, such as MDP, can bond extremely well to zirconia, so these are key ingredients to look for. We all have our own preferences, but there are now many excellent products on the market.