Our Evolving Understanding

ID: As you test materials, how universal are you finding universal adhesives to be?

JP: One of the concepts behind universal adhesives is that they can be used for all substrates, ranging from dentin and enamel to zirconia and metal. However, many do not perform well clinically on all substrates. Even for indirect restorations, the success of an adhesive depends on its pH. Most universal adhesives need to be mixed with a dual-cure activator because they are not very compatible with dual-cure composites. For example, if we are performing a buildup for a crown on an endodontically treated tooth, it may debond if it isn't mixed with an activator. There are some newer single-bottle options that do work without an activator, however, and there is even a new two-bottle universal adhesive that performs very well in vitro. We apply a hydrophobic bonding resin over the first layer to make the adhesive more compatible with various wet substrates. Another issue with universal adhesives is presented by the ones that contain silane to bond to ceramic materials. The goal of these products is to eliminate a step in the procedure—the application of a separate silane primer-but the research indicates that silane is not compatible with the composition of many universal adhesives, so it is not effective. However, the newest versions of these materials now incorporate different silanes that are effective when mixed into the composition of the universal adhesive, so they can be used for bonding glass matrix ceramic materials.

ID: With a universal adhesive, should clinicians utilize a self-etch or etch-and-rinse protocol?

JP: Clinical studies have been critical to improving our knowledge regarding the best approach; we need to understand the shortcomings of in vitro studies. Five-year follow-ups on universal adhesives indicate that the materials are not behaving very well when they are used with self-etch techniques because they cause quite a bit of marginal staining and problems around the cavosurface margin.^1,2 According to the results of these studies, we really need to etch the enamel for universal adhesives to behave the way that we want them to, so either an etch-and-rinse or selective enamel etching technique should be utilized.

ID: When using a universal, is it preferrable for the dentin to be wet or dry?

JP: Based on the work of John Kanca, DMD, and others, we have been taught for years to work with moist dentin.³ The newest adhesives, however, contain water, so there is no need to leave the dentin moist. Research has shown that, when compared with dry dentin, bonding to moist dentin does not improve the retention of composite restorations after 18 months.⁴ Obviously, you do not want to over-dry, but we do not recommend leaving the dentin as glisteningly moist as was customary for the older-generation universal adhesives.

ID: What is the potential of these newest universal adhesives regarding chemical adhesion to dentin?

JP: Everybody talks about this because virtually all of the newest universal adhesives contain a famous molecule called 10-MDP. However, it is important to note that the mechanism of chemically bonding to dentin requires the presence of calcium. If you etch dentin, the first thing that you remove is the calcium, so the top layer becomes totally decalcified, and there is no way that any adhesive can chemically bond to it. Therefore, the potential to chemically bond to dentin only exists if you apply a self-etching adhesive to it. We recommend selectively etching the enamel, not the dentin, and then applying the universal adhesive to both the enamel and dentin.

ID: Are universal adhesives needed for zirconia restorations?

JP: Clinical studies are not easy to conduct for this subject, but most in vitro research—including a study led by Alessandro D. Loguercio, PhD—has found that primers containing silane and 10-MDP behave very well and increase the longevity of bonds to zirconia.⁵ The 10-MDP molecule chemically bonds not only to dentin but also to zirconia. Thus, we do not need a universal adhesive to bond to zirconia. In addition, most universal adhesives are light cured, so if you apply them to a zirconia restoration and then use a self- or dual-cure resin cement, what will happen to the adhesive? It is not cured at all, so what will happen to it over time? We do not know. If you want to add some extra retention to a zirconia crown through chemical bonding in case your preparation is not ideal, what we currently recommend is to just use one of the new silane plus 10-MDP primers according to the manufacturer's instructions.

ID: What is needed for lithium disilicate restorations?

JP: For lithium disilicate, some manufacturers are no longer even recommending the use of an adhesive. In fact, they are recommending using only the same solution that I just mentioned: silane and 10-MDP. In vitro studies with aging for more than 1 year show that the use of a silane and 10-MDP solution preserves bonds longer than universal adhesives do.⁶ As a result of this research, we no longer teach the use of adhesives to bond glass matrix ceramics. Just etch the intaglio surface of the restoration with hydrofluoric acid, and the only material that needs to be applied to the intaglio surface before the resin cement is a thin layer of a silane plus 10-MDP primer.

ID: Shifting away from universal adhesives, do we really need matrix metalloproteinase (MMP) inhibitors, such as chlorhexidine, for dentin adhesion?

JP: This has been claimed for more than 12 years, but all of the studies supporting it are in vitro. MMPs are just enzymes that degrade collagen, and if a low pH, such as from an acid etch, activates them, then they will be able to degrade dentin collagen. They are present inside many other tissues besides dentin. We know that, in vitro, they may trigger the degradation of the hybrid layer; however, most clinical studies show that, in vivo, they do not. Clinical trials with follow-up after several years have shown that the current MMP inhibitors do not improve any characteristic of the bonding procedure over time; however, the research in this area is ongoing.^7-9

ID: Does immediate dentin sealing improve outcomes clinically?

JP: Dentists around the world use immediate dentin sealing. When Pascal Magne, DMD, PhD, introduced the technique, I was a reviewer of the first paper.¹⁰ It was beautiful, and the rationale made sense. Basically, it involves immediately placing an adhesive resin over crown preparations to prevent sensitivity and seal out bacteria before placing the temporary crown. Another layer of adhesive is applied just prior to bonding or luting the final crown. The in vitro research provided excellent information supporting the concept until we started to look at clinical studies, none of which really corroborated the in vitro findings.^11,12 Immediate dentin sealing does not improve the clinical performance of ceramic restorations. The results are the same without it. The biologic rationale behind the concept is sound; it just does not seem to improve outcomes clinically.

ID: What are the findings from clinical studies regarding the use of glutaraldehyde-containing desensitizers underneath composite restorations?

JP: Some may find it surprising, but they are similar to those regarding MMP inhibitors and immediate dentin sealing. We recently published a clinical study of posterior composite Class I and Class II restorations.¹³ Half of them had glutaraldehyde-containing desensitizers, and half of them did not. The outcomes were exactly the same for both groups. We examined two types of sensitivity: cases in which we applied the stimulus and cases in which it was spontaneous. Neither made a difference. Applying a glutaraldehyde desensitizer did not change the outcomes of any patient whatsoever. There are other clinical trials, but none of them really shows that glutaraldehyde, which is recommended for preventing sensitivity under posterior composite restorations, is effective in that regard.

Jorge Perdigão, DMD, MS, PhD, is a professor of operative dentistry at the University of Minnesota School of Dentistry and has delivered more than 280 lectures internationally as well as published more than 180 scientific articles.