To Etch or Not to Etch?
Robert G. Ritter, DMD
Adhesive dentistry came into existence in the late 1950s. By today’s standards, it was crude and only allowed for the placement of acrylics with phosphoric acid conditioner. The methodology was unique: place an acid conditioner to allow micromechanical adhesion of another material. The results were limited and did not last over the long term. However, they did set the stage for the age of adhesive dentistry.
While adhesive dentistry has continued to evolve, one of the limiting factors has been our ability to acid condition the surface of tooth structure—either dentin or enamel. Back in the mid 1980s, the dental community was using mild acid conditioners to avoid harming the dentin. We know that acid conditioning the enamel gives us the best long-term surface for adhesion due to its high inorganic content. Dentin, on the other hand, has a high organic content, thus making long-term bonding more questionable. Bonding to dentin was even more unstable due to the inconsistencies of bonding to the smear layer that was left on the tooth after grinding with diamonds and carbide burs. The first dentin bonding agents did not remove the smear layer.
The next generation of adhesives changed the game by using the phosphoric acid to remove the smear layer, giving the primers and adhesive resins the chance to penetrate into the dentin, surrounding the collagen fibrils, and curing the plastic resins, creating a hybridized plastic dentin. While this approach was initially met with great skepticism, it was eventually proven to be an effective method in laboratory studies. Long-term in vivo studies have shown this approach to have issues. Bond degradation by matrix metalloproteinases (MMPs) and water trees can affect the long-term bond of restorations, thus calling into question whether acid conditioning with phosphoric acid on dentin is the best long-term solution.
In the mid 1990s, Japanese manufacturers released a new class of adhesives that did not require phosphoric-acid application before the adhesive was applied. They were called self-etching primers. This prevented complete removal of the smear layer; rather the smear layer was incorporated into the hybrid layer. Newer approaches during that time were to pre-etch the enamel with phosphoric acid, prepare the tooth for the restoration, then apply the self-etching primers to all tooth surfaces to gain the maximum advantage in bond strength, while keeping the possibility of postoperative sensitivity to a minimum.
With the advent of the newest universal adhesives, dental manufacturers have given us materials that can now be used either with or without phosphoric acid before the application of the all-in-one adhesives. This gives clinicians a choice. To avoid the MMP formation, and not completely remove the smear layer, we can now selectively etch the enamel only, where we get our best bonds, and then utilize the self-etching adhesives on every surface. In turn, this will decrease the potential of postoperative sensitivity, gaining the best long-term bonds to both enamel and dentin, and also potentially using the same adhesive for both direct and indirect procedures. While the long-term studies have not been realized to this point, this approach seems to be the most prevalent among dental practitioners and clinical researchers. With time, this approach will either be fully legitimized or altered for the benefit of our patients.
About the Author
Robert G. Ritter, DMD, is in private practice in Jupiter, Florida. He is a member of the American Academy of Restorative Dentistry, the American Academy of Esthetic Dentistry, The American Academy of Cosmetic Dentistry and the International Association of Dental Researchers.