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 firstname.lastname@example.org.
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Q For many years, I have been using aluminum shell crowns, cemented with zinc oxide and eugenol cement, as provisional restorations for crowns. I have had only minimal problems using these temporary restorations unless I leave the temporary on the prepared tooth for several weeks. When that happens, the new crown is usually too high and the contact areas are often too tight or too loose. I like the inexpensive, easy-to-use characteristics of aluminum shell crowns, but the esthetic result is poor. What do you suggest as a simple, fast technique that I can delegate to staff easily?
A Let's look at what a provisional restoration is supposed to accomplish. A provisional restoration for a crown or fixed prosthesis:
• Maintains proper contact relationships with adjacent teeth. If fitted perfectly, the shell will maintain contact areas to an acceptable level, but most shells are not fitted well enough and do not maintain the correct contact relationship.
• Keeps the prepared teeth in a calm state by sealing the margins with a sedative cement. The shell crown can keep the tooth preparation in a calm state for a few days, and then the restoration starts to leak because of dissolution of the cement and subsequent lack of fit.
• Maintains the correct desired occlusal relationship. It is unlikely that the shell crown will maintain correct occlusal relationship for more than a few days, because the opposing teeth make a dent in the crown and weaken underlying cement, and the teeth extrude.
• Resembles the color of the natural teeth. Obviously, an aluminum shell cannot simulate the color of the tooth, since it is metal in color.
• Serves diagnostic purposes for esthetics and occlusion. The shell crown cannot serve well for diagnostic purposes for esthetics and occlusion.
Therefore, does an aluminum shell crown routinely accomplish the purposes intended for a provisional restoration? The answer is no!
You have been very lucky to have had minimal challenges with the aluminum shell crowns concept.
Currently, most dentists are using bis-acryl resin for provisional restorations. This type of resin has low exotherm, good color retention, moderate strength, and good fit. However, it is relatively expensive compared to the technique you have been using. You have been paying a minimal amount per provisional restoration plus the cost of the temporary cement. A typical bis-acryl resin provisional costs several dollars. I will describe the procedure which can easily be delegated to a competent staff person.
Provisional restorations made in the previous manner require a few minutes to make and cost a few dollars, but they satisfy the previously described characteristics of an ideal provisional restoration. They have a few weaknesses, including wear on occlusal surfaces if allowed to serve more than two weeks, and only moderate strength for several units in a row.
If you want to adapt a moldable resin crown form to the prep, have the patient bite on the soft plastic to correct occlusion, light-cure it, polish, and seat it. The new 3M ESPE material, Protemp™ crown forms, is excellent. The Protemp crown forms cost about the same as the previously described and illustrated bis-acryl resin technique, and some dentists and assistants feel they are easier and faster to perform.
I suggest that you use resin provisional restorations made of one of the materials I have described, and save the aluminum shells for emergencies and short-term service only.
See our new video on provisional restorations and techniques. It includes close-up demonstrations of all types of provisional materials, cements, and techniques. It is an excellent overview for dentists or dental assistants, and it is a great training video.
For more information on V1928, "Effective Provisional Restorations," contact Practical Clinical Courses at (800) 223-6569 or visit our Web site at www.pccdental.com.
Q What is the cement of choice for routine cementation of porcelain-fused-to-metal crowns, and can I use the same cement for Lava™, Cercon®, or e-max® zirconia-based, all-ceramic crowns?
A By far the most commonly used cement in North America is resin-modified glass ionomer. Specifically, the most popular brand names are RelyX™ Luting Cement from 3M ESPE, and FujiCEM™ from GC America. Why have these cements been so popular and well accepted? The reasons are clear, and I have listed them below. Resin-modified glass ionomer (RMGI) cements are:
- Relatively insoluble
- Capable of providing slight bonds to tooth structure
- Similar to tooth structure in expansion and contraction
- Relatively easy to use
- Adequately strong
It is difficult to suggest any other desirable characteristics this category of cement does not have for routine cementation of opaque crowns (PFM, metal alloy, and strong, opaque all-ceramic crowns).
To answer your question, several studies have shown that RMGI cements are adequate for cementation of zirconia-based, all-ceramic crowns. However, there are situations for which these cements are not adequate. Among them are:
• Ceramic veneers — RMGI cements are relatively opaque and only moderately strong. Additionally they do not bond well to enamel and expand on setting enough to potentially crack weaker restorations. Therefore, resin cements, not RMGI cements, are best for ceramic veneers.
• Tooth-colored inlays and onlays — These restorations require optimum strength and the ability to be bonded into place. Resin cements such as Calibra from DENTSPLY Caulk, Insure from Cosmedent, NX3 Nexus Third Generation from Kerr, RelyX™ veneer cement from 3M ESPE, or Variolink II from Ivoclar Vivadent are preferable for tooth-colored inlays and onlays. They provide strength, come in several colors, are light- or dual-cure, and possess translucency variability.
• Lower-strength ceramic crowns — Leucite containing crowns (such as IPS Empress from Ivoclar Vivadent) are beautiful, but they are only moderate in strength and exhibit expansion during setting. These restorations also require more strength than RMGI can provide. Again, I suggest the resin cements I recommended in the previous paragraph for these restorations.
I suggest that RMGI is the cement of choice for routine cementation of PFM and zirconia-based restorations because of its desirable characteristics.
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 Clinicians Report (formerly Clinical Research Associates), which since 1976 has conducted research in all areas of dentistry and publishes its findings to the dental profession in the "Gordon J. Christensen CLINICIANS REPORT" monthly newsletter. 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.
Making Bis-acryl Provisional Restorations
• A preoperative impression is made of the original tooth anatomy (Figure 1). If a portion of the tooth is missing, place a piece of light-cure composite resin in the void, and cure it to provide near normal tooth anatomy before making the impression. This impression can be made inexpensively by heating some pink base plate wax and molding it to the tooth or teeth being treated and at least one tooth on each side of the teeth to be prepared. Alginate can be used easily, but it is messy. The most adequate method is use of a rigid, fast-setting vinyl polysiloxane (VPS) interocclusal record material which is available from almost every impression material company (Figures 2, 3, 4, and 5). Blu-Mousse® from Parkell is the most popular. Of course, your cost to make the impression goes up if you use VPS, since you have the cost of the tray, the impression material, and the bis-acryl resin. The cost of these materials is about $6 to $10 for one tooth.
• Prepare the tooth or teeth.
• Make a normal impression of the area involved. Polyether or VPS are the most popular materials.
• Place bis-acryl resin into the impression in the area of the prepared teeth (Figure 6) and seat the loaded impression material and bis-acryl into the mouth. Leave a small amount of bis-acryl on your finger to approximate the time the bis-acryl sets.
• Remove the impression with the set bis-acryl in it from the mouth.
• Trim, smooth, and polish the provisional restoration (Figures 7 and 8)
• Seat the provisional with the provisional cement of your choice. Either noneugenol cement or zinc oxide and eugenol cement is OK. If you are using eugenol-containing provisional cement, do not seat the final restorations for about two weeks, by which time the free eugenol has been deactivated by combining it with the zinc oxide.