Ask Dr. Christensen

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 ...

I am confused about all-ceramic crown cementation. When should I use conventional cement, such as resin modified glass ionomer, and when should I use resin cement? I hear different suggestions from various sources.

Answer from Dr. Christensen ...

When cementing a porcelain-fused-to-metal (PFM) crown or fixed prosthesis, the cement selected should relate to the degree of retention needed and the level of preventive characteristics desired for the restoration. When cementing an all-ceramic crown, you add another dimension. The type of cement selected must be related to the retention needed for the restoration, the level of preventive characteristics desired for the crown or fixed prosthesis, and the strength of the crown or fixed prosthesis.

The strength of all-ceramic crowns varies significantly, but all of them are weaker than PFM restorations. The weakest type of all-ceramic restoration is the fired ceramic crown, which is cemented over a dentin surface. The next weakest type of all-ceramic crown is the cast ceramic restoration, such as IPS Empress. In spite of the claims made by some manufacturers of dentin-bonding materials, the adhesion of these materials to dentin diminishes over a period of service. If an all-ceramic veneer is cemented over acid-etched enamel, the overall bond of the restoration can be as strong as the bond of enamel to the dentin of a natural tooth, since the bond of resin cement to the etched enamel does not degenerate over time. The preceding statements describe why ceramic veneers are retained so well when cemented with resin cement over acid-etched enamel, and why some crowns come off during service when cemented with resin cement over dentin surfaces.

You did not state what type of all-ceramic crown you are using. The most commonly used type of all-ceramic crown is the cast ceramic crown such as IPS Empress. Cast ceramic crowns have excellent aesthetic characteristics, but they are not as strong as some other all-ceramic crown types. When cementing IPS Empress or other brands of cast ceramic, it is preferable to use resin cement. The choice of resin cement provides better strength to the overall restoration and greater potential for long-term service, than if the crowns had been cemented with conventional cement.

If you are using other popular all-ceramic crowns, such as Procera, Cercon, Cerec in Lab, or Lava, conventional cement, such as resin reinforced glass ionomer works well. These crowns are not as strong as PFM, but they are stronger than the cast ceramic restorations.

Please remember that all-ceramic crowns have improved significantly over the past 15 years, but a small percentage of them will break in service. Depending on the brand, the failure rate will be 2 to 3 percent during the first year of service. I suggest the fee for all-ceramic crowns should be a few percent higher than typical overhead expenses demand. This slightly higher fee allows for replacement of the few restorations that break in early service without an additional charge to the patient.

Question ...

Self-etching primers are supposed to reduce or eliminate post-operative tooth sensitivity. I am using Panavia F and some other resin cements used with self-etching primers, and I still have occasional post-operative tooth sensitivity. It doesn't go away for many weeks, and I have had a few cases where endodontic therapy was required. What is the reason for this problem?

Answer from Dr. Christensen ...

If you are using a self-etching primer that is separate from the cement, the answer to your question is clear. When the primer is placed on the tooth preparation, the tooth surface must be kept completely free of water, blood, saliva, or any other moisture. If moisture is present, the self-etching primer is dissolved off the tooth surface, leaving the dentinal canals in the same open condition as if a total-etch primer had been used. Most resin cement components are irritating to the dental pulp. If the dentinal canals have been opened with acid etch, the resin cement can cause tooth sensitivity.

A relatively new resin cement, Unicem from 3M ESPE, has self-etch primer combined with the cement, and the self-etching primer is not a separate liquid. It is less likely that any water contamination can influence the primer/cement when this concept is used. Initial clinical reports have been very promising concerning the lack of post-operative tooth sensitivity when using this type of resin cement. It appears to be more foolproof than using resin cement that requires the placement of a self-etching primer before placing the resin cement.

If you continue to use a resin cement brand that has a separate bottle of liquid self-etching primer, I suggest that you keep the area around the crown margins completely free of water/blood/saliva contamination during the cementation procedure. It may be necessary to use packing cords, styptics, or vasoconstrictors to reduce fluid presence, and, therefore, the post-operative tooth sensitivity. Also, do not make the tooth preparation too dry before cementation. The sequence for cementation using a two-step self-etching procedure should be:

1. Clean the tooth preparation with pumice and water.
2. If it is likely that water contamination will be present, pack the gingival tissue with cord.
3. Make the tooth surface wet with water, not saliva.
4. Gently blow the visible water off the tooth preparation, but don't over dry it.
5. In the case of Panavia F, the primer is ED Primer. Apply the liquid self-etching primer on all of the tooth surfaces where the cement will be placed. Leave the ED Primer on the tooth structure for the prescribed time.
6. Prepare the cement, and place it in the restoration.
7. Gently blow air on the primer that is on the tooth preparation surface, using a high-velocity suction system to pick up the excess solution. Self-etching primers can be irritating to gingival tissues.
8. Seat the crown, letting the excess cement flow around the periphery of the crown. Cure the excess cement and remove the excess cement.

Crowns or fixed prostheses cemented with the preceding technique should not have post-operative tooth sensitivity.

A new Practical Clinical Courses video demonstrates the popular all-ceramic crowns, makes suggestions about the indications and contraindications for each, and demonstrates cementation procedures: V19-99 "Successful All-Ceramic Crowns and Fixed Prostheses." For more information, contact Practical Clinical Courses (PCC) at: (800) 223-6569, fax (801) 226-8637, or visit our Web site at www.pcc dental.com.

Dr. Christensen's views do not necessarily reflect the opinions of the editorial staff at Dental Economics.