Ongoing Changes in Fixed Prosthodontics, 2007
Gordon J. Christensen, DDS, MSD, PhD
Fixed prosthodontic procedures are a major portion of the general practice of dentistry, as well as the most significant portion of the specialty of prosthodontics. This area of dentistry has seen little alteration until recent years, in which many changes have taken place. Some of these changes are well-proven, while others are in the process of being accepted or rejected. More changes are occurring in fixed prosthodontics than in almost any other area of dentistry, in my opinion.
In this article, I address (in alphabetical order) the many significant changes taking place in fixed prosthodontics, identify the state of the changes in each area and make suggestions regarding the importance and potential influence of the changes.
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ALL-CERAMIC CROWNS AND FIXED PROSTHESES
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In the past, all-ceramic crowns had the reputation of having moderate-to-low strength but near-optimal esthetic results. As a result, most dentists avoided using all-ceramic crowns in the posterior portion of the mouth and emphasized their use as anterior restorations. The advent of all-ceramic crowns using zirconium oxide substructures has changed that reputation. In speaking with audience members at my continuing education presentations, I find that most dentists have used some brand of zirconia-based all-ceramic crowns. Among the popular brands are Lava (3M ESPE, St. Paul, Minn.) Cercon Zirconia (Dentsply, York, Pa.), Everest (Kavo Dental, Lake Zurich, Ill.), and IPS e.Max (Ivoclar-Vivadent, Schaan, Liechtenstein). To date, the reports I have received from practicing dentists about the clinical characteristics of the zirconia-supported all-ceramic crowns and fixed prostheses are positive.
A representative of Glidewell Laboratories (Newport Beach, N.Y.), one of the largest dental laboratories in the United States, reported to me that in 2006, porcelain-fused-to-metal restorations composed 61 percent of the crown restorations that Glidewell Laboratories provided to dentists, and full-metal crowns constituted only 7 percent of the laboratory’s production (J. Shuck, oral communication, April 2007). Because of the diverse geographic representation of this laboratory’s clients, these statistics probably represent trends in the entire country. Additional information showed that use of nonmetal crowns has continued to grow during the past few years. In 1997, 20 percent of crowns were nonmetal. In 2006, 32 percent were nonmetal, a percentage that held true for the first quarter of 2007 (J. Shuck, oral communication, April 2007). I predict that the trend toward all-ceramic crowns will continue to grow, stimulated by patients’ desire to eliminate metal from their mouths, esthetic concerns, laboratories’ promotion of the concept, overall excellent service and optimal esthetic appearance.
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COMPUTERIZED FABRICATION OF CROWNS AND FIXED PROSTHESES
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Evolving over nearly 25 years, the computer-aided design/ computer-aided manufacturing (CAD/CAM) concept is rapidly gaining in popularity. When the scanning and milling devices are installed, programmed, supervised and used properly, CAD/CAM-produced crowns and fixed prostheses can be made more quickly and predictably than those made by hand.
The most popular substrate for milling crowns and fixed prostheses is zirconium oxide, but other materials also are used for CAD/CAM design and milling. Laboratory owners are excited about this change, and I predict not only that this trend will grow, but also that CAD/CAM restorations will gradually overtake porcelain-fused-to-metal restorations. The speed of fabrication and the predictability of the products ensure the future of this concept.
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CERAMIC VENEERS
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The widespread availability, esthetic beauty and physical success of conservative ceramic veneers have made this type of restoration extremely popular. For teeth requiring only slight-to-moderate rehabilitation, clinicians often use veneers instead of crowns, thus broadening the alternative treatments that are available for esthetic rehabilitation of some broken-down, discolored, malformed or grossly restored teeth. On the negative side, in my opinion, clinicians have increased their use of ceramic veneers too much, treating many situations with veneers that could be treated with less aggressive therapy, such as bleaching, minor orthodontics, incisal contouring or combinations of these treatments. However, veneer use will continue to expand. Patients find veneers desirable because they are a conservative measure. Furthermore, veneers are clinically successful and esthetically superior to most crown types, and they will remain as an expanding and viable part of fixed prosthodontic service.
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ELECTRIC HANDPIECES VERSUS AIR-DRIVEN HANDPIECES
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Electric handpieces have been the most popular tooth-cutting mechanism in numerous countries for several years. However, clinicians’ use of electric hand-pieces is just beginning to mature in the United States, where air rotors and air motors long have been the dominant instruments for cutting tooth preparations. I would estimate, on the basis of information gleaned from participants in my continuing education courses, that less than 50 percent of U.S. dentists use electric handpieces at this time.
Once practitioners try electric handpieces, they can see their advantages firsthand. These devices are quiet during use; they possess high torque (power), even at low speed; and they are concentric if they are maintained well. The disadvantages are equally evident after minimal use. Electric hand-pieces have larger heads than air-driven handpieces; they heat up if not maintained well; and if soft tissue or intraoral objects, such as cotton rolls, in the path of the rotating instrument make contact with the bur, the result is a significant challenge because of the high torque of the handpiece and the inability to stop the rotating bur rapidly. I predict that the use of electric handpieces, both low- and high-speed, will continue to grow at a moderate pace, and that these handpieces eventually will dominate the U.S. handpiece market.
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DECLINING USE OF FIXED PARTIAL DENTURES FOR SINGLE-TOOTH REPLACEMENT
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Because of the successful use of root-form dental implants to replace missing teeth, most patients prefer and demand implants with crowns or fixed prostheses on them, instead of conventional fixed prostheses cemented to prepared abutment teeth. It appears that the only significant remaining candidates for placement of three-unit fixed prostheses are patients with severely broken-down teeth that are in abutment locations, patients with inadequate bone to support implants who refuse to undergo bone grafting, or patients whose existing fixed prostheses require replacement. Is there any question that this trend will continue? The less invasive nature of properly placed root-form implants has been well-proven. When using implants, dentists can avoid affecting the structure of teeth that would have been abutments and allow the natural teeth and implant-supported crowns to function as single teeth. I predict that the use of three-unit fixed prostheses replacing single missing teeth will continue to decline.
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IMPRESSION TECHNIQUES
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Vinyl polysiloxane and poly-ether impression materials are at the center of almost all of the impression techniques for fixed prosthodontics. However, on the horizon is an entirely new clinical concept in which a scanner is used to make multiple pictures of the tooth preparation(s) and surrounding teeth, the opposing arch and the interocclusal relationship. The digital information is transferred to a cooperating laboratory. CAD/CAM devices produce models of the prepared teeth, the surrounding teeth and the opposing arch mounted in the correct interocclusal relationship. The restoration(s) are fabricated on these models.
At this time, the iTero system (Cadent, Carlstadt, N.J.) is available, and 3M ESPE will introduce another scanner system to the profession soon.
Will this concept overtake the well-known and proven ways of making impressions? In my opinion, the possibility of scanning instead of making intraoral impressions with elastomers has a high possibility of making a major impact on the profession. However, a few years will be necessary for this to occur, because of the cost of the new concept and the relative success of current impression-related techniques.
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METAL POSTS VERSUS FIBER-REINFORCED RESIN-BASED COMPOSITE POSTS
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Most experienced dentists were taught to use custom cast metal posts on endodontically treated teeth, requiring a reinforced attachment of the coronal portion of the build-up to the root structure. Although this concept is successful, its use has reduced markedly as the prefabricated post-and-core technique has gained in popularity. Most dentists’ material of choice for posts for at least the past 20 years has been metal, including stainless steel (nickel-chrome), titanium alloy and pure titanium. However, resin-based composite posts rapidly are becoming the most popular. Why? These posts do not impart an objectionable color to the tooth. They are flexible and, when they receive a traumatic blow, the tooth does not fracture vertically. Although resin-based composite posts are somewhat weaker than some metal posts, trauma to teeth restored with these posts almost always leaves the affected tooth still restorable. Their use will continue to expand.
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TOOTH PREPARATIONS
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Although some companies making zirconia-substructure restorations do not emphasize it, all-ceramic crowns and fixed prostheses require deeper tooth preparations to allow for the thickness of zirconia substructures underneath the esthetic veneering ceramic. Manufacturers recommend a 0.3-mm thickness of zirconium oxide on anterior teeth and a 0.5-mm thickness on posterior teeth. When one compares tooth preparations for zirconia-based prostheses with those for conventional porcelain-fused-to-metal prostheses, the all-ceramic crowns require deeper tooth structure removal on the mesial, distal and lingual aspects of the preparation to achieve optimum thickness of substructure and veneering ceramic.
As the popularity of all-ceramic restorations continues to increase, deeper tooth preparations will be used more frequently. This necessity probably will limit the use of all-ceramic crowns for younger patients who have larger pulps.
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CONCLUSIONS
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Fixed prosthodontics procedures compose a major part of dental practice. Numerous changes in techniques, materials and devices have made crowns and fixed prostheses more predictable in service, faster and easier to fabricate, and more esthetically pleasing. I predict that the need and demand for crowns and fixed prostheses will increase.
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FOOTNOTES
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Dr. Christensen is the director, Practical Clinical Courses, and co-founder and senior consultant, CRA Foundation, Provo, Utah. He also is the dean, Scottsdale Center for Dentistry, Ariz. Address reprint requests to Dr. Christensen at CRA Foundation, 3707 N. Canyon Road, Suite 3D, Provo, Utah 84604.
The views expressed are those of the author and do not necessarily reflect the opinions or official policies of the American Dental Association.
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Suggested Readings
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Christensen GJ. Choosing an all-ceramic restorative material: porcelain-fused-to-metal or zirconia-based? JADA 2007;138(5):662–5.[Free Full Text]
Christensen GJ. Is now the time to purchase an in-office CAD/CAM device (published correction appears in JADA 2006;137[3]:309)? JADA 2006;137(2):235–8.[Free Full Text]
Christensen GJ. Is the wide range in crown fees justifiable? JADA 2006;137(9):1297–9.[Free Full Text]
Christensen GJ. Are veneers conservative treatment? JADA 2006;137(12):1721–3.[Free Full Text]
Christensen GJ. Are electric handpieces an improvement? JADA 2002;133(10):1433–4.[Free Full Text]
Christensen GJ. Laboratories want better impressions. JADA 2007;138(4):527–9.[Free Full Text]
Christensen GJ. Post concepts are changing. JADA 2004;135(9):1308–10.[Free Full Text]
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