Ask Dr. Christensen

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 info@pccdental.com.

by Gordon J. Christensen, DDS, MSD, PhD

Q I have used two brands of zirconium-oxidebased, all-ceramic bridges and crowns and have had the same challenges with both brands. The margins fit well, but the crowns or bridges rock in a facial-lingual direction, and I must be careful when cementing them to avoid having the restorations rock and open the margins on the side opposite the rock. What can be done to avoid this situation, or is it something that cannot be avoided?

A You are not alone in your frustrations. I have heard the same question in many of the CE courses I have delivered around the country, and I have experienced it myself. I will explain the reason for the internal misfit and what you can do to avoid or reduce the continuation of this annoying problem. I have personally found that seating some zirconia substructure restorations can require significantly more time than seating similar restorations made from porcelain-fused-to-metal because of the uncertainty involved when seating rocking restorations.

In discussions with the respective zirconia all-ceramic restoration companies, they assure me that the problem can be overcome by proper laboratory technician control of the computer program when planning the milling of the substructure. As you know, when you request the fabrication of a zirconia-based, all-ceramic restoration, you make a conventional elastomer impression of the tooth preparation(s); an opposing arch impression, usually in alginate; and an accurate interocclusal recording.

The laboratory technician pours the cast in appropriate stone (Densite), scans your die(s), and constructs a digital image of the die. Any tooth preparation has some irregularities caused by the irregular diamond particles on the diamond-rotating instrument, the presence and removal of dental caries, previously placed restorations, or many other causes. Additionally, some tooth preparations may have mild to severe undercuts and other irregularities for a variety of reasons. If these features were left in the scanned image of your tooth preparations, the resultant milling of the die would be difficult at best, and the resultant substructure might or might not be able to be taken off the die.

You are familiar with "die spacing," used when making a metal casting for a crown. The technician usually places several layers of lacquer or fingernail polish on the surface of the die, resulting in a layer of "paint" about 50 to 100 microns thick. This space is supposedly created to allow space for the cement and the escape of the cement during restoration cementation.

To allow adequate margin fit, all-ceramic, milled restorations are made to have a smooth surface internally. They are not made to fit the undercuts and irregularities on the die. The technician sets the computer to make your margins fit very well, but a space is created between your tooth preparation and the internal surface of the milled zirconia substructure. The amount of space is under the control of the technician. If the space is large, the margins of the restoration may fit well, but the internal surface will not fit well. The result is the "rocking restoration."

I agree that this problem is a significant one, and technicians should be aware of the difficulty dentists experience when a "rocking restoration" is returned to them for seating. When a rocking restoration is encountered, I suggest checking the fit of the restoration on the facial and the lingual surfaces, while firmly holding the restoration in place. Assuming that the restoration(s) fit well on the facial and lingual surfaces, I advise cementing every other crown while evaluating the facial and lingual fit, and then adjusting the intermediate crown contact areas to provide optimum contacting surfaces.

If rocking restorations are a continuing problem for you, I suggest that you meet with the laboratory technician and explain your challenge. You will find that the technician can change the computer program to make the internal surface of the restorations fit better, and reduce the rocking problem.

Our organization, Practical Clinical Courses (PCC), has several videos that show zirconia-based, all-ceramic restorations with close-up, detailed clinical video. Two that are directly related are V19-15, "3M ESPE Lava"!: The Beautiful PFM Replacement" and V19-20, "IPS e.max¿, A Complete All-Ceramic Restorative Solution." For more information, please contact Practical Clinical Courses at (800) 223-6569 or visit our Web site at www.pccdental.com.

Q I have heard that zirconia is zirconia, and that I should not be concerned about the source of the zirconium oxide. Also, I have been told two completely opposite things about sandblasting -- some say I should and some say I should not sandblast the internal surface of zirconia frameworks to increase retention in situations where tooth preparations could use more retention. What are the correct answers to these questions?

A First, zirconia is not zirconia. I have discussed the question with several manufacturers of zirconium oxide for milling, and I have received the same answer. The scientists from each company have developed their own desired characteristics of their zirconium oxide materials. As you know, some are pigmented and some are not. There are also other dissimilarities. Some labs are telling dentists that any zirconia is adequate. I would be wary of this statement.

I suggest that you discuss the matter with your technician, and that you recommend that he or she use the zirconium sold through the company you have selected for the prostheses.

Relative to the second portion of your question, I have a more complex answer. Each of us encounters clinical situations in which the tooth preparations have inadequate length. Most authorities agree that about 4 mm of tooth structure or build-up from the gingival margin to the occlusal surface is a minimum amount of structure for optimum retention. Additionally, many tooth preparations are by necessity less parallel than desirable. Currently, popular cements do not bond well to zirconium oxide, and other retentive methods should be considered.

Make the tooth preparations as long as possible, 4 mm or more. Make the tooth preparations as parallel as possible. If these characteristics are inadequate, do the following: Tell the laboratory technician that in the "green" unsintered stage -- before final firing of the substructure -- diamond scratches should be placed in the internal surface of the substructures. These scratches make micro cracks in the structure, but they fuse or "heal" when the final firing of the substructure is accomplished. The micro cracks are not present in the fired substructure, but roughness made by the rotary diamond instrument is still present in the internal surface of the substructure, and mechanical interlock is present.

I suggest cementation of adequate retention zirconiabased restorations with conventional resin-modified glass ionomer (RelyX"! Luting or GC FujiCEM). This type of cementation provides strength, fluoride release, and some bond to tooth structure. I suggest crowns lacking retention be cemented with resin cement (RelyX"! Unicem, Multi-link Automix, PANAVIA"! F) for additional strength.

Sandblasting the internal surface of zirconia-based restorations with standard 50-micron-diameter aluminum oxide at 80 psi is contraindicated. Some companies recommend only mild blasting of the internal surface of crowns with low pressure applied to aluminum oxide, but the "evidence-based" research is on both sides of the questions. Until further data and clinical experience are available, I recommend blasting very lightly with low air pressure aluminum oxide for the purpose of removing organic debris left from trying the restorations in the mouth.

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 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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