Ask Dr. Christensen

by Gordon J. Christensen, DDS, MSD, PhD

In this monthly feature, Dr. Gordon Christensen addresses the most frequently asked questions from Dental Economics readers. If you would like to submit a question to Dr. Christensen, please send an email to info@pccdental.com.

Question ...
Lately, some of my Maryland Bridges have been coming off. I am using standard acid-etching and a typical popular American brand of bonding agent (acid etch-wash-prime-bond). Am I using the wrong materials? What are the best materials for bonding Maryland Bridges in 2001?

Answer from Dr. Christensen ...
The most likely reason for debonding of Maryland Bridges is inadequate tooth preparation. I recommend placing grooves and/or box forms in the enamel surfaces of the abutment teeth. These retentive features, combined with a good bonding agent and a current generation resin cement, provide adequate retention for most Maryland Bridge situations.

Originally, recommendations for Maryland Bridge preparations did not include the grooves and box forms. As a result, retention was primarily obtained from the cement's retention to the acid-etched surfaces of the tooth. Often, this retention is not enough. A significant increase in retention for the fixed prosthesis is provided by addition of shallow retentive and resistance forms in the enamel. These retentive features resemble the typical configuration of a standard three-quarter crown tooth preparation, but they are much more shallow.

Assuming that you have produced some retention in the tooth enamel, selection of the resin cement is not as critical. The most popular resin cement for Maryland Bridges is Panavia (Kuraray) in its various forms, the most current of which is Panavia F, used with the self-etching primer, ED Primer (Kuraray). This dual cure resin cement is well proven to provide a mechanical as well as a molecular bond to sandblasted metal, as well as a minimal film thickness and acceptable strength properties. An opaque form of Panavia F is available if you need to block the color of the metal. Base metal has an excellent chemical bond to Panavia. If additional retention is needed for noble or high noble metal frameworks, tin plating of the sand blasted metal will double the bond strength of Panavia cement to metal as shown in Clinical Research Associates studies. The 4-meta cement, C & B Metabond (Parkell), has also shown an impressive bond and clinical success for cementation of Maryland Bridges.

One of our recently produced videos, C900B - "Bonding Indirect Restorations Without Sensitivity" directly relates to this subject and shows the clinical technique.

In summary, with adequate tooth preparation and use of a current generation bonding agent and resin cement, Maryland Bridges should be highly successful from a retention standpoint.

Question ...
My lab technician and I have had discussions regarding the type of impression tray best suited for use with vinyl polysiloxane (VPS) impression material. What is your opinion on this, and is there any research on the subject?

Answer from Dr. Christensen ...
The most commonly used full arch impression concept in the United States is a stock resin tray with a putty initial impression first, followed by a wash of a lighter viscosity material. The putty is either allowed to set before the wash material is placed and the impression made, or the putty and wash material are allowed to set at the same time (so called simultaneous impression). Popularity does not necessarily mean that these techniques give the best results. Impressions made as described above seldom have excellent rigidity, because the plastic trays are flexible. If the putty is allowed to set first, distortion of the underlying set putty material can be present if too much force is used when the wash material is inserted. However, one advantage of this technique is that the cast can be removed easily from the impression because of the flexibility of the tray.

Some dentists prefer metal perforated impression trays for final VPS impressions. Impressions made in metal trays have a somewhat better chance for accuracy than those made in plastic trays; however, the impression material is difficult to remove from the tray. Also, because of the overall rigidity of the metal tray impression combination, the chance of fracturing the stone cast is present. Many dentists think that impressions made in stock trays are less expensive than those made in custom trays, but that assumption is incorrect. The cost of a VPS impression made in a stock tray is double that of an impression made in a custom tray. Counting the time for a dental assistant to make a custom tray and all of the materials needed, the custom tray and impression material cost range from $4.49 to $8.89, while the stock tray and impression material cost range from $6.60 to $18.60. What type of tray and impression material do I prefer? I have used the following technique for thousands of units of crowns and fixed prostheses with almost no misfits.

1. Preliminary cast made from alginate impression made by a dental assistant or dental hygienist on the diagnostic appointment.

2. Triad Light Curing Tray (Dentsply) made by a dental assistant in a few minutes between the diagnostic appointment and the tooth preparation appointment.

3. Impression made in two viscosities of VPS (heavy viscosity in the tray and medium viscosity in the impression syringe). The impression materials are inserted together, and allowed to set simultaneously. The brand of VPS is up to your own judgment. For years, I have used the GC America brand, Exaflex. However, Clinical Research Associates' research has shown that numerous brands of VPS are comparable in their accuracy and stability (see the CRA website www.cranews.com for details).

At this time, most of us are making more conventional crowns and bridges than in the past, as well as more implant-supported crowns and fixed protheses. The need for predictable impression accuracy and stability is present. Custom light curing trays and VPS impression materials provide impressions that are unprecedented and unexcelled in their ability to produce predictable impressions and the resultant well fitting restorations. For additional information on making perfect impressions, please see my recent video C101A - "The Perfect Impression," which shows my favorite successful techniques.

Dr. Christensen is a practicing prosthodontist in Provo, Utah. He is the founder and director of Practical Clinical Courses, an international continuing-education organization for dental professionals initiated in 1981. Dr. Christensen is a co-founder (with his wife, Rella) and senior consultant of Clinical Research Associates, which, since 1976, has conducted research in all areas of dentistry and publishes its findings to the dental profession in the well-known CRA Newsletter. He is an adjunct professor at Brigham Young University and the University of Utah. Dr. Christensen has educational videos and hands-on courses on the above topics available through Practical Clinical Courses. Call (800) 223-6569 or (801) 226-6569.

Dr. Christensen's views do not necessarily reflect the opinions of the editorial staff at Dental Economics.

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