Resin-Bonded Zirconia Bridges

ID: Knowing that a proper protocol is so critical, what considerations are involved when choosing between a single-wing or double-wing bridge?

MK: In our experience, we have seen that single-retainer resin-bonded bridges work much better. Using two retainers can lead to stress between them. For example, if you replace a lateral incisor with a double-wing pontic, because the canine moves to the lateral direction and the incisor to the anterior direction, the stress between those two retainers can cause the restoration to debond. That is how we were handling these restorations when we started in the 1990s, and we encountered quite a lot of problems. When we noticed that some of these restorations that were transferred into a unilateral bonded restoration behaved very well, we began using mostly single-retainer resin-bonded bridges, and the results have been exceptional.³

ID: What are the important considerations when selecting which cement to use?

MK: Over the past 25 years, it has become clear that we need to utilize two steps to bond to zirconia. The first step is air abrasion and surface activation, and the second step is utilizing a luting agent that contains a phosphate monomer (MDP).² In the 1990s, only one product (PANAVIA^™, Kuraray Noritake) contained MDP. However, after the patent was released, other companies were able to develop luting agents that contained MDP. Now, every cement includes an adherent in its luting system, so the important factor is the MDP. Different products may bond differently, but the key is to utilize a system with MDP in it.

ID: How do you treat the teeth?

MK: We bond to the enamel, which needs to be cleaned with pumice or a cleaning product first. Then we etch it with a phosphoric acid solution of 30% to 40% (usually 35% or 36%) for 30 seconds, and we rinse it. Thoroughly rinsing the acid off is very important; the washing time should be at least 15 seconds. The enamel is then thoroughly dried, and the adhesive system is applied. That is it. Any dentist should know how to bond to enamel, but mistakes can be made, such as not washing long enough to remove all of the acid etching gel remnants from the microporosities. In addition, if dentists etch enamel that has not been properly cleaned, there may be some plaque or calculus, even a thin layer that is unnoticeable, and they will not achieve a good etching pattern.

ID: What preparation design is used? You mentioned earlier that mechanical retention is not necessary.

MK: There is no mechanical retention at all. We did not use any when we started with alumina ceramic 30 years ago, and it was still not necessary when we began using zirconia ceramic 20 years ago. We place a little pinhole at the tubercle, which is to provide a defined position for seating purposes, and then we perform a very small proximal box preparation—a shallow box preparation—within the enamel to strengthen the ceramic in the connector area. Those are the two features. Otherwise, it is just a very shallow veneer preparation in which we remove the superficial enamel, which has less prism or even no prism. Then we are finished. The retainer will always make the tooth slightly thicker, but that is fine. That can be an issue in the occlusal contact zone, in which case we may need to create additional space via orthodontic pretreatment options, but otherwise, removing enamel for the retainer is not necessary.

ID: In the case of a missing lateral incisor, do you prefer a cantilever off of the central incisor or the canine?

MK: I usually prefer the central incisor. Why? Because we have a larger bonding surface and a longer proximal contact area. The canine's triangular shape makes its proximal contact area smaller, and thus, its proximal connecter is shorter. In addition, we would rather not interfere with the canine guidance, and the canine retainer wing might be visible if the patient is standing and someone is sitting or if someone in front of the patient looks from an angulated direction. The retainer wing is almost never seen if it is placed on the central incisor. In rare cases when the central incisor is compromised due to endodontic treatment, trauma, or decay and the canine is healthy, then we would use the canine for retention. I am aware that many dentists believe that the canine is preferable because it is more stable, among other reasons, but conversely, I believe that the central incisor is preferable because the incisors move in the same direction during functional movement of the mandible.

ID: What type of zirconia works best? Can you layer over the top of it?

MK: The scientific evidence is only clear for 3Y zirconia. We started with 3Y because nothing else was available during the first generation. The second generation was more translucent but was still 3Y zirconia. When the third generation, which was 5Y zirconia, came to the market, it was less than optimal because the strength was significantly reduced and there was no phase transformation. For those reasons, we never used it. If you examine the data published in the literature, to my knowledge, not a single study shows that one of the more modern, more translucent zirconias works as well as 3Y zirconia. Therefore, we still use 3Y zirconia, but we veneer it. However, we only veneer it labially, so the rest of the restoration is completely 3Y zirconia that is polished, not glazed. We know from the literature that polishing makes zirconia less abrasive to the antagonist. Labial veneering creates the interesting possibility of being able to easily replace the veneering if the adjacent teeth undergo changes in color several years later. You just cut it off like a veneer and place a new veneer in a better-matching shade on top of it. We have done this several times, particularly for very young children who have received these restorations. For a 10-year-old patient, you cannot expect the restoration to match perfectly after 10 to 15 years, but with this design, we can remove the veneering without the need to remove the zirconia framework.

ID: What are the minimal connector area size and wing thickness requirements?

MK: We always use at least 3 mm of height and 2 mm of thickness for the connector area. Usually, we get the designs from the dental technician and then double-check the construction, so the design can be finalized before the restoration is milled. We double-check the connector size and the thickness of the retainer wings, which we aim to set at 0.7 mm. Using these dimensions, during the last 20 years, we have seen that if a resin-bonded bridge undergoes trauma, such as from an accident or a fistfight, it debonds but does not fracture. If that is the result, we can re-bond it, which is good news for the patients. With the old alumina ceramic, we always saw fractures in such cases.

ID: You said that no mechanical retention is necessary for these restorations, but does it help the chemical retention to fire ceramic onto the inside of the zirconia wing?

MK: It only helps the dental laboratory's wallet because they can charge more. When we have seen debonding with these restorations, the resin is still on the zirconia. The cohesive strength of the resin to the enamel is the weak link. We have measured the bond strength of the resin to the zirconia at 40 MPa to 45 MPa, whereas the bond strength of the resin to the enamel is only approximately 30 MPa. Therefore, a material placed in between the zirconia and the enamel cannot make the restoration any stronger than the resin itself. Nothing can enhance the cohesive strength.

ID: What are the long-term success rates of resin-bonded zirconia bridges?

MK: We published the 10-year success rates for the anterior region in 2017,⁴ and we will soon publish the 15-year success rates. We have more than 400 of these restorations—many of which are long-term-in the mouth, and we have not seen any fractures. Only approximately 8% debonded in 10 years, and most of those were situations in which the patient acknowledged a clear cause, such as a bar fight or a mother accidentally getting hit while playing with her child. In very few cases did a patient report that the restoration just fell off. We have seen a few instances of chipping, but we also see those with natural teeth, and we can always repair chipped zirconia by either bonding it again or placing a little bit of composite resin after the zirconia has been air abraded and conditioned with an MDP primer.

ID: Have you also studied the success rates of these restorations in the posterior region?

MK: We published our first such pilot study in 2022, which included approximately 27 restorations of premolars and canines.⁵ The mean observation time was just short of 5 years, and all of them were successful. The oldest has now been in situ for approximately 12 years. We used monolithic zirconia for the premolars and labially veneered zirconia for the canines. Of course, utilizing the premolars for guidance instead of the canines is a compromise. These cases were handled this way because, for various reasons, placing implants was not possible. We are running another randomized clinical trial right now with 30 more patients, and none of these have failed yet either. I believe it is quite astonishing, or at least, it is very promising.

ID: For a resin-bonded zirconia bridge to replace two missing teeth, would you recommend placing one double-wing bridge or two single-wing bridges?

MK: We recently published an article about this topic in the International Journal of Prosthodontics,⁶ and we will be publishing another article soon about the treatment of an 8-year-old girl who lost her central incisors in an accident. After she and her parents were satisfied with our mockup, we created two single-retainer zirconia restorations with a smart feature that I described in my book Resin-Bonded Fixed Dental Prostheses.⁷ The feature is a sort of interlock between the restorations. The two teeth cannot move apart, but there is a certain degree of freedom when the patient is chewing. Of course, the teeth can move individually 50 μm to 100 μm, which otherwise would cause stress, but our interlocking design features a small vertical concavity in one pontic and a small convex extension in the other. It worked very well for this patient as well as others. For this case involving two central incisors, the interproximal space was approximately 17 mm. Regarding cases involving two missing teeth in the posterior region, we have replaced two premolars with this concept as well. We have not had many such cases, but they have all been successful.

ID: For a patient who is that young, this seems like a really impactful option.

MK: That is why I really want to get this knowledge out to dentists. It is so sad that children like this are sometimes sent away until they are old enough for an implant when there is often a very viable possibility to help them.

Matthias Kern, Prof, Dr Med Dent, Dr Habil, is a professor and chairman of the Department of Prosthodontics, Propaedeutics and Dental Materials at the Christian-Albrechts University in Kiel, Germany, and has published more than 400 scientific articles.

For more information about the topics covered in this Q&A, visit: youtube.com/c/ProfessorMatthiasKernEnglish