Different ways to etch teeth for adhesive dentistry create different outcomes. We explain each and the benefits so you can make the best decisions possible for your restorations.
Etching a tooth's surface to prepare for bonding is a dental procedure that changed restorative dentistry. Since the 1960s, creating a simple mechanical bond with teeth by etching the surface paved the way for today's adhesive dentistry, making bonding, porcelain veneers, dental sealants, and placing orthodontic brackets possible.
However, many variables affect etching, from the tooth surface you need to etch, the type of etchant you choose, and how many bottles are included in the etching system. Today we take a look at some of these differences and why you should know about them.
The Difference Between Etching Enamel vs. Etching Dentin
Perhaps one of the most significant variables for etching's effectiveness is the tooth surface the dentist needs to prepare for bonding. Enamel, made of compact crystalline structure-hydroxyapatite, has a distinct composition from dentin, made of hydroxyapatite and collagen-infused together. Enamel has robust intermolecular forces and a high-energy surface, while dentin has weak intermolecular forces and a low-energy surface. Each requires a different type of etching to create a successful bond.1
Moreover, the dentin composition varies from the surface to the interior depths, which affects the bond you can achieve. Unlike enamel, dentin has a smear layer, organic contents, and fluid in the dentinal tubules. These tubules' density varies as you go deeper into the dentin, and the water content increases. The ability of the restorative material and adhesive system to permeate the intratubular dentin is what creates the bond. When the dentin suffers decay, it tends to mineralize and reduce permeability. Moreover, dentin thickens as it ages, reducing its permeability, even when "healthy."2
Since the 2 types of tooth structures are so different, the substrate you want to bond to dictates your protocol. Per Nathaniel Lawson, DMD, Ph.D., Assistant Professor and Director of the Division of Biomaterials for the UAB School of Dentistry, etching dentin and etching enamel are different tasks.
When etching enamel, Dr Lawson creates some surface roughness to achieve micromechanical retention of his cured bonding agent. In this case, he needs something acidic enough to dissolve the mineral content of enamel partially.
"Phosphoric acid is very capable of etching enamel. Some self-etch adhesives/primers can also create surface texture on enamel. However, some of the universal adhesives are not sufficiently acidic to create surface texture on enamel. With these materials, I etch the enamel separately with phosphoric acid," Dr Lawson says.
Etching dentin is a different story, Dr Lawson explains. Etching dentin with phosphoric acid partially demineralizes it and exposes collagen. The adhesive can then penetrate the exposed collagen and form a hybrid layer. This hybrid layer can be several microns thick and allows a solid bond to dentin.
"Self-etch adhesives can also partially demineralize dentin, but not to the same extent as phosphoric acid," he says. "But the monomers in many new universal adhesives are capable of binding to calcium in the mineral content of dentin. So even though they form thinner hybrid layers (about half a micron), they can form a chemical bond with some of the remaining minerals in the dentin."
Dr Lawson says that one of the technique-sensitive aspects of etching is over-etching dentin. If too much collagen is exposed, then the adhesive cannot completely infiltrate the collagen.
"This exposed collagen may be hydrolytically or enzymatically degraded," Dr Lawson says. "Therefore, the development of a self-limiting etchant would be beneficial. "
The Differences Between the Types of Etching Protocols
To respond to the etching needs of the tooth's surfaces, dental products manufacturers have released different products and protocols over the past few decades. There are 3 clinical routines associated with etching products: total-etch, self-etch, and selective-etch. Each has a different protocol, with advantages and disadvantages3:
Total-etch: In this protocol, the clinician puts etching gel on the entire tooth preparation, which can include enamel and dentin in some cases. While the total-etch advantage ensures the etching pattern on the preparation, the disadvantage is that the mixture of tooth surfaces requires different timing to achieve ideal conditions for bonding. In addition, if there is a problem with the total-etch technique on the dentin layer, patients can experience postoperative sensitivity.
Self-etch: This protocol involves using a single-step product that combines the etchant and the bonding agent, including a primer for the dentin layer, in one step. The advantage is that it is time-efficient and creates more predictable outcomes for dentin etching, but the disadvantage is that it doesn't do as well etching enamel.
Selective-etch: In this third etching protocol, the etching gel is placed on the enamel only, and the dentin surfaces are sealed instead of etched. The advantage is that it reduces the potential from postoperative sensitivity generated from problems in the dentin layer. Still, the disadvantage is that it could fail to etch some enamel surfaces enough to achieve a proper bond.
Selective-etch, the newest of the protocols, is an option for working surfaces composed of enamel and dentin. Also called universal adhesive systems, these are known as multi-purpose adhesives and take the 1-bottle approach to the different types of substrates. These newest versions of universal adhesives have different monomers incorporated into them than the previous versions to simplify the etching process to dentin. An example is Methacryloyloxydecyl Dihydrogen Phosphate (MDP), which is a hydrophilic monomer with mild-etching properties. MDP enables universal adhesives to be used with any etching technique. These new adhesive systems reduce microleakage, enhance the flow of resin into fissures, and decrease or eliminate postoperative sensitivity.4
So, Which One is Best for Etching?
Like many things in restorative dental care, the answer to which type of protocol to choose is answered with an unsatisfying version of, it depends. Per the Compendium of Continuing Education in Dentistry, it comes down to personal preference. However, the authors did go on to suggest that etch-and-rinse bonding systems (total-etch) were preferred for indirect restorations where a lot of enamel was still present, and self-etch adhesives were better than total-etch for direct composite restorations, mainly when there was a lot of dentin in the preparation surface.5
In other words, there are some indications where a total-etch will be more effective than a self-etch and vice versa. However, the literature suggests that selective etch and improved monomers that can create chemical bonds in the dentin layer are heading in the right direction.6