Universal Adhesives

Nathaniel Lawson, DMD, PhD (NL): There are two primary requirements for an adhesive to be considered universal. First, it must be able to be used in any type of etching mode, with or without phosphoric acid, on both the enamel and dentin surfaces. Presently, many clinicians would agree that universal adhesives with pH levels closer to 3 are less ideally effective than self-etching materials with pH levels around 2 for self-etching enamel. Universal adhesives work best in the selective-etch or total-etch modes. The second requirement of universal adhesives is that they can be used for both direct and indirect restorations. They have primers in them that allow them to bond to substrates other than tooth materials. Most have primers for bonding to zirconia, and some have primers for bonding to glass-based materials. In addition, many universal adhesives are compatible with dual-cure cements. Despite all universal adhesives being somewhat acidic, which previously was understood to mean that an activator would be required for compatibility with dual-cure cements, some require a separate activator, but some do not.

ID: You mentioned universal adhesives being less effective for self-etch procedures. Are you seeing clinical evidence of this over time?

NL: Two recent clinical trials-one at our university and one at the University of Minnesota-have compared universal adhesives in this regard. Our study tracked self-etch and total-etch, and theirs studied both of those as well as selective-etch. Both studies demonstrated better margins and better retention of Class V restorations when phosphoric acid was used on the enamel (ie, the total-etch in our study, and the total-etch and selective-etch in Minnesota's study). So, there is clinical evidence of this.

ID: What is the evidence regarding the primers used in universal adhesives?

NL: We have done some studies that we have never published. Attia and Kern published a study on the results of bonding to lithium disilicate and zirconia with universal adhesives when compared with traditional ceramic primers.¹ They found that a universal adhesive with a silane primer did not bond well to lithium disilicate; however, another one with zirconia primers bonded very well to zirconia. Those results are in line with what I've always believed, although we have never fully studied it, which is that bonding to glass-ceramic-based materials with universal adhesives is less than ideal. Conversely, a recent study by Yao and colleagues showed that a new silane technology added to an experimental universal adhesive allowed it to not only bond to lithium disilicate but even endure thermocycling.² So, perhaps new silane formulations will be more effective than previous formulations.

ID: You also mentioned dual-cure activators. What is the evidence surrounding these?

NL: Universal adhesives can be used for indirect restorations by applying the adhesive to the tooth and then using a dual-cure resin cement on top of it. The issue with previous, more acidic self-etching adhesives and dual-cure cements was that the acidity of the adhesives could inhibit the polymerization of the self-curing mechanism of a dual-cure cement. To compensate for this, dual-cure activators needed to be added from a separate bottle. The exact function of these dual-cure activators is debatable, but primarily, they create free radicals to initiate the polymerization of the dual-cure cement. Interestingly, although the pH level of an adhesive is what theoretically determines the necessity of a dual-cure activator, we have identified universal adhesives that make different recommendations regarding that need despite advertising identical pH levels. In our laboratory, we have found that some adhesives that indicate requiring a dual-cure activator actually do not. Furthermore, some product lines have eliminated the recommendation to use a dual-cure activator for newer versions. Many of these may not have even changed anything in the formulation; the manufacturers simply realized that the dual-cure activator was not really needed.

ID: When you mention pH levels and their implications, do you trust manufacturers' marketing materials for the measurements, or do you confirm pH levels in the laboratory?

NL: I use pH paper to measure the levels. I am hesitant to risk damaging a pH probe by placing it in an adhesive material. There isn't a great source of comparative, third-party independent verification of the pH levels of different adhesives.

ID: Let's return to the discussion of indirect restorations. Clinically, would you feel comfortable bonding a porcelain veneer with a universal adhesive?

NL: No. To be totally honest, I would not. That is a very tricky question, however, because I do trust the data in the Yao study. If I were working at a DSO and had some lithium disilicate anterior crowns that were more or less retentive, and I just wanted to bond them, then I might use a universal adhesive. However, for a veneer, onlay, etc, I must admit that I would be nervous. I do not place veneers, but I still use a separate silane for lithium disilicate onlays. I did know one dentist who was unaware of phosphoric acid and used universal adhesives in self-etch mode no matter how many times I told her that the bond strength would be better if she etched the enamel margins. So, some dentists might be fine with the bond strength achieved by the silane in universal adhesives.

ID: Over-etching dentin and then using a universal adhesive can be detrimental, correct?

NL: Yes, definitely. I have not seen any new studies addressing that, but older studies have shown over-etching to be a problem.

ID: Regarding the formulation of universal adhesives, are two-bottle systems better than one-bottle systems?

NL: That is an interesting question, and one could even ask if the two-bottle systems should even still be considered universal at that point. The use of one-bottle versus two-bottle systems has been hotly debated. Biomimetic dentists, for example, strongly prefer two-bottle systems because they do not like hydrophobic resin mixed with hydrophilic primer. Personally, I remember speaking to a chemist for a manufacturer several years ago and asking, "How do you justify having a hydrophilic primer and a hydrophobic resin mixed together when the creation of hydrophilicity in the hybrid layer may permit water to get in through the dentin tubules and allow hydrolytic degradation?" The assertion has been that, when you apply a one-bottle universal adhesive, some of the more hydrophilic monomers go down into the wet dentinal collagen first, and the more hydrophobic monomers stay up toward the top, creating this gradient of hydrophilicity and hydrophobicity in the hybrid layer, but I do not know if this has been completely verified by research. A study by Dailing and colleagues confirms that some monomers exhibit different levels of hydrophilicity and hydrophobicity³; therefore, I suppose it is possible that they can separate in the hybrid layer and create a gradient more similar to what you would see with a two-bottle system where there is a hydrophilic layer first and a hydrophobic layer on top protecting the hybrid layer from water degradation. I would love to see this proven.

ID: An analogy has been used that a one-bottle adhesive system is like salad dressing. You can shake it, but it's difficult to envision it ever mixing perfectly.

NL: I like that analogy. For a long time, we did not have much proof that this was really a bad thing. However, in a study published by Sauro and colleagues, they tested the bond strengths achieved by single-bottle and two-bottle bonding agents, then hooked up the pulp chambers to fluid, applied pressure for 3 years, and showed that the water degradation during that time had decreased the bond strength of the single-bottle system but not the two-bottle system.⁴ Still, although those results are compelling, they are difficult for me to completely accept because I have been achieving excellent clinical results with universal adhesives for so long.

ID: Regarding compatibility, can one manufacturer's universal adhesive be reliably used with another's self-adhesive cement, or should clinicians stick with one manufacturer?

NL: That's a great question. For many years, I advocated mixing and matching because, for direct restorations, I saw no problems with light-cure materials. However, once you get into dual-cure chemistry, it makes more sense to stay in-brand with the materials. If you had asked me this 3 years ago, I would have given a different answer, but once you've performed bond-strength testing on a range of different materials, you understand that mixing and matching just gets complicated.

ID: Finally, what is one thing in this area that you'd like to study further?

NL: One question that warrants further exploration is whether or not clinicians should light-cure the adhesive on the tooth prior to using a dual-cure resin cement. In the laboratory, we achieve higher bond strength values if we light-cure the adhesive on the tooth. However, in practice, I worry about building up the film thickness, so I do not. Our laboratory testing has involved light-curing the adhesive on the tooth and then dark-curing the cement as well as dark-curing the adhesive and then dark-curing the cement. However, we do not have a reliable way to differentiate between the results of light-curing the adhesive on the tooth and then light-curing the cement on top of that versus dark-curing the adhesive and then light-curing everything together. I just hope that light-curing everything together will somehow lead to higher bond strength.