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The safety of amalgam has been upheld consistently by national health organizations. What do you see as amalgam’s future as a restorative material in the next decade?
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Although amalgam has been used by the dental profession for more than a century, its use as a restorative material has been questioned a number of times. The first time, of course, was in the middle of the 19th century when it was introduced in the United States. Based on the fact that it did contain mercury, the dental profession was divided as to the professionalism of using this material. Also, as compared with the exactness of the gold foil then in use, some members of the profession considered the amalgam techniques "sloppy and unprofessional."
Then, in the 1950s, the safety of using amalgam once more was brought under discussion. This time, the charge of potential toxicity was met with a series of research publications. In each and every case, all the biological data revealed no toxic response to amalgam restoration placed into patients.1,2
During the last several years, amalgam has been challenged once again.3,4 These more recent objections were based not only on the potential toxicological effects of mercury vapors from the amalgam restorations but also on amalgam’s lack of esthetic appeal. It is important to note that no scientific document to date has implicated amalgam in the development of any systemic diseases. Yes, increased levels of mercury in the blood and urine can be associated with amalgam restorations, but the amount measured has never been related to any specific diseases.5,6
It is interesting to note that the use of amalgam as a restorative material has decreased perceptibly during the last half-decade. More than concern over the use of a mercury-containing alloy, the primary reason for this decline is related to the use of a material that does not resemble the physical characteristics of tooth structure. The use of stainless-steel crowns on primary teeth has been all but eliminated because of their unnatural appearance.
While most manufacturers have reported a decrease in the sale of amalgam alloys, dentists have not refrained from using them at all that great a rate. I make this observation for a number of reasons. First of all, the only other direct restorative material available to the general practitioner is resin-based composites. The restorative process associated with resin-based composites is considerably more technique-sensitive and time-consuming than that associated with amalgam. Consequently, many clinicians have opted to continue using amalgam, at least for the present.
IN SPITE OF ENCOURAGING DATA, IT IS PROBABLE THAT AMALGAM WILL CONTINUE TO BE USED FOR A CONSIDERABLE TIME IN THE FUTURE.
It is interesting to note that while the physical and mechanical characteristics of resin-based composites have been improved considerably, the same cannot be said for the techniques associated with their insertion and finishing. During the last two years, however, a number of manufacturers have introduced posterior resin-based composites, which are considerably different. Specifically, these materials possess handling characteristics similar to those of a freshly triturated mass of amalgam. The material is inserted and condensed using procedures long familiar to the clinician. Slight modifications in procedural techniques allow the clinician to complete the restoration in considerably less time than was previously possible with resin-based composites.
In spite of this encouraging information, it is probable that amalgam will continue to be used for a considerable time in the future. In fact, it is probable that it will be part of the dental armamentarium for the next decade. There are several reasons for believing that its use will continue. To begin with, resin-based composites are as yet more technique-sensitive than is amalgam. Failure to follow a number of well-prescribed rules will all but guarantee clinical failure in a relatively short period. In addition, regardless of the exactness by which the resin-based composite is placed, its durability track record has been shown to be less than that of amalgam. Problems that are more abundant with resin-based composites include postoperative sensitivity, leakage and secondary caries.
In all likelihood, however, amalgam’s rate of use will continue to decline appreciably. The rate at which it will be replaced during the next several years will depend on how soon the use of resin-based composites can be simplified and made less technique-sensitive. It also depends on when the dental schools will take a more positive attitude toward the potential for resin-based composites and include them as part of students’ training. Amalgam is coming to an end, but it will be a while before that happens.
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Dentin-bonding agents have been improved to a degree that allows bonding of virtually any resin-based material to all tooth structures. What modifications do you see occurring in the next several years in these materials?
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The concept of acid etching and bonding resins to enamel can be attributed to Michael Buonocore. More than 40 years ago, he published an article describing a method by which acrylic resin could be bonded to enamel.7 Within a decade (1963), Rafael Bowen developed the composite resin.8 The importance of such a material was so apparent that it immediately replaced all direct restorative materials that preceded it. Unfortunately, however, it was nearly a quarter-century before a system was developed that would permit resin-based composites to bond to enamel. The combination of all these concepts, then, dramatically changed the way restorative dentistry is conducted.
Perhaps the next important milestone in the area of esthetic restorative dentistry was the development of the hybridization concept introduced by Dr. Nobuo Nakabayashi and colleagues9 in 1982. Essentially, they demonstrated that the application of phosphoric acid to the surface of dentin followed by the application of a dentin-bonding agent produced a surface impermeable to caries-producing microorganisms.10 By sealing the dentinal tubules and diffusing into the intertubular and peritubular dentin, the bonding agent protects the pulpal tissue from the oral environment. In addition, this process also prevents postoperative sensitivity; it prevents fluid flow within the dentinal tubule and consequently eliminates the potential for negative pressure on the odontoblastic process, which in turn causes postoperative pain.11,12
The adhesion of dentin-bonding agents and the overlying resin-based composite can be attributed to the interrelationship between the collagenous fibers and the polymerized bonding agent. The adhesion is so effective that when it is carried out properly, it becomes virtually impossible to separate the materials in the interfacial region. Failure will occur either cohesively in the dentinal substructure or in the overlying composite itself.
With this in mind, it is easy to understand that improvements in the adhesive qualities of dentin-bonding agents will not happen in the foreseeable future. Instead, the focus will be placed on simplifying the application process itself. Considerable progress has already been made in this. For example, while the dentin-bonding agents of yesterday consisted of two or more components, current formulations contain only one. The chemistry of the two- or three-bottle systems has been incorporated into one bottle. Unfortunately, however, some clinicians have experienced problems with this consolidated system, such as lack of consistency in bonding effectiveness and increased postoperative sensitivity.
Major efforts to improve the bonding process in the future will consist in trying to accomplish a number of things. The first will be to eliminate the need for etching the preparation before applying the dentin-bonding agent. Already, a limited number of systems permit the procedure to be carried out without a separate acid-etching procedure. In such systems, a conditioner that does in fact contain some type of organic acid first is applied. Other systems contain the acid in the dentin-bonding agent but are effective only in conjunction with dentin.
Furthermore, the newer systems will be more conveniently packaged so that application of the bonding agent can be carried out in a simpler fashion. The process will consist simply of removing the applicator from the container, applying the agent to the tooth surface and then immediately applying the bonding agent. Already, at least one system now on the market is approaching this concept. The greatest improvement in the future will consist of a simple one- or two-application process that carries with it full assurance that the resilient adhesion will be optimized and consistent from patient to patient.
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There has been concern that some of the resin-based composite materials may prove to be either toxic or allergenic. What are your thoughts regarding this issue?
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Over the years, a limited number of people have suggested that resin-based composites may be toxic in one way or another to the patient. But at present, none of the reports related to potential allergy resulting from exposure to resin-based composite restorations has demonstrated cause and effect. While some of the ingredients in the unpolymerized material may have some allergenic potential, the amount necessary to cause a reaction is substantially greater than the amount available in even multiple restorations. Perhaps the greatest concern about possible allergenic responses can be related to the inhalation of the fine particles during cutting and finishing procedures.
Whenever cutting or removing an existing resin-based composite, the dentist is strongly urged to ensure that water spray and an evacuation device are held adjacent to the restoration. Incidentally, it is very important that the dentist polymerize the resin-based composite restoration as much as possible, to minimize the percentage of the unreacted and, therefore, possibly allergenic components. On the patient’s first recall visit, the dentist should evaluate the condition of the gingival tissue to eliminate the (extremely small) possibility that the patient may be allergic.
TODAY IT IS POSSIBLE TO GENERATE VENEERS, INLAYS, ONLAYS AND CROWNS BY MEANS OF A COMPUTER.
TODAY IT IS POSSIBLE TO GENERATE VENEERS, INLAYS, ONLAYS AND CROWNS BY MEANS OF A COMPUTER.
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Could you comment on the future of computer-generated restorations?
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Research by numerous investigators, both industrial and academic, has been directed toward the development of systems that could generate restorations by means of a computer. After years of investigation, a number of practical systems have been developed. Although numerous problems were in evidence at the time of introduction, continued research has resolved many of them.13,14 Today it is possible to generate veneers, inlays, onlays and crowns by means of a computer. Restoration of other classes of cavity preparations, such as II and V, also can be readily achieved in this way.
One of the greatest advantages of such a system is that restorations can be accomplished in one appointment. Furthermore, no physical impressions or temporary restorations are necessary. Finally, the entire procedure can be carried out without the assistance of a laboratory technician at chair-side. Time for completion of a ceramic restoration commonly averages one hour.
Clinical research has demonstrated that the marginal integrity of the ceramic restorations produced with the use of computers has improved with some modification of the cutting or milling device.15,16 It should be pointed out that the quality of the restorations generated in this manner also improves with clinical experience. The marginal gap between the restoration and the margins of the cavity preparation easily can be less than 100 micrometers.
Continued research is directed at generated systems that would effectively and efficiently generate bridges, as well as crowns. There is every reason to believe that such efforts will bear fruit within the next four or five years. It is interesting to note that such technology already has been made available to the dental profession. Unfortunately, the complexities of generating multiple units from a solid piece of ceramic material so far have kept the quest from reality.
The computer-assisted design–computer-assisted manufacture, or CAD–CAM, system certainly has other uses. From a surgical point of view, it offers the technology for generating osseous structures such as portions of the mandible and even the calvaria. Reconstructive surgery related to repositioning or remodeling of various facial bones certainly would be within the realm of possibility with the CAD–CAM system. Devices for computer generation will continue to develop over the following decade and ultimately will result in a significant change in the way in which clinical dentistry is practiced.
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Do you foresee any totally new restorative materials emerging in the future?
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The development of any totally new restorative material in the near future is quite unlikely for a number of reasons. First, creating new and innovative systems requires a tremendous amount of time and research effort, as well as money. The time from initial conception to marketing of such an idea commonly is at least five years and more typically is 10 years. As totally new concepts are kept in secret while they are being developed, there does not appear to be anything brand-new on the horizon.
There is still another major reason why totally new restorative materials will not be generated for the foreseeable future. The stringent requirements set down by the U.S. Food and Drug Administration for the introduction of new materials make it extremely expensive and time-consuming to market systems that are major deviations from traditional materials. It is not surprising that the basic formulation of bisglycidyl dimethacrylate, or Bis-GMA, still serves as the backbone of all new and modified resin-based restorative systems. Introduced to the profession nearly 35 years ago, the Bowen resin8 (or modifications thereof) continues to be the basic component in nearly all polymeric restorative systems based on esthetics.
On the other hand, what I can foresee is the introduction of techniques that will make the success of restorations more constant and uniform. The introduction of the packable composite, for example, has allowed many more clinicians to place the material without the many problems associated with conventional posterior resin-based composites. Dentin-bonding agent delivery and application will be made considerably easier and faster. Computerized clinical techniques associated with the fabrication of restorations will be improved to the point that many more clinicians will make them part of their daily practice.
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Truth in advertising of dental products appears to be a growing issue in the profession. What are your thoughts on this potentially controversial issue?
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It would appear that there have been some disappointing changes in the fields of marketing and advertising. Some manufacturers of restorative materials advertise their products using testimonials of clinicians with national reputations. In some ways, the technique approaches that of hiring sports superstars to promote the use of high-fiber breakfast cereals, although the superstar may know very little about nutrition, diet or much of anything that relates to good health. The idea, of course, is that a person with great public recognition should get the attention of those who are in the market for a new and better product.
Such a concept may sell products—but, unfortunately, this approach tells clinicians little that they need to know as it relates to performance and efficacy. It attracts on the basis of emotion rather than facts and science.
Before a clinician decides on the acquisition of a new system or material, it is very important that he or she request documentation related to the product. Without doubt, the preference should be for clinical data. While data on laboratory and mechanical properties are very important, they do not predict clinical performance. Such numbers are quite important to the manufacturer, which is trying to improve its product; unfortunately, however, the resultant modification of properties is no guarantee that the restorative system will behave better. In fact, the improved properties, ironically, may move clinical performance in the opposite direction. Finally, it should be kept in mind that it would be somewhat unusual for the person making the recommendation to do so without any strings attached.
Controlled, independently performed clinical research studies are the only basis for dependable clinical claims made by a manufacturer. One reason there is not more research-based product information available to the practitioner is that such investigations are timely and expensive. Under most conditions, it is virtually impossible for reliable data to be generated in less than two years—sometimes three.
If the manufacturer has clinical data, the clinician probably will not have to request them. In light of the importance of such documentation, the manufacturer will take the initiative and offer it even before the clinician has the opportunity to request it. Incidentally, the report by itself is not as important as is the manner in which the study was accomplished. It is important for the reader to evaluate how the study was conducted, the number of restorations included in it, and the techniques used for measuring the results. Finally, the reader should make an effort to determine if the results justify the claims being made by the report’s author.
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During the last several years, a number of manufacturers have introduced metal-free bridges. How successful do you believe they have been and what changes, if any, do you see in them in the future?
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The development of resin-reinforced fibers has opened the door of opportunity for many clinicians, permitting the development of fiber-reinforced posts and metal-free bridges, to name two examples. Both systems have been quite successful in restorative dentistry.
The metal-free bridge offers a number of advantages over the conventional porcelain-fused-to-metal restoration. To begin with, the absence of a metal substrate provides a greater opportunity for increasing translucency and natural appearance. Porcelain-fused-to-metal restorations commonly require considerable opaquing, which may lead to loss of translucency. In addition, the marginal integrity and adaptation of a metal-free bridge can be considerably better than those of a porcelain-fused-to-metal-bridge. With metal-free restorations, margins frequently are placed above the gingival crest. Adaptation can be resolved in part by adhesive bonding.
There have been increasing concerns about the potential for metal sensitivities. Metals giving rise to this concern include nickel, beryllium, mercury, palladium and copper. A metal-free bridge obviously sidesteps this issue. Another positive feature of this novel restorative system is that it has better shock absorption capabilities than a porcelain system, regardless of what the substrate may be. A polymer occlusal surface will absorb considerably more masticatory energies, thereby reducing the potential for osseous degradation at the implant/bone interface.
WITH A METAL-FREE BRIDGE, THERE IS LESS POTENTIAL FOR WEAR OF THE OCCLUDING STRUCTURE, WHICH OBVIOUSLY HAS BEEN A MAJOR SHORTCOMING OF THE CERAMIC RESTORATIVE SYSTEMS.
Finally, with a metal-free bridge, there is less potential for wear of the occluding structure, which obviously has been a major shortcoming of the ceramic restorative systems. And such a bridge offers the clinician the ability to repair fractured surfaces intraorally without removing it for laboratory processing—certainly an advantage.
The number of clinicians using metal-free bridges has been growing. While the cost to either the patient or the practitioner in terms of laboratory fees is not less than that of conventional systems, the properties discussed previously certainly have encouraged the acceptance of this relatively new system. While some clinicians have experienced great success with this concept, others have been somewhat disappointed, for a couple of reasons.
- – First, there appear to be differences in wear resistance among the various materials currently on the market; some are significantly more resistant to degradation than others.
- – Second, some of those using the system have experienced sensitivity and debonding of the prosthetic device from the retainers or abutment teeth. This debonding can be related to one of two things. The first is the use of the wrong type of luting agent for cementing the appliance. It is absolutely mandatory that a resin-based composite luting agent is used; no other material will suffice. Zinc phosphate cement and glass ionomer cements are definitely contraindicated. The reason for this, of course, is that the elastic modulus (or stiffness) of the metal-free bridges is only 40 percent that of gold-based alloys and perhaps as much as 20 percent that of base-metal compositions. This simply means that the metal-free restorations can bend under occlusal load as much as 2.5 times as much as gold-based alloys. Therefore, it is mandatory to use a technique that will ensure adhesive bonding of the appliance to the surface of the prepared retainers. Failure to achieve this objective will result in the separation of the appliance from one or more of the retainers. The possible cause of debonding is the preparation. Inlay-retained preparations may offer insufficient surface area for long-term retention. In this regard, a full coronal preparation is suggested.
The concept of metal-free bridges is here to stay. Undoubtedly, gradual changes will be made in the fibers now being used, as well as in the polymers with which they are being manufactured. Fibers with different mechanical characteristics and morphology will increase these bridges’ potential for extended applications.
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For the last 50 years, zinc phosphate cement has been the luting agent of choice for all indirectly placed restorations. Recently there has been an emphasis on resin-based composite and glass ionomer systems. In the future, what changes can we expect?
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Zinc phosphate cement has been used continuously for a little more than 100 years. During this time, only one major modification was made to this material: the addition of magnesium oxide to the formulation to better control the rate of reaction. Nearly five decades ago, acrylic resin was introduced as a possible substitute for the zinc oxide–containing cement. While initial retention was encouraging, the concept failed almost immediately. Leakage, secondary caries and loss of retention caused its demise.
Since then, glass ionomers have been added to the list of possible luting agents. In addition, the resin-based cement has returned, but with different techniques and overall composition. The new luting agents were based not on acrylic resin, but on Bowen’s formulation, which consisted basically of Bis-GMA. Both the glass ionomers and the resins were successful. In fact, today, the majority of all indirectly placed restorations are cemented with glass ionomers, resin-based composite cements or a combination of the two.
The advantage of the glass ionomers is that they, more than any other type of luting agent, release sufficient fluoride to render the tooth less prone to secondary caries. The resin-based composite cements offer numerous advantages. These include adhesion, insolubility, ease of mixing and excellent color-matching ability. The combination of all these characteristics substantially increase resin-based composite’s potential for extended longevity. The techniques associated with the resin-based composite cements enhance the longevity of the pulpal tissue.
The future of cement will undoubtedly witness vast improvements in terms of effectiveness, ease of handling and reduction of technique sensitivity. I predict, however, that little change in the basic formulations will occur within the next 10 years.
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