A closer look at what makes dentin bonding so much more complex than enamel bonding.
Finding the best way to achieve strong bonds on tooth structure is a continuously winding road dating back decades. The solution for adhesion to enamel came in 1955, when Michael Buonocore, DDS, discovered that acid etching enamel with 85 percent phosphoric acid improved acrylic bond retention.1
While enamel bonding didn’t become widely used until approximately two decades later, it revolutionized restorative dentistry. Today, phosphoric acid is still widely used (in 35-40 percent concentrations) to etch enamel to enable retention of resin-based adhesives. With just a few simple steps, a reliable enamel bond can be created.
“You can clearly say that bonding to enamel is easier than bonding to dentin,” Dr. Jason Goodchild, director of clinical affairs at Premier Dental, says. “We’ve known that since the 1950s-that the best bond occurs on enamel.”
But what about adhesion to dentin? This has been a greater challenge and has proven to be more difficult and less predictable.
“Bonding is so technique sensitive, and enamel and dentin are two very different surfaces,” Dr. Goodchild says. “Enamel is highly inorganic and has very little organic material. Dentin is 30 percent collagen and 20 percent water. This makes dentin a hydrated tissue, so you’re trying to bond-and bonding is hydrophobic-to a watery substance, which is inherently difficult.”
While achieving reliable bonds on dentin can be daunting, understanding the differences between enamel and dentin-and the challenges they create-can assist clinician’s in successfully executing consistent results.
The challenges of dentin
While enamel is composed of approximately 92 percent inorganic hydroxyapatite, dentin is only around 50 percent inorganic.2 Dentin’s structure is also more complex than that of enamel. Dentin contains fluid-filled tubules, and the hydroxyapatite crystals in dentin are not regularly arranged (as in enamel) but are random in distribution. In addition to being more complex in structure than enamel, dentin’s composition-and subsequently, its peremeability-varies by the depth and region of the tooth. These variations make dentin must less predictable and uniform than enamel, thus making it harder to ensure reliable bonds.
The success of a dentin bond relies heavily on the successful creation of the hybrid layer. The hybrid layer is the highly organic interface between the dentin substrate and the adhesive resin, a hydrophobic, acid-resistant collagen network that is exposed by etching.3 To create the hybrid layer, mineral components in the dentin are replaced by resin monomers, forming a polymer-collagen biocomposite. Dentin bonding can be achieved with etch-and-rinse or self-etch adhesives, but both techniques share a common goal: To remove the smear layer and minerals to expose the collagen network of the dentin, creating an avenue for the adhesive resin to infiltrate the collagen matrix and form the base of adhesion for the restoration.
Removing the smear layer is a particularly important step for forming a hybrid layer, as it can fill the dentinal tubules and form smear plugs, decreasing dentin permeability by up to 90 percent.4 This can prevent the adhesive resin from permeating into the dentinal tubules, posing a threat to bonding.
However, even with effective smear-layer removal and hybrid-layer formation, variations in dentin structure, dentinal tubule permeability and moisture can affect bonding, making achieving uniform bonds a challenging endeavor. These physiological variables, combined with limited procedural time, varying polymerization rates, and biodegradation of resin-dentin interfaces can lead to unpredictable results.3
“Whether we are talking about fillings or restorations on the direct side, or crowns on the indirect side, most of that bonding is into dentin, which creates challenges,” Dr. Goodchild says. “Enamel is easy-you etch enamel, you create an etching pattern, and then you have resin microtags and it locks itself in. That’s pretty simple.
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“But dentin is not so easy,” he continues. “The challenges of dentin include: Do we etch or not? How long do we etch for? What level of dryness or wetness does the dentin need? Plus, application times-is it one coat or multiple coats? There are so many factors involved in that process. There are so many things to consider.”
With four categories of dentin adhesives, there are indeed many factors to take into account. Falling under the etch-and-rinse and self-etch strategies, the categories include three-step etch-and-rinse; one-bottle etch-and-rinse; self-etch primers; and self-etch (or all-in-one, universal adhesives).
Each comes with its own pros and cons. For example, while much simpler than three-step systems, one-bottle systems sometimes lead to postoperative sensitivity due to polymerization shrinkage. And, while self-etch adhesives have lower bond strengths to both enamel and dentin, they are easy to use and result in less instances of postoperative sensitivity, which is a critical concern.
“Another thing that is worth noting is, because dentin is a living tissue, one of the things we’re always worried about is postoperative sensitivity,” Dr. Goodchild says. “So, how you treat dentin and how you mitigate sensitivity from both the direct and indirect sides. If you’re cementing a crown-a definitive restoration with all kinds of materials-you want to be careful not to cause post-op sensitivity, or, when it occurs, at least understand if it’s going to be permanent or just going to be a few hours of discomfort.”
Addressing enamel and dentin differences
Despite the differences between dentin and enamel bonding, it’s impossible to address one without the other.
“Dentin and enamel are always connected,” says Dr. Goodchild. “You’re always going to have both, even if it’s a crown prep where you just have the littlest sliver of enamel on the outside, and 99 percent of it is dentin. I try to treat each part individually-put a little etchant on the enamel part and keep the etchant off the dentin.
“So that’s the idea of selective bonding or selective etching-you treat them a little bit differently, even in the same tooth,” he explains. “That can add some procedural steps and procedural time, and if dentists are on cruise control and just do it all the same every time, that does create a bit of a challenge.
It’s easy to get on cruise control with enamel bonding, since it’s so much simpler than dentin bonding. Compared to the numerous steps necessary when addressing dentin, enamel bonding is a breeze. Isolation, application of acid etchant for 10-15 seconds, rinse, and apply bonding agent. But when dentin is involved, it’s important to take the extra steps, regardless of the headache.
“Wouldn’t it be nice if we had the one-size-fits-all solution where you could do everything the same way,” Dr. Goodchild laughs. “That’s why there are so many different generations of bonding agents. I think that has led to the move towards universal bonding, where the one bottle can do everything-total etch, self-etch, even no etch. It does everything for you.”
But, Dr. Goodchild cautions, even with universal bonding, the different substrates need to be treated individually.
“Personally, I don’t like to etch on dentin, and prefer a self-etch product like Clearfil SE Bond (Kuraray America), which is a two-bottle system,” he says. “But although I’m a no-etch-on-dentin kind of guy, I really believe we need to put etching on enamel.”
In the end, it’s important to react to the type of substrate that is in front of you, whether the prep be exclusively dentin, enamel, or a combination of the two, as every situation is different.
“I always believe that the best approach is a tailored approach to each clinical situation,” Dr. Goodchild concludes. “The take-home message is, no two clinical situations are identical, and you really have to take each one individually to get the best possible outcome.”