Ask Dr. Christensen

Aug. 1, 2003
My laboratory technician condemns the "double-arch" technique as substandard and inaccurate. I must admit I have had some failures when using this type of impression.

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 [email protected].

Question ...

My laboratory technician condemns the "double-arch" technique as substandard and inaccurate. I must admit I have had some failures when using this type of impression. What is your opinion about the comparison of the double-arch technique to standard impression procedures using a separate impression for the upper and lower arches?

Answer from Dr. Christensen ...

You are asking a biased, but experienced practitioner. I have personally seated thousands of crowns using the double-arch procedure with a success rate of nearly 100 percent. I attribute that success not only to an excellent laboratory technician, but also to my own adherence to several significant rules. I will relate those below:

1) Canine rise occlusion is preferred, and if a group function occlusion is present, less accurate occlusal registration and more adjustments at seating are expected.

2) One tooth preparation only is preferred in a double-arch impression. Fortunately, 80 percent of fixed prosthodontics in the U.S. are single units. If two units are necessary, it is desirable for stability to extend the impression further around the arch toward the opposite side. I suggest making the impression nearly to the midline. I prefer to use full-arch custom trays if three or more units are required.

3) There must be adequate space distal to the most posterior teeth to allow the connector of the two sides of the tray to be in place without inhibiting mouth closure.

4) Occlusion should be stable, without significant shifts either laterally or from centric relation to centric occlusion.

5) When the impression is removed from the mouth, there should be no contact between the tray and tooth structure. Soft-tissue contact with the tray is usually acceptable.

6) Many laboratory technicians misunderstand pouring of the casts. The arch with the tooth preparations is poured first and allowed to set. The base of that arch is poured next, followed by the opposing arch, which is then mounted to a simple articulator. The laboratory procedures can be accomplished in two or three separate pours; I prefer two pours.

7) If stone or plaster runs anterior or posterior to the teeth that are in the impression, the excess material must be removed or the occlusion will be too high.

I would not change from the double-arch technique to any other currently available technique. It is simple, fast, accurate, and kind to patients.

Our newest video is on double-arch technique and all of the new concepts with it. The video will update your technique and educate your staff. C902A "Simple Double Arch Impressions for Dentists And Technicians" (VHS) is available from Practical Clinical Courses. Please contact (800) 223-6569, Fax (801) 226-8637, or visit our Web site at

Question ...

SS White recently introduced a new bur, the Smartprep. The manufacturer states it is designed to remove dental caries without disturbing healthy dentin. Is this concept a viable one, or should I continue to use burs and spoon excavators to remove caries?

Answer from Dr. Christensen ...

Clinicians and researchers have promoted several beliefs about removal of dental caries during my career. These concepts vary considerably and range from radical removal of all apparently carious tooth structure to allowing frank caries to remain under restorations. Quite obviously, there is not just one correct answer to the posed question.

Many clinicians remove caries with a slow-speed round bur, using gentle force and a "painting" motion. This concept is successful, but it requires changing to a low-speed handpiece, placing a bur in it, and changing tactile feeling from the light touch of high-speed air rotor cutting to a lower speed, gentle, more moderate cutting force.

Some clinicians prefer to use a sharp spoon excavator to remove caries. This concept does not require changing burs, and it has other advantages. Clinicians soon develop delicate tactile sense about the hardness of dentin. An experienced clinician knows nearly exactly how much force to place on the spoon to cause the optimum removal of carious dentin. Some clinicians prefer to leave a slight amount of "leather-like" carious dentin, knowing that it is carious, but anticipating subsequent remineralization of the carious tooth structure. Leaving carious dentin avoids pulp exposures in many cases. Other practitioners scrape the dentin until all soft material is removed, often causing a pulp exposure. Which technique is best? I prefer the conservative one, having had routine success when a minimal amount of "leather-like" dentin is left under the restoration.

In severely carious teeth, indirect pulp capping is indicated; considerable carious dentin is left under a provisional restoration, and a final restoration is placed after several weeks or months of remineralization.

The Smartprep bur is made of relatively soft polymer, and it is a clever idea. Used on a low-speed handpiece, it provides removal of carious dentin to the level it can be cut with the soft bur. The hardness of carious dentin ranges from 0 to 30 on a Knoop hardness scale, and most normal, non-carious dentin has a hardness of about 70 or more on the Knoop scale. The bur is said to cut material up to 50 Knoop hardness. However, hardness and wear resistance are not necessarily related. Clinicians must use the bur a few times before finding the level of caries removal it allows in relation to their beliefs about caries removal. This level may or may not agree with your own personal beliefs of how much carious dentin to leave or remove. When used correctly and with low cutting forces, the bur accomplishes very adequate removal of moderately wear resistant carious dentin. Of course, the bur wears out rapidly when it is used on sound dentin or enamel. In some situations where local anesthetic is contraindicated or not desired, the Smartprep bur can be used without anesthetic.

In summary, this novel new bur concept is relatively inexpensive and good if you are dissatisfied with the unpredictability of your current caries removal technique.

A popular recent PCC video related to this subject is C102B "Sealants And Tooth Cutting Techniques" (VHS). This up-to-date video compares the viability of various tooth cutting concepts, including air abrasion, with leaving undetected caries under sealants. For more information, please contact Practical Clinical Courses at (800) 223-6569, Fax (801) 226-8637, or visit our Web site at

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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