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QUESTION
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Porcelain has been used for the last half-century as a restorative system because of its esthetics and wear-resistant properties. Do you believe there are alternatives available that will challenge the role of ceramic materials?
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ANSWER
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Porcelain has been an integral part of dentistry for many years. In fact, the first denture base material was constructed of porcelain. Porcelain teeth and even ceramic inlays have been part of clinical dental practice since the turn of the 20th century. Today, ceramic materials are used for porcelain crowns, veneers, porcelain fused-to-metal restorations, inlays and onlays and other specialized techniques. The success of porcelain can be attributed to its great potential for esthetics, tissue compatibility, insolubility and wear resistance.
While dental porcelains have been modified to a state of near-perfection, they also have a number of decided disadvantages. The first and most serious is their tendency to abrade all structures against which they occlude. Most clinicians have experienced conditions in which the unglazed surface of porcelain has effectively destroyed the occlusal surfaces of natural teeth and various types of nonporcelain restorative systems. While glazing of porcelain can prevent such hazardous results, retention of the glazed surface is far from guaranteed. Once an interruption of the glaze occurs, the deleterious effects of abrasion will begin.
Another potential problem lies in the fact that underlying supporting structures deteriorate more quickly under porcelain-based dentures than under acrylic resin–based dentures. The higher rate of loss associated with porcelain can be attributed to its energy absorption characteristics, in contrast to those of acrylic resin.1 In the case of porcelain, the energy of mastication is readily transferred through the porcelain and into the tissue substrate. In the case of acrylic, a considerable amount of the energy is absorbed by the polymer rather than being transferred away. This energy, which is transferred to the underlying osseous structure, causes structural loss.
The problem may be of even greater concern as it relates to dental implants. Using the same argument of energy transfer or absorption, the excess energy in the case of porcelain crowns is transferred to the implant and into the interfacial region between the surrounding osseous structure and the surgically placed device. Again, the polymeric materials tend to serve as a shock absorber for the energy.
This particular principle was demonstrated at the University of Alabama in 1991.1 When the amount of deflection of a substrate was measured, the resin restoration always transmitted less energy than did a corresponding restoration of porcelain. The amount of difference between the two materials was not great, but it was always there.
An additional problem with porcelain is that all corrections in contour and finishing must be done in the laboratory. Repairs of fractures or additions of material must be accomplished extraorally. Finally, owing to their lower potential for energy absorption, porcelain restorative materials are more prone to fracture. Ceramic inlay restorative material serves as an excellent example.
In light of these potential problems associated with porcelain, many companies and academic investigators have long searched for polymer substitutes. Considerable progress has been made during the last 10 years, but the problem of wear resistance has been overwhelming. While some forms of postoperative heat treatment appeared promising,2,3 long-term clinical results demonstrated that this, in fact, was not the truth.4,5 Further research indicated that wear resistance was improved, but the amount fell far short of what was necessary to compete with porcelain.6
Recently, a limited number of polymer-based materials that demonstrate excellent wear resistance have been clinically tested and evaluated.7 All of them, while polymer-based, are substantially different (a few examples: Cristobal+, Dentsply/Ceramco; belleGlass HP, SDS KerrLab; Sculpture FibreKor, Jeneric/Pentron). Perhaps the most significant influence on wear resistance, as well as on esthetics, is the elimination of oxygen during the curing process. When all oxygen is eliminated from the internal aspect of the restoration, the resin matrix becomes considerably stronger and more wear-resistant. In essence, the presence of oxygen-containing voids, regardless of their dimension, tends to inhibit the polymerization process of the resin with which they are in contact. If nitrogen or some other inert gas is used to replace a normal oxygen-containing atmosphere during the curing process, the oxygen is effectively removed. Also, if curing is conducted under pressure and sufficiently high temperatures (125 C, for example), the curing rate is extended beyond resin-based composites’ usual curing rate. Interestingly, the elimination of air-containing voids during the curing process also improves the esthetics of the material, by enhancing the translucency of the restoration and thereby giving it a more three-dimensional effect.
Finally, the wear rate of resin-based composite is clinically acceptable. In an eight-year clinical study, researchers found the wear rate of such a system to be only 1 to 2 micrometers per year more than that of enamel.7
Resin-based composite systems offer another advantage over conventional ceramic agents. The fact that their filler particles are submicrometer in dimension considerably reduces the possibility of their abrading the structures with which they may be in occlusion.
In conclusion, during the past several years, a major effort has been made to make polymers more similar to porcelains and vice versa. The newer resin-based composite systems discussed in this article represent the results of such efforts. The properties have been improved sufficiently so that a limited number of polymer-based materials are, in fact, clinically acceptable for use on posterior teeth. While the inherent properties of these materials have been improved, many of the advancements can be attributed to the specific methods of curing.
Undoubtedly, the near future will witness further improvements in these systems and, consequently, a greater acceptance among clinicians and laboratory technicians alike.
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