Ask Dr. Christensen

July 1, 2006
In this monthly feature, Dr. Gordon Christensen addresses the most frequently asked questions from Dental Economics® readers.

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 e-mail to [email protected].

Question ...

When cementing Lava or Cercon® all-ceramic crowns, I find that the restoration margins fit very well, but the restoration rocks back and forth when lateral force is placed on it before cementation. Why is this? Is the internal fit of these all-ceramic restorations similar to porcelain-fused-to-metal crowns?

Answer from Dr. Christensen ...

You are correct in your statement that the marginal fit of these zirconium-oxide supported crowns is excellent. The computer-directed milling process produces margins that are often superior to the well-known fit of PFM crowns. However, the internal fit of computer-directed milled crowns is, of necessity, made to be somewhat loose. The design of these crowns is purposely developed to have a slight internal space between the tooth preparation and the internal surface of the ceramic crown.

What does that internal space mean to you when seating the crowns? Check the fit of the crown as usual. Assuming it fits, cement the restoration. Hold it in place with force directly down the long axis of the tooth preparation, until the initial cement set has occurred or you have light-cured the cement depending on the type of cement you have used. At that time, you may release the force from the restoration.

If you direct the patient to bite on a cotton roll or other object to hold the restoration in place while the cement sets, there is a strong possibility that the restoration will not fit somewhere on the margins. It will have been pushed in the direction of the force until the internal wall of the restoration stops the movement, and the margin will not fit.

Assuming a careful cementation procedure has been accomplished with the cement of your choice, these all-ceramic restorations appear to be among the best tooth-colored restorations dentistry has had to offer the public, and they are competitive with PFM crowns.

One of our newest videos, V19-15 “3M ESPE Lava - The Beautiful PFM Replacement,” shows, step-by-step, the entire clinical procedure for the zirconium-oxide supported crowns and fixed prostheses in close-up video. For more information, contact Practical Clinical Courses at (800) 223-6569 or visit our Web site at

Question ...

I have observed that the wear of Class I and Class II composites is more than enamel, and that after a period of service in the mouth, there is a step from the remaining enamel to the resin restoration in most cases. What is the significance of this wear?

Answer from Dr. Christensen ...

You may have noticed that these materials have improved over the years, and the step you described has become less with the newer generations. The original resin-based composites that were promoted for Class II use had high wear, often described as “plucking,” in which the glass-filler particles were released from the body of the material as the resin wore while chewing. The apparent result was significant restoration wear. The clinical significance was often severe. As the intracoronal portion of the restoration lost contour, the restored tooth and the opposing tooth extruded into one another. This extrusion created the potential for the development of abusive nonworking contacts during chewing. Nonworking contacts can cause cusp breakage and temporomandibular disorders. The described situation was essentially the collapse of the occlusion caused by excessive wear of the resin-based composite.

As each new generation of resin-based composite has come onto the market, many of the brands have improved wear resistance. How has the wear resistance been improved? The overall size of the glass-filler particles has been reduced. More microfill particles (usually silicon dioxide) have been incorporated into the resin. The newer nanofill or nanohybrid resin-based composites have more particles in the sub-100 nanometer range. The percentage of filler particles has been increased. These changes have resulted in increased wear resistance.

An older product that has shown low wear over its many years of service in Class I and II locations is Heliomolar from Ivoclar Vivadent. This product has been classified as a microfill resin. The term “microfill” is confusing because the microfill resins of 25 years ago would probably be called nanofill resins if they were introduced today. A newer resin-based composite that also is showing excellent wear resistance is 3M ESPE’s product, Filtek Supreme Plus. This resin is being called a nanofill and has nanofill particles throughout the body and incisal materials. Many other new products also have good wear-resistance, and their physical characteristics should be considered by practitioners as they attempt to select a new resin-based composite.

I encourage manufacturers to continue to work on making the wear of these materials exactly match the wear of enamel. Currently, the only material that matches enamel wear closely is Type II gold alloy, and its use in dentistry is unfortunately decreasing.

Most of the current, so-called universal-use, resin-based composites wear slightly more than enamel, and the step you describe will be visible after a few years of service. However, it is so small that the clinical significance is minimal to none in most cases, in my opinion.

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.

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