Bart Van Meerbeek, DDS, PhD, professor in the Department of Oral Health Sciences at KU Leuven (University of Leuven) in Leuven, Belgium
Inside Dentistry (ID): With today's universal adhesives and self-etching primers, is etching dentin a thing of the past?
Bart Van Meerbeek, DDS, PhD (BVM): Some may feel this way, but I would say no. Although dentin can be etched via an etch-and-rinse procedure, we prefer to use a self-etch procedure because it provides an opportunity for additional chemical interaction involving primary ionic bonding, which can contribute to a very stable, durable bond. As soon as dentin is etched with phosphoric acid, the fibril meshwork is exposed, and the main bonding mechanism depends on micromechanical interlocking via the interdiffusion of monomers into that exposed network of collagen fibers. However, the current generation of universal adhesives with 10-MDP—he most efficient and functional monomer that we have available today—can provide this additional opportunity for chemical interaction via the self-etch procedure, so our preferred protocol involves selectively etching enamel and then performing a self-etch procedure on both the etched enamel and the unetched dentin.
Despite this assertion, we should respect all of the scientific data that we have available from laboratory and clinical research. Clinical studies going on more than 10 years have demonstrated that if you use an etch-and-rinse procedure, which means that you also etch the dentin, you can also achieve a durable bond. These data are product-specific, however, and should not be considered valid for the entire category of adhesives.
More recently, we have found that the immediate bond strength to dentin is higher when a universal adhesive is applied in an etch-and-rinse approach. However, we have also seen that the strength of these bonds decreases after artificial aging in the laboratory, so the long-term bond strength ends up being similar to that which is produced by a self-etch procedure. This suggests to me that perhaps we cannot conclusively say that etching dentin should be forgotten. Actually, we need to better understand how 10-MDP, as a functional monomer of universal adhesives, reacts with phosphoric-acid exposed collagen. Dedicated etch-and-rinse adhesives do not contain 10-MDP, which may explain the better performance of 10-MDP-based universal adhesives on dentin when applied in etch-and-rinse mode.
Overall, the new generation of universal adhesives is clinically attractive because they allow dentists to adapt their bonding protocols to specific situations by choosing either etch-and-rinse or self-etch procedures. When I see younger dentin, I utilize the self-etch procedure, but when I see sclerotic dentin that is highly mineralized, I tend to etch it with phosphoric acid. The same universal adhesives can be used for either option.
ID: Why is "etch-and-rinse" considered to be better terminology than "total-etch"?
BVM: The term "total-etch" is commonly used in many parts of the world, but we believe that the word "total" refers to the simultaneous application to both enamel and dentin, whereas a self-etch adhesive is also simultaneously applied to enamel and dentin. Therefore, we prefer to differentiate by specifying "etch-and-rinse" to indicate that a rinse phase is required (as opposed to a self-etch procedure that does not require water rinsing and could even be considered as "etch-and-dry"). In Europe, etch-and-rinse is the terminology that is now being used in the scientific literature.
ID: Can etching dentin be detrimental?
BVM: This depends on the age of the dentin. For a young patient whose dentin is visually highly permeable, you should be careful not to irritate the pulp. If you are close to the pulp and can see any direct color from it shining through, I recommend avoiding etching dentin and considering locally applying a protecting base. In any case, certainly, a rule should be to always limit the etching time to 15 seconds because the longer and deeper that you etch, the more difficult it is for the resin monomer to infiltrate the exposed collagen mesh. When performing etching procedures, we recommend that you begin by etching the enamel and finish with the dentin. Then afterward, you can wait 15 seconds and rinse everything off.
ID: You mentioned that universal adhesives achieve a higher immediate bond strength to dentin when an etch-and-rinse approach is used but that it decreases after aging. Could you elaborate?
BVM: I cannot speak for all of the products on the market, but we have tested several different 10-MDP-based universal adhesives. Our most recent research has shown repeatedly that, yes, if you etch dentin, the immediate bond strength is usually higher, but it balances with the strengths achieved by self-etch procedures upon aging. That element of the testing is important. We always measure the immediate bond strength as a reference, but we also challenge specimens with significant aging procedures in the laboratory. We go beyond ISO standards to 25,000, 50,000, or sometimes even 100,000 thermocycles. Furthermore, we make microspecimens rather than using whole teeth, so there is direct interaction with water. The interface is directly exposed, which is a relatively severe test. These challenges are important in studying long-term durability.
ID: Are three-step total-etch/etch-and-rinse adhesives still the gold standard in your opinion?
BVM: I always say there are gold standards that, indeed, involve a three-step, etch-and-rinse adhesive, but I do not want to generalize their success to all three-step adhesives. We have done extensive literature reviews analyzing both laboratory and clinical data, primarily the latter, regarding retention rates for Class V restorations, and within that group of three-step, etch-and-rinse adhesives, there are big differences, so it is problematic to say that the whole group is the gold standard. Regarding bonding to dentin, I also consider a two-step, self-etch adhesive as a gold standard, on the condition that you selectively etch the enamel beforehand.
The new generation of universal adhesives is still, in my opinion, a tradeoff. In the multistep system, the primer has a different function from that of the bonding agent. If you combine them, as is the case with a universal adhesive, you are making a compromise. That is why we have seen the development of new two-step universal adhesives that come with a primer, are still 10-MDP-based, and can be used in an etch-and-rinse or self-etch mode. Application of the primer is followed by a separate adhesive resin, so both functions are independent from one another. The application procedure is slightly more complicated, but the resulting durability of the bond achieved should be better.
ID: Which material is best to rehydrate overdried dentin?
BVM: A water-based 10-MDP primer works best. The 10-MDP molecule has unique properties. I even use this type of primer as a decontamination agent. If you have contamination—for example, when you cannot place a rubber dam or do not have the field under complete control—use a water-based 10-MDP primer. You can use them to rehydrate dentin if overdried, and you can use them to decontaminate surfaces.
ID: What else makes 10-MDP products so special?
BVM: Interestingly, 10-MDP is one of the only monomers that combines primary chemical interaction and ionic interaction—which results in a stable bond—together with some etching effects. It still has a self-etching effect. Some monomers etch more significantly and will really decalcify the surface, others will only chemically interact superficially, but 10-MDP combines both the ability to etch relatively well, creating a thin hybrid layer, and the ability to chemically bond.
Of course, we all know the wet bonding technique today, but that was introduced when acetone-based adhesives were marketed. In our randomized controlled trials with multistep adhesives, we never applied the adhesives using a wet bonding technique. We were careful to never overdry. We gently dried dentin just until we could see that it had become dull and that the etched enamel exhibited a white frosted appearance, then we stopped. When you use a water-based primer, you get an automatic wet bonding effect. There is much less technique sensitivity.