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 e-mail to [email protected].
Q What is happening with the concept of making routine digital impressions for crowns and fixed prostheses? A Many dentists are unfamiliar with intraoral scanner impression-making technology, with the possible exception of making digital impressions for the CEREC in-operatory milling device available from Patterson. One scanning device was launched last year, the Cadent iTero"! (www.cadentitero.com). It allows making routine digital impressions of prepared teeth, the opposing arch, and the interocclusal relationship. The digital concept replaces the long-used impression techniques for crowns and fi xed prostheses using trays, vinyl polysiloxane, polyether, or other impression materials. The digital information recorded by the iTero is relayed to an associated laboratory, and the models are milled, along with copings for esthetic ceramic layering. The iTero concept has been accepted well by users who claim faster impressions than conventional polyvinyl or polyether techniques, good fit of Later this year 3M"! ESPE"! will launch to a small test market its version of the digital-impression technology, Brontes. The technique, as follows, is relatively simple and understandable:- Prepare the teeth. The margins and prepared tooth must be clearly visible to allow the scanner to record all of the details of the tooth preparation.
- Scan the quadrant or arch involved.
- Scan the opposing quadrant or arch.
- Make a digital interocclusal scan.
- Review the 3D model to assess the quality of the scan and the restoration.
- Complete the electronic prescription and upload (e-mail) it to an associated lab for fabrication of porcelain-fusedto-metal or CAD/CAM restorations.
- Receive the restoration(s) from the laboratory and seat them.
These devices are a definite step forward. Currently, they are not well known by dentists. The advantages are quite evident. It is unnecessary to place impression material in the mouth. This will eliminate the known uncomfortable sensations, occasional peculiar taste, and undesirable associated mess. The digital information can be relayed immediately to the laboratory. The procedure has the possibility of reducing the unpredictability of standard impression-making.
Cost of the devices may appear to be high. The Cadent iTero device is $18,000, and the price of the 3M ESPE device has yet to be announced. Time will be required to determine if the cost of digital impression-making is equal to, less than, or more than standard vinyl polysiloxane or polyether impression techniques.
Dentists often are shocked when they realize that a fullarch impression with either material made in a stock tray can cost $30 to $40. Impressions in custom trays cost about onehalf as much as impressions made in stock trays because less impression material is needed in the well-fitted tray.
We will watch the digital-impression market continue to develop with great interest. It obviously works, but how popular will the new concept be? I predict that the technology will catch on rapidly.
Our video, C101A "The Perfect Impression," shows a step-by-step, up-close, patient demonstration of the two-cord technique described above. For more information, call Practical Clinical Courses at (800) 223-6569 or visit our Web site at www.pccdental.com.
Q I have heard that packing cords used to retract soft tissue before making impressions for crowns and fixed prostheses are obsolete. Is that conclusion valid? If so, what are the alternatives to packing cords? A As you know, there are several methods to retract soft tissue when making impressions for crowns and fixed prostheses. Some are reliable, and others are relatively unpredictable. Laboratory technicians agree that the state of dental impressions for crowns and fixed prostheses is not good. Many impressions are inadequate, forcing technicians to guess the location of margins and occlusal relationships. The result is often poorly fitting or occluding crowns and the possibility of premature crown failure. I will discuss the various soft-tissue retracting methods and provide my observations on the relative effectiveness of the methods.(1) No tissue retraction before impressions. This concept is acceptable only when the restoration margins are supragingival; however, some dentists attempt to make impressions of tooth preparations that are subgingival without retracting the soft tissue. In my opinion, the chances for an adequate impression are minimal to none.
(2) Single-cord technique. Most dentists were taught this technique in dental school. If the gingival tissues are healthy and do not bleed easily, the single cord technique can be satisfactory. The signifi cant challenge observed with the single-cord technique occurs when the single cord is removed from the gingival sulcus, causing the gingival tissues to often bleed. Blood in the operating field usually precludes making an adequate impression.
(3) Laser or electrosurgery. For several years, I used this concept as my primary tissue-management technique. My impressions for crowns and fixed prostheses were very good to excellent most of the time. Frequently, though, when the gingival tissues healed, the planned level of the gingival tissues was not achieved. I remade numerous restorations because the gingival tissues had shrunk and did not cover the restoration margins. I now prefer to use electrosurgery or laser surgery as an augmentation to tissue retraction and not as the sole technique; however, I acknowledge that when laser or electrosurgery use is carefully controlled and meticulously accomplished, the technique can be successful. This is especially true on posterior teeth.
(4) Double-cord technique. I wrote about this concept three decades ago, and have found it to be one of the most predictable of all procedures for tissue management. A single cord, measuring in diameter about one-half of the depth of the gingival sulcus, is placed before making the initial tooth preparation. The initial tooth preparation is made to the level of the cord. When carefully placed, this cord stops at the epithelial attachment. This provides an assurance that the crown margin will be placed slightly subgingivally. A second cord of about the same diameter as the first -- and containing your choice of vasoconstrictor or styptic -- is placed on top of the first cord. Be aware that iron-containing styptics can stain teeth and the resultant translucent restorations, such as veneers or thin, all-ceramic crowns. This second cord compresses the first cord. The second cord is removed almost as soon as it is placed, leaving a clear vision of the gingival margins of the tooth preparation. The first cord is left in place during the impression, and is removed after the provisional restoration is made and cemented. The cement debris is removed at the same time as the first cord is removed.
(5) Noncord techniques. At least two products have found popularity and achieved relative success. They are Expasyl"! from Kerr and Magic FoamCord from Coltne Whaledent. When used properly, I think these products can be successful for one- or two-tooth preparations. The materials must be pushed in an apical direction before the impression is made. It has been my observation that when these products are used in multiple-tooth situations, the result is less predictable, more time-consuming, and more failure-prone than when using other procedures. In summary, after restoring thousands of teeth with crowns, I prefer the double-cord technique, augmented occasionally with laser or electrosurgery for tissue management during soft-tissue management.
Dr. Christensen is a practicing prosthodontist in Provo, Utah. 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.
