Spotlight on Adhesion and Bonding
Robert A. Lowe, DDS, FAGD, FICD, FADI, FACD, FIADE, FASDA
Diplomate, American Board of Aesthetic Dentistry
Charlotte, North Carolina
Restorative dentistry started moving away from cements and toward adhesion in the early- to mid-1980s, when bonding first to enamel and then to dentin was successfully achieved. Since that time, one goal of clinical and material science has been to simplify the placement process for composite resins and the delivery of indirect tooth replacements using resin cements. Of paramount importance is maximizing the adhesion of restorative material to tooth structure while decreasing technique difficulty and the possibility of postoperative sensitivity that can occur due to the improper use and application of bonding chemistries.
Dental amalgam is not affected by moisture and can be rapidly condensed and carved into a completed restoration. The direct placement of resin-based composite, as well as cementation of a crown with adhesive protocols, requires many more steps and attention to proper technique to achieve predictable clinical results. These include conditioning with acidic solutions to make tooth surfaces more micromechanically retentive and the application of primers and adhesives to form a “hybrid zone” where these “plastic” materials lock onto the tooth surface. For many types of direct composite resins, proper technique also includes layering the material required to complete the restoration. In short, adhesive bonding procedures may involve many independent procedural steps that must be carefully executed to ensure good clinical results.
Current Etching Protocols
Total-etch is a technique that has been used in adhesive dentistry procedures since it was introduced by Fusayama in 1980 and described by Kanca as the “wet bonding technique” in the early 1990s. Using 37% phosphoric acid simultaneously on enamel and dentin for a total of 15 seconds, the enamel and dentin is sufficiently demineralized for the adhesive process. To enhance the bond strength to dentin and decrease the chance of postoperative sensitivity, the dentin should remain moist. Smear layer and smear plugs are removed with total etch total-etch the resin to penetrate into the dentinal tubules and “lock on” to the tooth surface. A micromechanical lock is also formed to the etched enamel surface to help retain the restoration and prevent microleakage.
It is the author’s belief that self-etching adhesives were developed because some patients experienced postoperative sensitivity from total-etch procedures. While this was most likely a result of etching too long or not getting enough of the hydrophilic primers into the etched dentin surface, it was a common complaint heard by many dentists using the total-etch technique.
Self-etching bonding agents do not remove smear layer or smear plugs, but rather modify the smear layer. They bond well to dentin; however, the mild acidity of these products does not etch the enamel as well as phosphoric acid when a total-etch procedure is used. Currently, there are some newer self-etch products that are etching the enamel better than previous generations. It is still generally thought that despite the reduced technique sensitivity in placement and less chance for postoperative sensitivity in a self-etch technique, the marginal seal is not as good when compared with total-etch protocols because self-etch materials typically do not etch uncut enamel as well as phosphoric acid.
The selective-etch technique, which uses 37% phosphoric acid etch on enamel only, along with a self-etching bonding agent for both the enamel and dentin, gives the clinician the “best of both worlds.” Phosphoric acid on enamel etches sufficiently to increase micromechanical retention and seal of the tooth-restoration interface. In addition, the smear layer and smear plugs are left intact, and the smear layer is modified to bond to the surface of the dentin, decreasing the possibility of postoperative sensitivity due to technique or operator issues. In general, it is the opinion of this author that in cases in which enamel surrounds the entire cavosurface margin of the cavity preparation, the selective-etch technique will provide excellent clinical results with less chance for postoperative sensitivity issues with the patient.