DENTAL COMPOSITES/GLASS IONOMERS: CLINICAL REPORTS STEPHEN C. BAYNE
Department of Operative Dentistry School of Dentistry University of North Carolina Chapel Hill, North Carolina 27599-7450 Adv Dent Res 6:65-77, September, 1992
Abstract—Composites and glass ionomers have not been extensively tested in clinical trials for biological safety. Most clinical evaluations have looked at other factors, such as retention, wear, or color. The primary evaluation criterion used in clinical trials is post-operative sensitivity. Sensitivity does not seem to have any correlation to pulpal inflammation. Inflammation can be the result of mechanical, thermal, chemical, and bacterial insults. It is complicated for individual contributions to be separated in short-term studies. General usage of these materials over about 20 years indicates a high benefit-to-risk ratio. Despite some complaints of sensitivity with some glass-ionomer compositions, both composites and glass ionomers are relatively trouble-free. There is no evidence of short-term or long-term risk. Toxicological studies have focused almost exclusively on pulpal reactions. Systemic reactions have not been closely examined, although there is no suspicion of any problems after virtually billions of procedures in the United States. New glass-ionomer cements are similar to contemporary composite formulations. Continued development of these materials may ultimately produce an amalgam replacement material that is economically and philosophically desirable to general dentists.
This manuscript is published as part of the proceedings of the NIH Technology Assessment Conference on Effects and Sideeffects of Dental Restorative Materials, August 26-28,1991, National Institutes of Health, Bethesda, Maryland, and did not undergo the customary journal peer-review process.
omposites and glass ionomers have undergone considerable evolution in composition and properties during the past 20 years. At the present time, they represent large classes of materials' applications in dentistry. This necessarily makes it complicated for the clinical properties of these materials to be summarized. Specific application requirements and conditions must be considered. Composites are routinely used as Class IE, IV, and V filling materials, for foundations for cast restorations, as luting cements, for direct and indirect chair-side inlays, for veneering natural and prosthetic teeth, and for fabricating artificial abrasionresistant teeth for dentures. Although composites have shown reasonably long-term success in conservative Classes I and II filling applications, they ^re not considered replacement materials for dental amalgam. Glass ionomers are used for similar filling-material applications, luting, lining and basing, and for foundations. During the past three years, glass ionomer has proliferated extensively. Charbeneau et al. (1988) have reported extensive use of glass-ionomer cements. Phillips (1990) noted an almost 10-fold increase in glass ionomer use in a few years. Swift et al. (1990) indicated glass ionomer use among 85% of the dentists recently surveyed in the state of Iowa. Wilson et al. (1991) surveyed dental schools around the world and discovered that 90% of the curricula include the teaching of glass-ionomer procedures. The success of restorative materials has been difficult to quantify because of the large number of mitigating factors. These have been considered in detail by Anusavice (1989). Bayne (1991) has proposed that, via normalization techniques and Weibull statistical approaches, estimates of longevity can be calculated. He reports CL50 values (clinical longevity for 50% of restorations) for composites. Longevity estimates based on information collected by Maryniuk (1984) and Mjor et al. (1990) were used to develop a time-line for the 1980 half-lives of restorative dental materials (Fig. 1). With use of more recent CL50 values and information about high-copper dental amalgams from Letzel et al. (1990), the time-line was modified to indicate the excellent longer-term service provided by both amalgams and composites. Glassionomer cement half-lives were estimated. Actual half-lives do not necessarily influence clinical usage. Recently-reported survey information about dental procedures (Nash and Bentley, i991) shows some different trends. This information has been abstracted and re-presented as Fig. 2. Using ADA survey data from 1979 and 1990, they have been able to detect decreasing relative usage of dental amalgam and approximately level usage of dental composite per 100 patient procedures. Of the restorative procedures involved, dental amalgam still represents the most significant number of procedures. However, if it continues to decline, then one might expect that, by the year 2000, other direct restorative filling
materials such as composite and glass ionoitier would replace its functions. Although dental amalgam is considered safe for patient and dentist use, the Environmental Protection Agency is becoming vigilant in enforcing pollution laws regarding dental office effluents. Mercury-contaminated waste water may be the issue which turns the tide against dental amalgam use (Bayne, 1991). Safety and Efficacy Testing Evaluation of dental products for safety and efficacy has historically been the purview of both the American Dental Association (ADA) and the US Food & Drug Administration (FDA). In 1976, the FDA charge was modified to include consideration of dental products and devices. The FDA classifies materials as requiring documentation of (1) good manufacturing practices, (2) quality (by meeting defined laboratory standards), and (3) biocompatibility. The last category involves testing for toxicity based on (a) initial tests (screening) for indication of in vitro cellular responses, (b) secondary tests for evaluation of tissue responses, and (c) usage tests for detection of pulp responses (ADA, 1981). These tests are described in great detail in ADA/ANSI specification #41 (ADA, 1982) and have been critically reviewed by Craig (1989). Ideally, information should be collected on both the toxicity and allergenicity of dental materials. However, most tests concentrate on toxicity. A few screen for mutagenicity, carcinogenicity, or teratogenicity. No tests screen for allergenicity. All of these responses are of interest to dental manufacturers and to practitioners. Few major problems have been encountered with either composites or glass-ionomer cements. Therefore, there has been little impetus to pursue elaborate and expensive biocompatibility tests. However, the introspection with regard to dental amalgam is now forcing the dental community to document the relative safety and efficacy of all restorative materials more carefully.
Toxicology Toxicology testing does not include highly reliable procedures. Initial screening tests involve a range of possible procedures. However, even some of the more traditional procedures are now being re-examined. There is no simple way to screen and summarize the safety of composites or glass ionomers. This was emphasized recently by re-interpretations of Ames test results. Asby and Morrod (1991) questioned the validity of carcinogenic responses as maximum tolerated doses (MTD). They have suggested focusing on standard genetic toxicity tests, examining new transgenic rodent mutation assays, and more careful use of empirical markers. Above all, they emphasize the need for mechanistic studies of biological activities. Toxicological evidence must consider critical concentration effects, genotoxic vs. cytotoxic effects, and specificity of responses in certain test animals. Asby and Morrod (1991) state, "Thus, chemicals labeled carcinogenic by these [current] criteria may not present commensurate hazard to people exposed at lower, non-toxic concentrations." The MTD data from the Ames test may produce an exaggerated risk
ADV DENT RES SEPTEMBER 1992
estimation. It is important that we understand the relative risk at the concentrations experienced clinically. Testing at high dosages may produce responses in test animals that are not representative of human clinical responses. Concentration per se is not as important as the effect of concentration vs. time. Even potentially toxic amounts of materials appear to be well-managed by normal physiologic clearance mechanisms if the amount of exposureper unit time is low. In the dental situation, the concentration vs. time is mitigated by filtering effects of the dentin remaining between the source of the toxin and the dental pulp. At the present time, it is difficult to calculate the dentin effect. Dentin thickness has a substantial influence in protecting the pulp by discouraging diffusion and/or filtering materials. In most restorative dentistry situations, the cavity preparation includes a smear layer of cutting debris that may also remain as a partial barrier against diffusion into dentin. Stanley (1990) has emphasized that, in the case of fluid flow, the smear layer represents 86% of the resistance to diffusion or fluid movement. He argues that the presence of the smear layer and the amount of remaining dentin thickness (RDT) significantly affect the interpretation of any toxic events involving dental materials and the dental pulp. A classic experiment by Mjor et ai (1977) examined the relative outcomes of cell culture, implantation, and pulp tests for the same set of dental materials. They demonstrated different rankings for all three materials in all three tests. This evidence emphasized the need for understanding the relative value of each of these evaluations. As previously noted, there are observed differences in materials properties before and after setting, and as a function of dentin filtering. All of these alter the relative ranking of biological outcomes for different materials. Beside concentration and dentin effects, one must also consider a number of procedural influences at the site of a restoration. For instance, a composite restoration typically includes (a) acid-etching procedures, (b) enamel and/or dentin bonding materials, and (c) light-curing steps. Any of these may produce pulpal reactions that are otherwise not associated with the filling material. Finally, one must also consider the effects of removing previous restorative materials on the outcome of the new restoration. More and more restorative dentistry involves rerestoration rather than initial treatment for dental caries. Remnant effects of restoration removal are not yet wellunderstood.
Classifications of Dental Materials Hazards Restorative dental materials potentially affect not only the patient but also the dentist, dental assistant, and/or dental hygienist. An outline of these interactions is presented in Table 1. Jacobsen and Hensten-Pettersen (1989) have collected substantial information on the (1) occupational health hazards to dental personnel and (2) adverse patient reactions to dental materials involved with orthodontics in Norway. There were relatively high numbers of instances of contact dermatitis and inhalation reactions due to the use of acrylics and composites. This represents one of the first extensive efforts at recording hazards to personnel. Most published information about dental materials such as
Posterior Composites High Copper Amalgams Porcelain/Ceramic
Porcelain/Ceramic Posterior Composites High Copper Amalgams
Glass lonomers Fig. 1—1980 (top) and 1990 (bottom) estimates of restorative materials' comparative longevities. composites has focused on adverse patient reactions and has considered only local reactions involving pulp changes. There are potential systemic reactions as well, stemming from events occurring on occlusal surfaces or other interfaces that may release materials into the lungs, the gastro-intestinal tract, or the soft tissues of the mouth. Very little consideration is given to those types of events. All responses which are possible are quite varied and may be expressed as acute and/or chronic ones. Pulpal changes are most often recorded as patient sensitivity (pain) or pulpal inflammation. The latter is difficult to monitor clinically and can only be detected after tooth removal and histological analysis. Sensitivity (pain) is easy to detect, but may include a range of response levels. At the present time, the consensus of scientific evidence about dentin physiology is that fluid flow in the dentinal tubules alters pulpal pressures, that those changes are detected by mechano-receptors within the pulp, and that the changes are reported by specific nerve fibers as pain. Fluid flow may be produced by changes in temperature (Brannstrom and Johnson, 1970) involving hot or cold, by changes in moisture such as local desiccation, by osmotic imbalances created by local concentrations of salts or sugars, and/or by mechanical changes in the tooth. For the most part, these stimuli are encountered on an acute basis. Inflammation of the pulp, on the other hand, is related to other stimuli involving (a) mechanical, (b) thermal, (c) chemical,
Fig. 2—Dental materials usage in the United States in 1979 vs. 7990. (From Nash and Bentley, 1991) or (d) bacterial events, or (e) nerve stimulation. Mechanical trauma may be produced acutely during cavity preparation or chronically by occlusal problems. Thermal trauma may occur acutely during temperature changes due to cavity preparation or chronically due to thermal cycling related to different food temperatures. Chemical trauma may be produced acutely by diffusion of unreacted components before the setting of a restorative material or chronically by elution of mobile constituents over long times. Finally, bacterial endotoxins have been indicted as key causes of pulpal inflammation associated with leaking restoration margins. The endotoxins are produced as cellular wall degradation products from bacteria in the mouth or trapped under the restoration. Recent evidence indicates that pulpal inflammation may be of neurogenic origin. Endotoxins are most likely responsible for the greatest number of pulpal inflammation problems. Sensitivity is almost exclusively related to fluid flow events. In the absence of sensitivity, most restorations are considered clinically acceptable regardless of the pulpal condition.
CLINICAL REPORTS Clinical studies of composites and glass ionomers have never included the sophistication to provide prevalence information about biological responses. For the most part, the scientific foundations for dental clinical research have evolved during the