Ask Dr. Christensen

by Gordon J. Christensen, DDS, MSD, PhD

Question ...
Recently, I attended a continuing education course where the instructor stated that self-etching primers did not create adequate etch on enamel surfaces. He suggested I use standard phosphoric acid on enamel, even when I am using self-etch materials on the overall preparation. I have been using self-etching primers alone without any problem, and I would appreciate your comments on the validity of the instructor's statement.

Answer from Dr. Christensen ...

The allegation that self-etching primers do not adequately etch enamel is partially true. External enamel surfaces on a mature person contain significant amounts of fluoride. This layer is as thick or thicker than a human hair. The fluoride content resists acid etching, especially by products that are not as aggressive as phosphoric acid used for the "total-etch" concept. Etching uncut enamel surfaces with some brands of self-etching primers can result in a less aggressive etch of enamel than is desirable. On the other hand, etching cut enamel with self-etching primers results in very adequate bonds to enamel, equivalent to the "total etch" products. Clinical Research Associates recently published a newsletter that shares further information on this subject, and you may contact (801) 226-2121 for more information. I suggest that the "total etch" concept with phosphoric acid should be used for any situation where the major portion of the retention of the restoration is expected to come from the "bond" and not from mechanical retention produced by a bur. In my opinion, the clinical situations that require standard acid etch with phosphoric acid are veneers that have been prepped all in enamel, sealants, incisal or occlusal enamel walls of class V preparations, or any other area that requires major retention from the enamel etch.

I am not suggesting using the "total-etch" concept in typical class I or class II situations, or in any other location where mechanical retention makes use of self-etch very adequate. An obvious question that is related is - what do you do with a veneer prep that is mostly dentin with only a small amount of enamel remaining? I suggest that the "cut enamel" surface of the veneer prep should be selectively etched with standard phosphoric acid, using a controllable gel, such as Ultradent Products Ultra Etch. Wash the area rapidly, avoiding getting the diluted acid on the dentin surface. Place a self-etching primer over the entire preparation surface, including the etched enamel. If the preparation surface is mostly dentin, I suggest using bur retention on the lateral borders of the tooth preparation to augment the minimal retention afforded by the dentin bonding.

In summary, self-etched primers used on cut enamel provide adequate retention for most situations. However, when you find that most of a tooth preparation has been cut into dentin, and mechanical retention made by a bur or undercut is not present, I suggest the following technique. Use conventional acid etch on the remaining enamel. After the total etch has been washed off, accomplish self-etch on all of the tooth preparation, including the previously etched enamel. The result is optimum mechanical retention on the etched enamel and production of a non-sensitive restoration due to the remaining dentin plugs allowed to remain because of the self-etch of the dentin

Our video, V1584 "Veneers — All Types," shows the best methods to bond veneers using the concept described above. You can contact PCC at 800-223-6569 or visit our Web site at www.pccdental.com.

Question ...
The companies selling all-ceramic crowns have been promoting their products for multiple unit fixed prostheses. I am anxious about the ability of these restorations to serve, since I have had numerous failures over many years with all ceramic restorations. Can I trust an all-ceramic fixed prosthesis?

Answer from Dr. Christensen ...

I also have had many failures of all-ceramic restorations, but as I interpret the current state-of-the-art clinical research, we are into a new generation of all-ceramic restorations. Zirconia framework multiple-unit restorations have had significant clinical research in Europe, and the results have been highly encouraging. In those studies, there has been very little fracture of frameworks over a several year period. The zirconia frameworks appear to be competitive with or stronger than similar frameworks made of conventional alloys.

When would you consider using an all-ceramic fixed prosthesis? When patients have known allergies to metals, it is foolish to make metal-containing restorations. You can usually identify these patients because they have skin reactions to metal jewelry. Most of these patients are women. I suggest having a question on your health questionnaire asking if they have any skin reactions to jewelry. We should offer the alternative of all-ceramic restorations to any person answering this question positively. From my observations in practice and from results when polling hundreds of continuing education participants, I am convinced that the need for all-ceramic restorations is high.

In summary of the answer to your question, yes, I suggest that all-ceramic fixed prostheses are now available that are competitive with porcelain-fused-to-metal restorations. Representative brand names are Cercon from Dentsply and Lava from 3M ESPE, and others are coming.

Our newest video, V19-99 "Successful All-Ceramic Crowns And Fixed Prostheses", covers this subject thoroughly. You can contact PCC at 800-223-6569 or visit our Web site at www.pccdental.com.