The Chemistry of Compromise: Why the Single-Bottle Adhesive is a Beautiful Lie

Dr. Melissa Seibert: Welcome to Dental Digest. This is a podcast with a mission of enabling you to stay on the cutting edge of evidence-based dentistry. I'm your host, Dr. Melissa Seibert. Today's episode is Part 1 of a two-part series featuring Dr. Bart Van Meerbeek, and it's hard to overstate just how significant this conversation is. Professor Van Meerbeek is truly one of the preeminent authorities in adhesive dentistry worldwide. His work has shaped how we understand bonding at a fundamental level — from dentin permeability and hybrid layer stability to why certain simplifications in adhesive dentistry come at a biological and mechanical cost. For decades, his research out of KU Leuven has set the scientific benchmark that the rest of our field measures itself against.

In Part 1, we take a step back and examine the core principles of adhesion. We discuss where modern adhesive dentistry truly stands today, what the evidence actually tells us about universal versus multi-step systems, and why ease of use is often a trade-off rather than a free upgrade. Bart walks us through concepts like hydrophobic layering, film thickness, stress distribution, and why proper technique still matters more than a single product. This is not a conversation about shortcuts. It's a conversation about understanding the biology and material science well enough to make deliberate, defensible clinical decisions.

Dr. Seibert: In your work on the adhesion degradation paradox, you've shown that the same hydrophilicity that enables dentin infiltration also accelerates hydrolytic breakdown. In 2025, do you see any realistic pathway out of this paradox, or is durability still fundamentally a compromise?

Prof. Van Meerbeek: Yeah, I think it is a compromise. Quite often it is said that this new generation of universal adhesives is the way to go, and I think so in respect of ease of use. But on the other hand, we should still realize these are — I call them trade-off type adhesives — where you go for ease of use, but you pay a price for it. And I'm not just saying this; we have evidence. We have tested so many times. The immediate bond strength with these universal, ease-of-use adhesives is significantly lower than when you go for a two-step procedure. That's why we claim you need these two phases. You need a primer to be applied — the adhesion promoter, as I call it — and this needs to be followed by an adhesive resin that polymerizes very well. That has to do with hydrophobicity — it should not absorb too much water — and on top of that should have a certain thickness.

That thickness is very important because it can function as a kind of stress reliever, a shock absorber, between the adhesive interface, the dentin substrate, and the composite. The composite is going to pull on the interface, creating shrinkage stress immediately and later on as well. The adhesive resin layer works as a buffer. If it is too thin, it will not do that. Some products come on the market where they claim a very thin film thickness as an advantage. I think basically the opposite — it's a disadvantage. If it is too thin, it will not polymerize well, it will not be stable, and at the moment you apply your composite on top of it, you have a bigger risk that it will debond, that it will be pulled away from the surface.

Regarding hydrophobicity, we have clearly learned — and this has to do with the kind of testing too. We are aging, we are challenging our specimens by water storage, thermocycling. Water storage on its own is a severe test, a little further from what happens clinically, because we have these very tiny sticks exposed to water directly, which is not what happens in the mouth. But the more water they absorb, the strength and durability degrade. The interface degrades. For that reason, a good hydrophobic layer that doesn't absorb too much water is better for durability.

Dr. Seibert: There's a lot to unpack. For dentists listening — what are some of the popular universal adhesives used today? Here in North America the popular ones are Scotchbond Universal by Solventum (3M), and Kuraray makes a universal adhesive too. In Europe, what are the popular universal adhesives that you see being used?

Prof. Van Meerbeek: Being from a university, I have a little difficulty really naming what is most popular. I don't have a view on market figures. But I can imagine, like you mentioned, in the US it's Scotchbond Universal — and nowadays Universal Plus — that is one of the popular ones. Scotchbond Universal was the first really branded as a universal adhesive on the market, and it is a very stable product. We dare to name certain products as a gold standard, but these are multi-step adhesives. I do not yet want to say that any one particular product is a gold standard universal adhesive. That is too early, especially because we lack long-term clinical data, while we have it for multi-step adhesives. By long-term I mean beyond 10 years, or around 10 years.

If you look at Class V clinical trials, you can clearly see the loss rate with universal adhesives is larger than with multi-step. But within the universal adhesive class there are differences. There are several others I consider universal adhesives — G-Premio BOND, Clearfil Universal Bond, more nowadays. Regarding what is most popular, I have no view on market figures.

I have not that much against one-step universal adhesives, because we always recommend: either go for a two-step universal adhesive, or go for a one-step universal adhesive but always apply a flowable composite on top of it. The flowable then serves as your bonding resin or adhesive resin, making it actually a kind of two-step system. That's not wrong — we teach here in Leuven that the first layer in the box is always a flowable composite. If you combine that with a one-step universal adhesive, fine.

But there are differences between universal adhesives that aren't realized enough. Most contain 10-MDP as functional monomer, but what we do not know is the content — the percentage of 10-MDP. I can assure you, because these are things we are studying now, that there are big differences. Some adhesives contain much more 10-MDP than others. Sometimes in our lab we see differences in bonding effectiveness we can't always explain, because we don't know the compositions completely — but it can be related, certainly when applied in self-etch mode, to the percentage of 10-MDP inside the product.

This is not released by manufacturers — understandable — but it's an important factor. We are working in this light, and it's too early to make public, but we see a lot of differences. One universal adhesive can be totally different in performance from another. So currently, dentists think "this is a universal adhesive, I can apply it, it contains 10-MDP" — but if it contains only 2–3%, or it contains 10%, it can mean a big difference in performance. You will only notice this in self-etch mode, because then 10-MDP plays its biggest role. In etch-and-rinse mode it will be more difficult to differentiate between universal adhesives.

Dr. Seibert: Just to recap — the advantage with universal adhesives could be ease of use, but potentially we might be accepting some compromises. Is that fair?

Prof. Van Meerbeek: Yes. Versatility is also a very important advantage. You can choose as a dentist how to use the adhesive, depending on personal preference. I always said we were in favor of good chemical bonding, so I favor more the self-etch technique on the dentin — not on the enamel. Enamel is clear; enamel needs phosphoric acid, you need etch-and-rinse. But on dentin, you could say you would like to go for self-etch because of the mechanism behind it.

But here's the message I want to bring: you can let your choice depend on the type of dentin you have to bond to. If it is very young dentin — for a young patient, or for a patient suffering from an eating disorder where all the enamel at the palatal side is completely dissolved, with very young, very permeable, often very sensitive dentin where sometimes you even see the pulp shining through a little pink — that is when I go for a self-etch technique. I go for the mild type of interaction because I certainly do not want to further irritate that pulp.

On the other hand, when I have to restore a tooth with a typical sclerotic dentin — dentin that has been exposed for a long time, discolored, brown, glassy — I know this is difficult to bond to. In that case I would go for an etch-and-rinse approach, because I want a deeper interaction on the dentin. The same is true if you've removed an amalgam restoration: underneath, you have corroded dentin with a tattooing effect — pigments and corrosion products in there. Often, even without anesthesia, you can do it because the pulp is completely retracted. In that case, go for an etch-and-rinse, a more aggressive treatment. That is the philosophy I think is good — and why I like that universal adhesives let you choose, not based on personal preference but on the kind of tissue you're confronted with.

Dr. Seibert: You talked about adding flowable on top of a one-step universal to optimize it. Can you spend a little more time on that? For many dentists in North America this might come as a surprise.

Prof. Van Meerbeek: The biggest disadvantage of one-step universal adhesives is the film thickness, and this is sometimes interpreted wrongly market-wise as an advantage. Most one-step universal adhesives have a thickness, with gentle air-blowing, of about 10 micrometers. Ten micrometers is very thin. You can light-cure it, but it will never cure well because you have oxygen inhibition. So you haven't really stabilized the hybrid layer. The actual interaction, the actual bond, is not very stable at that moment — and then you put composite on top of it. The composite shrinks. It produces tensile stress on that interface, which can result in debonding. That's the C-factor playing a role in narrow cavities with a lot of polymerization shrinkage stress.

If you have a flowable composite on top of that, you make a buffer layer that better stabilizes the adhesive. Flowable composite also shrinks, but in a layer with a big free surface, it can shrink without producing too much tensile force on the interface. That's the first thing — it protects the hybrid layer underneath. On top of that, a flowable is quite hydrophobic, has some filler, has stronger properties. So it works as a shock absorber, a stress reliever. With a two-step system where you have a primer and adhesive resin that creates a certain thickness, you don't need that flowable composite in principle. But there's nothing against adding it. That's the basic idea: go for a one-step adhesive, but always apply a flowable composite on top.

Dr. Seibert: You talked about how these materials are hydrophobic. Salivary contamination can be detrimental. What should clinicians do when salivary contamination arises?

Prof. Van Meerbeek: It's not that long ago we did an interesting study where we looked at the best way to decontaminate. In the past, it was always: this happened, start over. The question was whether you have to do that or whether there are other ways. We learned, to our surprise, that when you use a 10-MDP-based solution — I'm going to call it that because that really is a very effective decontamination agent. This has to do with the functional monomer of 10-MDP. 10-MDP is a good chemical bonder, but at the same time it's a good etcher. This combination gives it a cleaning function.

We did the study. We tried it out. In the laboratory we have a lot of saliva — exaggerated saliva. To our surprise, if we just used a two-step system with a 10-MDP-based primer — and this is critical, the concentration of 10-MDP in the primer is high — this works perfectly as a surface decontamination agent. You don't have to worry that much anymore. You take your primer, you start priming with your brush, you decontaminate the surface, you gently air-blow, and you continue. There is a paper about the excellent decontamination capacity of 10-MDP.

I said a 10-MDP-based solution. If you take a one-step universal adhesive, of course it contains 10-MDP — but the concentration is lower because a one-step universal is a cocktail of different ingredients. If you use a primer that contains 10-MDP as the functional agent and not too many other ingredients, this is going to work much better than a one-step universal that contains other things too. In my opinion, a good 10-MDP-based primer is the best to decontaminate the surface. I cannot extrapolate that for blood contamination — I have no data — but I speculate it might have a similar effect.

Dr. Seibert: So a popular example would be Clearfil SE Bond 2?

Prof. Van Meerbeek: That is what we call the gold standard self-etch adhesive — Clearfil SE Bond 2, the newer version, basically similar to the original with claimed better polymerization capacity. Yes, the primer has quite some 10-MDP inside. There are others. You have G2-Bond Universal that came on the market — a two-step version based on similar technology. There you also have a separate primer with an adhesive resin, and the primer has more 10-MDP. There are fewer other ingredients in the way. A one-step universal adhesive is a cocktail of different things. It's nice to put things in one cocktail, but that's why it is a compromise.

If you have 10-MDP in a universal adhesive, that 10-MDP has to fight with other ingredients to reach the dentin surface. There are other monomers in the way, by which 10-MDP doesn't always reach the dentin to interact with. If you have a primer that contains proportionally more 10-MDP, it will reach the surface much more easily and interact much more. That is just simple common sense.

Dr. Seibert: Do the same rules apply if the composite gets contaminated? Can you scrub in a 10-MDP-containing primer?

Prof. Van Meerbeek: Yes, I think it works the same way. During restorative treatment, you apply a layer of composite and see some contamination. I would do the same — this is very fresh composite. Sometimes it's said we need the oxygen-inhibition layer to make sure the next layer bonds well to the previous one. That is also not really true. Even a fresh composite, even if that oxygen-inhibition layer is not there, has sufficient double bonds still available. If you decontaminate first with 10-MDP and then go on with your next layer of composite, you'll be fine.