Ask Dr. Christensen

Oct. 1, 2007
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].

Q I am concerned about the airborne microorganisms present in the aerosol produced when accomplishing many dental procedures. Breathing these bacteria all day cannot be good for me or my assistants. Is there any way to control or reduce the hazard of airborne microorganisms while cutting teeth or using an air-abrasion unit or an air-slurry polisher?A Dentistry is one of a very few professions or vocations that has the challenge you described. We place our face directly in the operating field, and in spite of using face masks, we breathe many of the organisms that are present in the aerosols produced by the air/water spray of the dental handpiece and other procedures. The challenge is further increased by the fact that few dentists or staff members wear a face mask properly. Most practitioners and staff persons do not fit their face masks tightly to their faces, thus allowing airborne microorganisms to enter into the breathing zone. Additionally, dentists and staff persons wear so called "scrubs" as clinical attire. These are short-sleeved, low-neck underwear usually worn by surgeons. The airborne microorganisms are unavoidably spread over the naked arms and neck of practitioners, unless long-sleeve, high-neck clinical gowns are worn. If scrubs are worn, a thorough washing of the exposed body parts should be done after performing clinical procedures.

What can be done to reduce the quantity of airborne microorganisms generated during use of handpieces, air abrasion units, or air slurry polishers? Does it not make sense to reduce the quantity of organisms present in the patient"s mouth before starting the procedures, thus reducing the quantity of organisms available to be incorporated into the aerosol?

I suggest the following simple and well-proven methods to reduce the microorganisms:

1) Clean the teeth. Usually, before definitive restorative treatment is started, a thorough scaling and polishing of teeth is completed. This procedure may be a separate appointment or part of the first restorative appointment. In most cases, the restorative appointment is planned for a few days after the prophylaxis procedures have been completed. However, plaque and debris accumulate rapidly on the teeth after the prophylaxis appointment. Sometimes patients require several appointments to accomplish a series of restorative procedures, and each time the patient comes in for additional treatment, a new coat of debris is on the teeth. Before doing restorative procedures, I suggest removing all visible accumulated debris using a rubber cup and standard prophylaxis paste. The task of removing the debris may be legally delegated to qualified dental assistants in most geographic locations. I suggest routinely doing this simple but effective staff procedure before any restorative appointment. The result is a significant reduction in organisms available for incorporation into airborne aerosols.

2) Use a preoperative mouth rinse. Most dentists do not have patients rinse their mouths with mouthwash before starting treatment procedures. It is very easy to provide a mouth rinse for patients before restorative or surgical procedures. What type of mouth rinse is most effective? Almost all of us use chlorhexidine gluconate 0.12 percent for various purposes. This is an extremely effective rinse for preoperative reduction of oral microorganisms. Two other commonly used and readily available rinses are Listerine¿ or Crest Pro-HealthTM. Both rinses have proven effectiveness, and their current formulations afford relatively acceptable reductions in oral microorganisms. Many other rinses can be used. Select the mouth rinse that you feel is most palatable and has supporting research.

• Use a rubber dam. This technique -- one that is frustrating to most dental students -- is actually of fantastic assistance in providing quality dentistry. To relate it to your question, it significantly reduces oral microorganism contamination. Unknown to many dentists are the new types of rubber dam, including nonlatex dams, varied colors of dams, several thicknesses of rubber dams, varied types of rubber dam holders, and other rubber dam accessories. Your local retailer can update you on the significant changes in the rubber dam area. Oral microorganisms are greatly reduced when using a rubber dam because of reduction of saliva in the operating field.

In my opinion, use of a rubber dam is an excellent way to reduce the availability of microorganisms and debris in the operating field during many oral procedures.

Our newest DVD on rubber dams is excellent for your staff members. See all of the new accessories and, most importantly, learn how to use a rubber dam effectively and rapidly. For more information on "Rubber Dam -- Still the Best Dry-Field Technique" (Item No. 3522), contact Practical Clinical Courses at (800) 223-6569, or visit our Web site at

Q Representatives of the companies promoting zirconia-based, all-ceramic crowns have told me that tooth preparations for these restorations are the same as those for porcelain-fused-to-metal crowns. After seating numerous crowns and fixed prostheses with zirconium-oxide substructures, I am questioning that statement because I am seeing a bright white "glow" in the gingival areas of the crowns where my tooth preparations were made as with PFM crowns. How should tooth preparations for zirconia-based, all-ceramic crowns be made relative to PFM crowns? A I have noted what you describe. The most popular zirconia-based, all-ceramic crowns and fixed prostheses are currently LavaTM from 3M ESPE, Cercon from DENTSPLY, IPS e.max from Ivoclar, and EVEREST from KaVo, listed in order of current popularity. The most popular brand, Lava, has pigmented zirconia substructures, so if your tooth preparation is too shallow, the pigmented color of the zirconia does not distract from the beauty of the restoration. Some other brands have white, nonpigmented zirconia. If the white zirconia substructure has not been pigmented -- and your tooth preparation is not deep enough -- the white glow you describe will definitely be present. On the other hand, if the preparation is adequately deep, even the white zirconia will not show through. The following are my recommendations for tooth preparation depth for all-ceramic, zirconia-based crowns:
  • Occlusal reduction. Companies are recommending 1.5-2.0 mm of occlusal tooth reduction. After placing many of these restorations, I suggest reducing the occlusal surface 2 mm, with the reduction following the natural contours and anatomy of the occlusal surface. This allows adequate space for optimum tooth anatomy, strength of the framework, and a beautiful esthetic result.
  • Axial surface reduction. This reduction should be about the same as for PFM crowns. If the tooth size and anatomy allow, I recommend 1.0 mm of reduction on the mesial, distal, and lingual surfaces, and 1.0+ mm on the facial surfaces. As you might expect, there are exceptions to my suggestions for smaller teeth for which deep reduction is not advised. The axial reduction should follow the original anatomy and contour of the tooth, which makes the facial preparation of lower molars and premolars slant to the lingual, and the lingual surface of maxillary premolars and molars slant toward the facial surface. These suggestions are similar to those for PFM crowns, but the lingual preparation is usually deeper for all-ceramic, zirconia-based crowns than the reduction for PFM crowns.

Tooth preparations as deep as I have suggested require noninvasive, nonabrasive tooth-cutting, signifi cant water spray during tooth preparation, excellent provisional restorations, and nonpulpally irritating final cements to avoid damaging the dental pulp.

A summary answer to your question is that tooth preparations for zirconia-based crowns need to be somewhat deeper than the well-known tooth preparations for PFM crowns to provide acceptable restoration color, proper anatomy and contour, and adequate strength for the restorations.

Practical Clinical Courses (PCC) has just finished producing an in-depth, close-up, live video comparing the most adequate tooth preparations for full-metal, PFM, and all-ceramic crowns and for ceramic veneers. For more information on Video 1925 -- "Optimal Fixed Prosthodontic Tooth Preparations," contact us at (800)223-6569, or visit

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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