Ask Dr. Christensen

Jan. 1, 2008
In this monthly feature, Dr. Gordon Christensen addresses the most frequently asked questions from Dental Economics readers.

by Gordon J. Christensen, DDS, MSD, PhD

In this monthly feature, Dr. Gordon Christensen addresses the most frequently asked questions from Dental Economicsreaders. If you would like to submit a question to Dr. Christensen, please send an e-mail to [email protected].

Q I see many ads for so-called “alginate replacements.” Are these materials better or worse than alginate for the many uses of alginate impressions? Alginate impressions make a mess in my office and require significant clean up by my staff.A Alginate impressions are made hundreds of times each month in the majority of dental offices. They are usually made by dental assistants or dental hygienists, many of whom have minimal education on the materials or techniques to produce adequate impressions. As any dentist would admit, many of the alginate impressions made by either dentists or staff members are inadequate in one or more ways. The errors observed in making alginate impressions are usually voids or folds, air bubbles, lack of border molding, debris left in the impression that results in a rough cast, separation of the alginate from the impression trays, or distortion related to late pouring of the casts. In answering your question, I will consider these problem areas in alginate impressions and compare them to alginate replacements.

Most of the alginate replacements are vinyl polysiloxane (VPS). Since you likely use this material for your fixed prosthodontic procedures, undoubtedly you are familiar with it. The impression-material companies have reduced the cost of alginate replacements to make them as close to the cost of alginate as possible. While the cost of alginate replacements is not as high as for conventional VPS used in all areas of prosthodontics, the cost is still considerably more than alginate. Currently, a typical full-arch alginate impression costs about 90 cents. A representative group of five alginate replacements cost from 2.5 to 4.5 times more than alginate for the same quantity. Do the advantages of using the VPS justify the obvious increase in cost? Let’s compare the two materials.

Voids or folds occur just as frequently with VPS as with alginate. There does not appear to be a major advantage with this challenge.

Air bubbles incorporated into the impression material are just as frequent with VPS as with alginate. In fact, there is a possibility of more bubbles with some of the VPS materials because of their hydrophilic nature.

Lack of border molding relates to the person making the impression, and not to the specific impression material used.

Debris left in the impression relates to the person making the impression failing to take the time to wash it out before pouring it. It has nothing to do with the impression material used.

Impression material separating from the tray is a result of inadequate holes in the tray and/or failure to properly use adhesive. Again, this problem is related to the person making the impression, not to the impression material used.

Distortion of the impression material before pouring is a definite problem related primarily to alginate impressions. VPS materials are highly superior to alginate in this regard since they can be poured at any reasonable time after making the impression.

In my opinion, VPS is superior to alginate in the four following characteristics:

  1. Routine excellent duplication of surface detail
  2. Ability to pour at any time
  3. Ability to be disinfected without distortion
  4. Lack of mess when making the impression

If you feel these positive characteristics outweigh the additional cost of the VPS materials, you have your answer. Most dentists still use alginate.

For detailed information on this subject, please see our newest DVD, V1927 — “Alginate Impressions — Predictable and Accurate.” Contact Practical Clinical Courses at (800) 223-6569 or visit our Web site at for more information.

Q I am confused about the various “generations” of bonding agents and which is the best to use with resin-based composite. It seems that every time I have the opportunity to read for awhile, I see another “generation” of bonding agents advertised to be better than the previous generation. What do you suggest? Is there any reason to change to another generation if I am having success with what I am using? A This is a confusing and market-driven question. Because it is, I am going to lead you through a series of thoughts and questions before I give you my candid answer to your question.

Why are you using a bonding agent? If you are a typical practitioner and are aware of the published papers on this subject, you are probably using the bonding agent primarily to prevent postoperative tooth sensitivity. Any of the generations of bonding agents, when used properly, will accomplish that need; however, some of the early generations were more difficult to use than many of the current ones.

I remember using an old product for several years, Scotchbond™ Multipurpose by 3M™ ESPE™. If I used all three components (acid etch, primer, and bond) and used them exactly as the manufacturer recommended — and if I placed two layers of bond, curing between the two — I had no postoperative tooth sensitivity. In fact, when a newer version hit the market, I wondered why I should switch to the new product. Scotchbond Multipurpose was a “total-etch” product, a category that has fallen out of use in North America because many dentists reported significant postoperative tooth sensitivity. This, presumedly, is because of improper or incomplete use.

Are you using the bonding agent to provide a bond to enamel and dentin? As you have probably heard me discuss in the past, your expectations for bond on enamel are justified. If you expect long-term bond on dentin, you will be disappointed. Anyone taking ceramic veneers off enamel and dentin knows that removal from enamel is extremely difficult. Meanwhile, removal from dentin requires only a brief and mild force from a hand instrument. Some in-vitro research projects in the literature appear to refute my previous statement, but the temperature changes in the mouth, the masticatory forces applied to dentin bonds, and the subsequent degeneration of dentin bonds speak for themselves. The various generations of bonds have improved the reported in-vitro bond values to dentin. But the bond to enamel, which has always been excellent, is about the same. In my opinion, the numerous generations of bonding agents do not offer clinically significant changes in their reported and observed bonds to enamel and dentin.

The various generations of bonds have evolved from three bottles to two bottles to one. Most chemists report that they would prefer to have the various ingredients of bonds in separate bottles, but some dentists report that they like simplification of the bonding procedure by reducing the number of bottles and applications.

Self-etch bonding agents have been proven to reduce the incidence of postoperative tooth sensitivity, and I use them myself.

Now, I will provide my answer to this confusing subject. If you are pleased with the bonding agent you are using, your restorations are staying in place, and patients are not experiencing postoperative tooth sensitivity — continue to use what you are using! Currently, I am using two-component, self-etch products. Occasionally, I will use a one-component self-etch product for indirect restorations. I will not change what I use just because there is another “generation” on the market.

Our new video on nanofill resin-based composites and techniques includes live, close-up demonstrations of all classifications of resin-based composites and the respective bonding agents.

For more information on V3519, “Predictable, Non-sensitive Resin-Based Composite Restorations,” contact Practical Clinical Courses at (800) 223-6569 or visit our Web site at

Dr. Christensen is a practicing prosthodontist in Provo, Utah, and Dean of the Scottsdale Center for Dentistry. He is the founder and director of Practical Clinical Courses,
an international continuing-education organization initiated in 1981 for dental professionals. Dr. Christensen is a co-founder (with his wife, Rella) and senior consultant of Clinicians Report (formerly Clinical Research Associates), which since 1976, has conducted research in all areas of dentistry and publishes its findings to the dental profession in the “Gordon J. Christensen Clinicians Report” monthly 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.

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