Adhesives versus cements

April 3, 2019

Which materials and procedures should be cemented, and which are better served by adhesion?

Cementation is a seemingly well-understood procedure. However, as products and materials evolve, the lines can get somewhat blurry between conventional cements, such as glass ionomer and resin-modified glass ionomer, and resin cements.

Adhesives

“Very broadly, when I think of adhesives, I think of direct dentistry,” says Dr. Jason Goodchild, DMD, director of clinical affairs for Premier Dental. “The patient has a carious lesion, you remove the part of the tooth that’s carious and you place the composite. In most cases, to place a composite correctly, you need an adhesive.”

More recently, adhesive dentistry not only involves direct dentistry, but it’s become very common for the indirect side as well.

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“When you think of adhesive dentistry, you also have to consider that definitive cementation can involve adhesion or bonding with common examples being resin cements and self-adhesive resin cements,” Dr. Goodchild says. “We’re talking about not just mechanical retention; adhesive dentistry involves chemical bonding of a restorative material to a tooth. If we are getting down to the product level, there are many types of adhesive products. Sometimes we would call them bonding agents, and in other cases, adhesive products may be part of a cementation system where a bonding agent and then a cement are applied. Still, there are other products where the adhesive is included in the cement itself; this last example is called self-adhesive resin cement.”

“As a general rule, if a material can be bonded, then always bond it,” says Augusto Robles, DMD, MS, associate professor and director of operative dentistry curriculum at the University of Alabama at Birmingham School of Dentistry. “It seals better, increases retention and makes the restoration stronger. In my practice I do veneers, crowns and onlays and they are bonded routinely.”

Adhesive formulations and features have evolved and are now more user-friendly.

“Again, thinking about bonding agents, we’ve had etch-and-rinse adhesives,” Dr. Goodchild observes. “We’ve had self-etch adhesives, and then, most recently, we have things like universal adhesives, which have fewer steps. They may be more technique-robust, which means they can be used for multiple clinical situations and can be a little less sensitive to operator error.”

Cementation

Cements are used for indirect dentistry, typically restorations made at a lab. But times are changing.

“Historically, the dental lab would have to make the crowns, inlays and onlays, but nowadays we have chairside milling machines that can help do that for us,” Dr. Goodchild says. “These are restorations that are made outside the mouth and then glued in the mouth. The glue, for an indirect restoration, is typically called a cement. Under the category of ‘cements’ there are sub-categories of glass ionomer, resin-modified glass ionomer, resin cement and self-adhesive resin cement.”

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Glass-ionomer cements are easier to use in situations where moisture is harder to control and when the material and preparation allows for it. And as formulations have evolved, their use becomes a bit more ambiguous.

“This is where  it gets a little bit sticky,” Dr. Goodchild explains. “Besides glass-ionomer-based cements, there are also adhesive cements. They factor in some of that same technology to not only achieve mechanical retention but also chemical retention, which would be adhesive.

“There's so much crossover now, and that may be the confusing part for a lot of dentists,” he continues. “It used to be when you placed a crown, the procedure involved putting cement in the crown and placing it on the tooth. Now, you must weigh several factors to select the most appropriate cement based on factors such as preparation height and taper, ability to isolate, esthetic and substrate. It really becomes a confusing landscape, especially when you factor in the number of available products and try to remember which one goes with which situation. When I have lectured on this topic, I sometimes title the talk something like ‘The Confusing Landscape of Cements’ or ‘Cements are Scary, Let’s Demystify Them’ because there are several categories of cements and each cement has its own unique indications and features and benefits.”

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Crossover confusion

While the concept of cementation seems fairly straightforward, indications and the abilities of a product can muddy the waters.

“Bonding is a very predictable, but very technique-sensitive, procedure,” Dr. Robles says. “Many clinicians look for the easiest or least complicated way in their practices, and usually this involves the use of a glass ionomer cement rather than an adhesive cement. If you are not bonding routinely, then I can see how the bonding protocol can become confusing.”

The best way to avoid confusion is by simply reading the directions. Unfortunately, that seemingly modest step may not be as instinctive as one might think.

“There's an old joke when we start talking about these kinds of things that become operator sensitive,” Dr. Goodchild says. “The joke is that most dentists don’t read directions. They learned how to use a product and think that, perhaps, those same directions apply for every other product that falls into that category. If I know how to use a bonding agent or an adhesive cement, well, shouldn’t they all just work the same way? I'll just handle them the same way. And, in fact, that couldn't be further from the truth.

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“They all have their own unique directions,” he continues. “Even within the same category, products from different manufacturers can have wildly different directions, and so it is vital-absolutely vital-that the directions get read and the dentist understands how to use the product correctly.”

Dental materials and processes continue to evolve, which is great for overall outcomes, but it can lead to confusion, especially with bonding agents and how to use phosphoric acid.

“When you think of adhesive dentistry, we've evolved from using phosphoric acid over the entire preparation to avoiding placing phosphoric acid to potentially using it selectively, helping to mitigate postoperative sensitivity and achieving great bond strengths for long periods of time,” Dr. Goodchild says. “So, there are changing formulations, changing indications. The chemistries for adhesive products have become more sophisticated. It’s become more important than ever to read directions because they’re not really apples-to-apples. Every single product could be apples-to-oranges, and that’s why you have to read the directions.”

Evolving materials are the biggest source of adjustment when selecting the right cementation product.

“On the indirect side, I think the biggest change has probably been in available substrates,” Dr. Goodchild says. “The newer crown materials may need some special product selection. You might want to use certain products with certain substrates because those are meant to go together. The confusion when definitively cementing crowns, inlays and onlays is how to match the right cement to the right substrate.”

“My personal rule is, ‘If it can be bonded, bond it,’” Dr. Robles adds. “Any restoration made from feldspathic porcelain and glass ceramics like leucite-reinforced, lithium disilicate, or lithium silicate should be bonded, meaning the dentist should use an adhesive cement. Zirconia can be bonded, but conventional cementation is probably sufficient, examples being glass ionomer, resin-modified glass ionomer cement, or the new bioactive cements.”

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One size doesn’t fit all

It would certainly simplify everything if only one syringe of cement and one bottle of adhesive were all that was necessary. Unfortunately, it’s rarely that simple.

“Dentists would probably love a cookbook approach to cementation,” Dr. Goodchild says. “Which is, ‘If I have this clinical situation, I do this.’ That takes a lot of the guesswork out of it, and I think a lot of people would love that, especially for efficiency sake and best practices.

“And that cookbook approach is what everybody would love, but every clinical situation is different; every patient is different,” he continues. “There are so many different substrates and so many different available products to choose from that the dentist must synthesize all the patient-, material- and substrate-related factors into a cementation approach that will produce optimum results. It gets challenging, and that’s perhap, where all this confusion comes from.

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“When you consider cementation in general, I think dentists really want a move toward the ease-of-use and universality-that is, it’s easy to use and it works in every situation and I don’t have to match the right clinical situation with the right material, so it takes care of all of that by having a universal approach,” he continues. “That’s been a real challenge for manufacturers. We would love to give dentists the one answer, the magic bullet, but it doesn’t exist. You can see that trend in adhesive dentistry for direct restorations, where we started with etch-and-rinse, then we went to self-etch and now we call them universals, and they’re allowing dentists to do a lot of different techniques with one bottle.”

Cement science is moving toward one-syringe-does-it-all, but don’t start purging the supply cabinet just yet.

“On the cement side, we started with zinc phosphate, then we moved to zinc polycarboxylate, then we moved to resin cements, resin-modified glass ionomer cements, and then into self-adhesive cements,” Dr. Goodchild explains. “We are moving toward this idea of, ‘If you could have one thing to deal with all these clinical situations,’ but the chemistry becomes more difficult. The chemistry can become more user-sensitive, especially when you bring in the nuances of each substrate, they have to really follow the directions to get the best results. Dentists aren’t always good at that, but I think that message is resonating more than in the past because as new substrates and products are introduced, operators have to stay on the learning curve to get the results they want and the outcomes our patients demand. Do we have the one thing for everything? We’re not there yet.”

Historically, cements were used for very clearly defined purposes. As science and materials evolve, giving dentists myriad material choices to involve glass ionomer-based cements as well as adhesive resin cements, the process of knowing which cement to use for what clinical situation has become more confusing. And while it’s overly simplistic, it’s fundamentally correct to think that cement products are still used for many of the same functions as they always have.