International Journal of Technology Assessment In Health Care, 6 (1990), 369-377. Copyright © 1990 Cambridge University Press. Printed in the U.S.A.
PRESENT AND FUTURE VALUE OF DENTAL COMPOSITE MATERIALS AND SEALANTS I. Leon Dogon Harvard School of Dental Medicine
Abstract This article reviews the development, composition, chemistry, recent technological advances, and extent ot use of composite resin restorative materials, adhesives, and pit and fissure sealants. The problems related to the clinical behavior of these materials in the oral environment are dealt with, and methods of minimizing their present deficiencies are suggested. Future directions that might be taken to improve these materials and solve some of the inadequacies that these materials exhibit are also discussed.
A composite material by definition is a material consisting of two or more components in which there is interatomic or molecular bonding between these components so as to provide overall properties superior to the constituents alone. Dental composite materials consist of a resin matrix and a filler that is usually glass, quartz, or a ceramic material. The molecular bonding is produced by the treatment of the filler with a vinyl silane material to create active sites on the filler particles enabling each particle to bond to the resin matrix. Composite resin restorative materials were first introduced in the United States almost 25 years ago. The materials available today contain primarily dimethacrylate resins such as 2,2-bis[4-(2-hydroxy-3-methacryloyloxy propoxy) phenyl] propane (bis-GMA) (5) or a urethane dimethacrylate. Because of the high viscosity of bis-GMA, diluent monomers with a low viscosity, such as triethyleneglycol dimethacrylate, are used. Plasticizers and more flexible dimethacrylates have been used in combination with the urethane dimethacrylates. These resin systems all shrink when polymerized. METHODS OF POLYMERIZATION
All composite restorative systems also include an inhibitor. For the chemically activated system, this prevents premature polymerization, and for the photo-cured materials, it provides prolonged storage life. Simply described, the method of polymerization in the chemically activated system is as follows: when the two pastes are mixed, the amine accelerator immediately begins "activating" the catalyst (benzoyl peroxide), causing it to form free radicals. The reaction to these radicals with the dimethacrylate resin molecules is temporarily prevented because the radicals react first with the inhibitors. When the small amounts of inhibitors are consumed, the catalystfree radicals then react with the resin and the vinyl coating of the filler particles to
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form chains of resin and coated fillers with cross-linkage between chains. This reaction proceeds during the hardening time, linking filler particles and resin molecules into a single, highly cross-linked chain that hardens for hours after initiation. This process does not, however, go to completion, and a significant percentage of the methacrylate grouped in the cured resin remains unreacted (3;44). An alternate means of polymerization is the use of benzoin methyl ether as the initiator and ultraviolet (UV) light as the activator rather than a chemical such as a tertiary amine. The benzoin methyl ether absorbs energy from the UV light until it is raised to the point where free radicals are generated. A third method of polymerization is by means of visible or white light as the activator. Light wavelength is usually around 400 or 500 nm. The initiator is a hydroquinone or other quinone that absorbs the energy from the white light within this specific spectrum until it is raised to the point where free radicals are generated that will react with the vinyl silane coating of the filler particles and form a highly cross-linked system. In the chemically activated restorative resin system, the polymerization reaction takes place almost uniformly throughout the bulk of the material, and curing is not dependent upon the thickness of the restoration. However, resins systems that are activated by UV or visible light polymerize only to a certain depth (14;41;42;43;45;46;51;52). The depth of cure is dependent on the depth of penetration of the activating light, composition of the material, light source and intensity, and the exposure time.
FILLER SYSTEMS
The major constituent, by weight and volume, of composite resin materials is the reinforcing filler particles. Modern composite systems contain fillers such as quartz; colloidal silica; silica glasses containing barium, strontium, or zinc; lithium-aluminum silicate; and, more recently, a synthetically produced porous zirconia silica filler. The filler imparts significant physical properties to the material, such as increased strength and modulus of elasticity, and reduces the polymerization shrinkage, the coefficient of the thermal expansion, the heat of polymerization, and the water sorption. Another advantage of most glass fillers is that they contribute radiopacity to the restorative material, facilitating detection of secondary caries as well as marginal overhangs and voids. These glasses are also softer than quartz, resulting in materials easier to polish. Prior to 1977, all composite resin materials were filled with inorganic particles between 1 and 100 um in diameter (15). These materials suffered from particle sedimentation, poor finishing, and low wear resistance. A marked improvement in the polishability, surface texture, and even distribution of particles was achieved by the use of microfine amorphous colloidal silica particles (31) with a mean particle size of approximately 0.04 um. Because of the small size of these particles, they have a marked effect on the viscosity of the resin during manufacture of the composite (13). Therefore, only a relatively small amount of the filler can be incorporated into the resin. To solve this problem, the manufacturers of microfilled composite material incorporate the colloidal silica particles into more dilute bis-GMA resins or use less viscous monomer systems. The filler monomer mixture is polymerized and crushed into relatively coarse organic particles containing the microfill particles (30). These are mixed with a monomer that also contains a filler fraction of collodial silica particles to produce the final product. The filler loading achieved by this process is slightly over 50% by weight. A further development has been the blending of conventional particle-sized fillers 370
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with colloidal silica, producing "hybrid" materials. The surface properties of these materials, however, do not compare with those of the microfilled materials and are less suitable for anterior use; however, they can offer radiopacity, whereas the microfine composite materials are not radiopaque. In recent years, manufacturers have concentrated efforts to reduce the particle size and size distribution of the conventional fillers as well as the introduction of new synthetic fillers that can be produced in relatively small particle size. Materials with average particle size of 0.1-5 (xm are considered small-particle composite materials. These tend to have a considerably higher inorganic loading and, thus, provide superior physical and mechanical properties coupled with surface finish superior to that of the conventional larger particle materials. Of equal importance to the reduction in particle size itself is the particle size distribution within the composite, which when optimal permits the composite to be compacted so that a much thinner interparticle resin layer is produced (17;37). The thinner resin layer provides greater wear resistance, which is of particular significance when these materials are used for posterior restorations. FILLER-RESIN COUPLING
Two possible mechanisms are available to achieve the bond between the resin matrix and the filler particles (47). A filler particle surface may be produced that resin can bond to mechanically (23;24;25), or a chemical bond may be achieved (6). Mechanical bonding can be performed either by sintering the glass (23) or by etching the glass (6). In both of these techniques, a porous structure is obtained into which the resin may penetrate and on polymerization mechanically bond the filler to the matrix. The chemical bonding method is the most frequently used method for bonding fillers to the matrix. This technique forms covalent bonds at the filler's surface by the use of coupling agents such as silane (4). Silane treatment may be done by aqueous solution or by dry-blending (41). More recently, synthetic zirconia silicate sintered fillers have been produced (16). As these filled particles are silane treated, the advantage of both mechanical and chemical bonding to the resin matrix is achieved. The bonding of the filler to the matrix has considerable clinical significance, as evidence suggests that debonding caused by hydrolytic degradation or slow crack growth is a major factor in wear of composite materials. POLYMERIZATION SHRINKAGE AND WATER SORPTION
The matrix resin of all composite restorative materials shrink volumetrically approximately 10% on polymerization (27). This shrinkage is markedly reduced by the incorporation of filler particles, and, therefore, the higher the filler loading, the less shrinkage should take place. The clinical significance of polymerization shrinkage is its effect on the enamel-composite resin interface and the formation of a marginal gap. This gap at the tooth restorative material interface may produce microleakage with resultant postoperative sensitivity and subsequent recurrent decay. "Contraction gaps" of up to 17 urn in commercially available composite materials have been demonstrated in laboratory studies (20). The use of the acid-etch technique, where enamel surrounds the preparation, will minimize this gap formation clinically. In areas where cementum or dentin interfaces are present, the careful use of dentin adhesives will reduce the incidence of microleakage. All composite resins absorb water from the oral environment and undergo INTL. J. OF TECHNOLOGY ASSESSMENT IN HEALTH CARE
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hygroscopic expansion (15). Linear expression, caused by water uptake ranging from 0.09-0.72%, has been reported (7). It has been shown experimentally that the marginal gap can in fact be closed by hygroscopic expansion of the composite resin material (1). However, most of these experiments on the marginal gap have been done without using the acid-etch technique to bond to the enamels surrounding the restoration. BIOCOMPATIBILITY OF COMPOSITE RESIN MATERIALS
Considerable histological evidence of pulp reactions to composite resin restorative materials is seen in the literature (10;27;32;33;48;49;50). The use of a liner or base on the dentin prior to the application of the composite is strongly recommended by all authors. Acid etching the enamel markedly reduces microleakage and the possibility of the invasion of bacteria. The secondary invasion of bacteria into these spaces is suggested by many authors (11;12;35) to be of primary importance in considering the biocompatibility of these materials. Incremental placement of composite materials and the careful use of dentin adhesives in areas where enamel is not present also helps to reduce the incidence of microleakage and secondary bacterial invasion. The most severe pulp reactions to the composite material constituents occur in inadequately polymerized materials. Unpolymerized monomer can penetrate through the dentinal tubules into the pulpal tissue with subsequent inflammatory response. CLINICAL USE OF COMPOSITE RESTORATIVE MATERIALS
Composite resin restorative materials were introduced in the United States approximately 25 years ago. Considerable progress has been made in the improvement of these materials particularly over the past 10 years. A greater understanding of the role of the various components that make up the composite materials has been achieved particularly in relation to the filler materials, where the greatest improvements have come. As an anterior restorative material, the composite materials are the restorative materials of choice in most cases. Together with the development of the acid-etch technique, these materials are used in a wide range of restorative procedures from Class III or V restorations to complex tetracycline masking and diastema closing. The microfill composites provide a restorative material of considerable esthetic quality. The range of hue, value, and chroma of these materials can be selected to match closely those of enamel and dentin, as required of a material that is needed to replace both tissues in a restoration (15). The color stability of these materials, although by no means perfect, has improved considerably over the earlier conventional composites. This is true particularly of the microfill materials that are polymerized by visible light (20). The clinical performance of conventional composite resins used as a restorative material in the posterior teeth has been extremely disappointing, particularly because of the excessive wear that occurs on the occlusal surface (22;39). In recent years, three different approaches have been taken in an attempt to improve the wear characteristics of the material. First, a variety of filler materials has been developed in an attempt to minimize occlusal wear. The second approach was the development of improved polymerization systems that include the use of UV and visible light photoinitiators. The third method used to improve the mechanical properties of the composite restorative materials has been reduction in the amount of resin exposed to the abrasive and erosive forces. This was accomplished by increasing the amount of filler loading, reducing the average particle size and optimizing the particle size distribution and particle filler surface so as to minimize the amount of matrix resin exposed to the abrading 372
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forces. More recent laboratory studies have shown that the new posterior composite materials that have made use of these improvements in composite technology are approaching the wear characteristic of amalgam (C. L. Davidson, personal communication; W. D. Douglas, personal communication). According to sales statistics of the American Dental Trade Association, over 40% of practicing general dentists in the United States are using composite materials developed primarily for posterior use. Because these materials are also used for "core" buildup and other nonrestorative use, it is estimated that these dentists are placing posterior composites in 15-25% of the posterior teeth that they restore. With few exceptions, therefore, there does not appear to be a widespread indiscriminate use of these materials in North America, but rather a judicious, selective use in the vast majority of practices that use these materials. Posterior composite materials are comparatively new, and techniques for their use are just being developed. They are also less forgiving as a restorative material than amalgam and require more exacting techniques for manipulation. Consequently, an acceptable restoration takes more time to complete. DENTIN BONDING
The bonding of a restorative material to dentin has been a goal of dental material scientists for many years. The advantages of such a material are numerous and include the preservation of dental tissue in that nonretentive cavities can be prepared leading to elimination of microleakage and its sequelae. A number of dentin bonding agents have the morphological features of the "smear" layer. The removal of the smear layer by acid treatment is a prerequisite for successful bonding of clearfill resin (Kuraray) (28). Pashley and others (9;36;38) showed that removal of the smear layer increases the permeability of dentin in vivo. A possible consequence of this is moisture contamination of the dentin surface from fluid moving out of the dentinal tubules. The recommended use of Scotchbond (3M Co.), however, requires that the smear layer remains intact. The adhesive resin infiltrates the surface of the smear layer and chelates to the liberated calcium. Gluma (Bayer U.K. Ltd.), a mixture of gluteraldehyde and 2hydroxyethylmethacrylate (HEMA) (36), fixes the smear layer with the gluteraldehyde prior to the application of the adhesive resin. Finally, adhesive such as Scotchbond 2 (3M Co.) requires the removal of the smear layer with a primer composed of a mixture of carboxylrc acids and the precipitation of HEMA onto the dentin surface to which the adhesive is then able to bond. Other adhesive systems require pretreating surfaces with acidified ferric oxylate solutions, then /V-phenylglycine, followed by PMDM (7). It appears, however, that the forces exerted by the polymerization shrinkage are considerably greater than the bonding forces of the dentin adhesive materials (18). Cavity shape and incremental layering of the restorative composite material can minimize the polymerization shrinkage (19). McLean et al. (34) proposed an alternative means of achieving bonding to dentin: the use of the glass-ionomer cement as a base to the composite resin. The advent of visible light cure made possible glass-ionomer materials with higher bond strengths to dentin than those obtained with conventional glass ionomer materials. This should make a very suitable technique for clinical use, as the bonding of the glass ionomer to the composite material is readily achieved. FUTURE PROSPECTS AND DEVELOPMENTS
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materials. Two major deficiencies remain in the use of composite restorative material for posterior use. These are polymerization shrinkage, which leads to marginal gap formation and microleakage, and the lack of resistance to wear. Both of these phenomena are primarily due to the matrix resin. Alternative reactive monomers may replace the bis-GMA and urethane dimethacrylates that are predominantly used today. Cook (15) suggested the use of fluorinated dimethacrylates because of their hydrophobic nature that may reduce marginal gap formation and microleakage. He further suggested departure from conventional resins by using novel monomers such as the unsaturated spiro-orthocarbonates that expand during cure, thus eliminating the marginal gap. Newer and superior dentin adhesive materials are in the developmental stage and should make bonding to dentin and cementum a practical realization. At this stage, the techniques for posterior composite placement are exacting and time-consuming; however, the potential for the development of materials that can be considerably more compatible to the oral environment and easier to manipulate is well advanced. FISSURE SEALANTS The pit and fissure sealant resins are basically identical to those used in the preparation of composite restorative materials. Both bis-GMA and urethane dimethacrylate diluted with the appropriate dimethacrylate resins are used for pit and fissure sealing. Chemically cured as well as visible-light-cured materials are widely used, and both types appear to have adequate retention over prolonged periods. Recently, glass ionomer pit and fissure sealants have been introduced. Their proposed advantage is the possibility of the transfer of fluoride ion from the fluoride glass present in the filler of the ionomer. Unfortunately, early results show the retention of this material in pits and fissures to be extremely low. However, high levels of residual fluoride may be retained for some period in the fissures, although this has not been clearly demonstrated. Of marked significance was the National Institute of Health Consensus Development Conference held on "Dental Sealants in the Prevention of Tooth Decay" in December 1984. The conference focused on seven questions. The answers to the more significant questions are summarized below. The panel concluded that pit and fissure sealants are needed because dental caries today is largely a disease of pits and fissures since the widespread use of fluorides has markedly decreased smooth surface caries. Sealants were found to be highly effective in preventing pit and fissure caries, with 100% protection in pits and fissures that remain completely sealed. The risks associated with the use of sealants were found to be minimal, and sealants are safe when properly placed with the state-of-the-art materials and procedures. The current status of sealant research has established the safety of the method with a high degree of confidence. Clinical investigators have shown that sealants are a highly effective means of preventing pit and fissure caries. The panel suggested that the following objectives should be a basis for future research: • improving current sealant technology, such as improved acid-etch methods and new materials, possibly with a fluoride releasing ability; • studying the effects of sealants on cariogenic bacteria; • collecting more data on cost-effectiveness; • developing new technologies for bonding sealants to enamel that would not require acid etching of the enamel or the strict avoidance of contamination of the enamel surface; • further understanding of the reasons for sealant underutilization; and • developing low-cost screening methods to identify children at high risk of developing pit and fissure caries. 374
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Experimental compositions of sealants withfluoride-releasingability are already being tested both in vitro as well as in pilot clinical studies. Early results with these materials are extremely promising (29) and may, in addition to occluding the pits and fissures, be a novel means of delivering aflowof fluoride ion into the oral environment. REFERENCES
1. Asmussen, E., & Jorgensen, K. D. A microscopic investigation of the adaption of some plastic filling materials to dental cavity walls. Ada Odontologica Scandinavica, 1972,30,3-21. 2. Asmussen, E. Composite restorative resins: Composition versus wall-to-wall polymerization contraction. Ada Odontologica Scandinavica, 1975, 33, 337-44. 3. Asmussen, E. Restorative resins: Hardness and strength vs. quantity of remaining double bonds. Scandinavian Journal of Dental Research, 1982, 90, 484-89. 4. Bjorksten, J., & Yeager, L. L. Vinyl silane size for glass fabric. Modern Plastics, 1952, 29, 124, 188. 5. Bowen, R. L. Synthesis of a silica resin direct filling material, progress report. Journal of Dental Research, 1958, 37, 90. 6. Bowen, R. L. Properties of a silica-reinforced polymer for dental restorations. Journal of the American Dental Association, 1963, 66, 57-64. 7. Bowen, R. L., Cobb, E. N., & Rapson, J. E. Adhesive bonding of various materials to hard tooth tissues, XXV improvement in bond strength to dentin. Journal of Dental Research, 1982, 62, 1070-76. 8. Bowen, R. L., Rapson, J. E., & Dickson, G. Hardening shrinkage and hygroscopic expansion of composite resins. Journal of Dental Research, 1982, 61, 654-58. 9. Bowen, R. L., Eick, J. D., Henderson, D.A., & Anderson, D. W. Smear layer removal and bonding considerations. Operative Dentistry Journal Supplement, 1984, 3, 30-34. 10. Brannstrom, M., & Nyborg, H. Pulpal reactions of composite resin restorations. Journal of Prosthetic Dentistry, 1972, 27, 181-89. 11. Brannstrom, M., & Nordenvall, K. J. Bacterial penetration pulpal reaction and the inner surfact of concise enamel bond composite fillings in etched and unetched cavities. Journal of Dental Research, 1978, 57, 3-10. 12. Brannstrom, M. Communication between the oral cavity and the dental pulp associated with restorative treatment. Operative Dentistry, 1984, 9, 57-68. 13. Brunner, H.,&Schutte, D. InGrundlagenundAnwendungen vonAEROSIL, Teil9:Hydrophobes AEROSIL, Herstellung, Eigenschaften und Verhalten. Degussa: Werk Rheinfelden, 1973. 14. Cook, W. D. Factors affecting the depth of cure of UV-polymerized composites. Journal of Dental Research, 1980, 59, 800-08. 15. Cook, W. D., Beech, D. R., &Tyas M. J. Resin-based restorative materials-A review. Australian Dental Journal, 1984, 29, 291-95. 16. Creo, A., & Steen, D. Technical Bulletin. St. Paul, MN: 3M Company, 1987. 17. Cross, M., Douglas, W. H., & Fields, R. P. Optimal design methodology for composite materials with particulate fillers. Powder Technology, 1985, 1-10. 18. Davidson, C. L., de Gee, A. J., & Feilzer, A. The competition between the composite dentin bond strength and the polymerization contraction stress. Journal of Dental Research, 1984, 63, 1396. 19. Davidson, C. L. Conflicting interests with posterior use of composite materials. In G. Vanherle & D. C. Smith (eds.), Posterior composite resins dental restorative materials, 1965,61-65. 20. Dippel, H. W., Borggreven, J. M., & Hoppenbrouwers, P. M. Morphology and permeability of the dentinal smear layer. Journal of Prosthetic Dentistry, 1984, 52, 657-62. 21. Dogon, I. L., Murray, L., Van Leeuwen, M. J., et al. Three year comparison of a light cured vs. chemically cured microfilled material. Journal of Dental Research, 1985, 353, 1603. 22. Eames W. B., Strain, J. D., Weitman, R. T., & Williams, A. K. Clinical comparison of composite, amalgam and silicate restorations. Journal of the American Dental Association, 1974, 89, 1111-17. INTL. J. OF TECHNOLOGY ASSESSMENT IN HEALTH CARE
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Dogon 23. Ehrnford, L. A method for reinforcing dental composite restorative materials. Odontologisc Revy, 1976, 27, 51-54. 24. Ehrnford, L. Composite resins with a condensable inorganic phase. Journal of Dental Research, 1981, 60, 1759-66. 25. Ehrnford, L. Dental composites reinforced with microporous sintered glassfiber networks. Swedish Dental Journal, 1983, Supplement 18. 26. Eriksen, H. M. Pulpal response of monkeys to a composite resin cement. Journal of Dental Research, 1974, 53, 565-76. 27. Glenn, J. F. Composition and properties of unfilled and composite resin restorative materials. In D. C. Smith & D. F. Williams (eds.), Biocompatibility of dental materials, CRC Press, 1982, 98-130. 28. Hosoda, H., & Fusyama, T. A tooth substance saving restorative technique. International Dental Journal, 1984, 34, 1-12. 29. Ishikawa, M., Dogon, I. L., Van Leeuwen, M. J., & Norris, D. A comparative study of two fluoride releasing sealant materials. Journal of Dental Research, 1989, Special Issue, 1178, 1014. 30. Jacobsen, P. H. The current status of composite restorative materials. British Dental Journal, 1981, 150, 15-18. 31. Jorgensen, K. D., & Asmussen, E. Occlusal abrasion of a composite restorative resin with ultra-fine filler —an initial study. Quintessence International, 1978, 9, 73-78. 32. Langeland, L. K., Guttuso, J., Jerome, D. R., & Langeland, K. Histologic and clinical comparison of addent with silicate cement and cold-curing materials. Journal of the American Dental Association, 1965, 72, 373-85. 33. Langeland, K., Dogon, I. L., & Langeland, L. K. Pulp protection requirements for composite resin restorations: A clinical and histological investigation. Australian Dental Journal, 1970, 15, 349-50. 34. McLean, J. W., Powis, D. R., Posser, H. J., et al. The use of glass-ionomer cements in bonding composite resins to dentin. British Dental Journal, 1985, 158, 410. 35. Mjor, I. The penetration of bacteria into experimentally exposed human coronal dentin. Scandinavian Journal of Dental Research, 1974, 82, 191-96. 36. Munksgaard, E. C , & Asmussen, E. Bond strength between dentin and restorative resins mediated by mixtures of HEMA and glutaraldehyde. Journal of Dental Research, 1984, 63, 1087. 3 7. Okazaki, M., & Douglas W. H. Comparison of surface layer properties of composite resins by ESCA, SEM and X-Ray diffractometry. Biomaterials, 1984, 5, 284-88. 38. Pashley, D. H., Kepler, E. E., Williams, E. C , & Okabe, A. The effects of acid etching on the in-vivo permeability of dentin in the dog. Archives of Oral Biology, 1983,28,555-59. 39. Phillips, R. W., Avery, D. R., Mehra, R., et al. Observation on a composite resin for Class II restorations. Journal of Prosthetic Dentistry, 1973, 30, 891-97. 40. Plueddemann, E. P. Silane coupling agents, New York: Plenum Press, 1982. 41. Reinhardt, K. J., & Vahl, J. Utersuchunger fullstoffhaltiger Und-freier Adhasive im Vergleich zu Einem Composite. Dtsch Zahnartzl Z, 1977, 32, 867-70. 42. Reinhardt, K. J., & Vahl J. Einfluss von Sauerstoff und Feuchtigkeit auf UV-Polymerisierbare Versiegelungsmaterialien. Dtsch Zahnartzl Z, 1978, 33, 384-87. 43. Reinhardt, K. J., & Vahl, J. Ein Vergleich lichthartendent un UV-Polymerisierbarer Versieglaer und Komposite. Dtsch Zahnartzl Z, 1979, 34, 245-50. 44. Rutyer, I. E., & Svendson, S. A. Remaining methacrylate groups on composite restorative materials. Acta Odontologica Scandinavica, 1978, 36, 75-82. 45. Ruyter, I. E., & Oysaed, H. Conversion in different depths of ultra-violet and visible light activated composite materials. Acta Odontologica Scandinavica, 1982, 40, 179-82. 46. Salako, N. O., & Cruickshanks-Boyd, D. N. Curing depths of materials polymerized by ultra-violet light. British Dental Journal, 1979, 146, 375-79. 47. Sonderholm, K. J. M. Filler systems and resin interface. In G. Vanherle and D. C. Smith (eds.), Posterior composite resin dental restorative materials, 1985, 139-45.
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Value of composite materials 48. Stanley, H. R., Going, R. E., & Chauncey, H. H. Human pulp response to acid pretreatment of dentin and composite restoration. Journal of the American Dental Association, 1975, 91, 817-25. 49. Stanley, H. R., Bowen, R. L., & Folio, J. Compatibility of various materials with oral tissues, II: Pulp responses to composite ingredients. Journal of Dental Research, 1979,58,1507-17. 50. Suarez, C. L., Stanley, H. R., & Gilmore, H. W. Histopathologic response of the human dental pulp to restorative resins. Journal of the American Dental Association, 1970, 80, 792-800. 51. Viohl, J. Wie tief Polymerisieren UV-hartende Kunststoffe? Dtsch Zahnartzl Z, 1978, 33,476. 52. Viohl, J. Polymerisationstiefe von Photopolymerisierende Fullungskunststoffen. Dtsch Zahnartzl Z, 1982, 37, 194-96.
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PRESENT AND FUTURE VALUE OF DENTAL COMPOSITE MATERIALS AND SEALANTS I. Leon Dogon Harvard School of Dental Medicine
Abstract This article reviews the development, composition, chemistry, recent technological advances, and extent ot use of composite resin restorative materials, adhesives, and pit and fissure sealants. The problems related to the clinical behavior of these materials in the oral environment are dealt with, and methods of minimizing their present deficiencies are suggested. Future directions that might be taken to improve these materials and solve some of the inadequacies that these materials exhibit are also discussed.
A composite material by definition is a material consisting of two or more components in which there is interatomic or molecular bonding between these components so as to provide overall properties superior to the constituents alone. Dental composite materials consist of a resin matrix and a filler that is usually glass, quartz, or a ceramic material. The molecular bonding is produced by the treatment of the filler with a vinyl silane material to create active sites on the filler particles enabling each particle to bond to the resin matrix. Composite resin restorative materials were first introduced in the United States almost 25 years ago. The materials available today contain primarily dimethacrylate resins such as 2,2-bis[4-(2-hydroxy-3-methacryloyloxy propoxy) phenyl] propane (bis-GMA) (5) or a urethane dimethacrylate. Because of the high viscosity of bis-GMA, diluent monomers with a low viscosity, such as triethyleneglycol dimethacrylate, are used. Plasticizers and more flexible dimethacrylates have been used in combination with the urethane dimethacrylates. These resin systems all shrink when polymerized. METHODS OF POLYMERIZATION
All composite restorative systems also include an inhibitor. For the chemically activated system, this prevents premature polymerization, and for the photo-cured materials, it provides prolonged storage life. Simply described, the method of polymerization in the chemically activated system is as follows: when the two pastes are mixed, the amine accelerator immediately begins "activating" the catalyst (benzoyl peroxide), causing it to form free radicals. The reaction to these radicals with the dimethacrylate resin molecules is temporarily prevented because the radicals react first with the inhibitors. When the small amounts of inhibitors are consumed, the catalystfree radicals then react with the resin and the vinyl coating of the filler particles to
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form chains of resin and coated fillers with cross-linkage between chains. This reaction proceeds during the hardening time, linking filler particles and resin molecules into a single, highly cross-linked chain that hardens for hours after initiation. This process does not, however, go to completion, and a significant percentage of the methacrylate grouped in the cured resin remains unreacted (3;44). An alternate means of polymerization is the use of benzoin methyl ether as the initiator and ultraviolet (UV) light as the activator rather than a chemical such as a tertiary amine. The benzoin methyl ether absorbs energy from the UV light until it is raised to the point where free radicals are generated. A third method of polymerization is by means of visible or white light as the activator. Light wavelength is usually around 400 or 500 nm. The initiator is a hydroquinone or other quinone that absorbs the energy from the white light within this specific spectrum until it is raised to the point where free radicals are generated that will react with the vinyl silane coating of the filler particles and form a highly cross-linked system. In the chemically activated restorative resin system, the polymerization reaction takes place almost uniformly throughout the bulk of the material, and curing is not dependent upon the thickness of the restoration. However, resins systems that are activated by UV or visible light polymerize only to a certain depth (14;41;42;43;45;46;51;52). The depth of cure is dependent on the depth of penetration of the activating light, composition of the material, light source and intensity, and the exposure time.
FILLER SYSTEMS
The major constituent, by weight and volume, of composite resin materials is the reinforcing filler particles. Modern composite systems contain fillers such as quartz; colloidal silica; silica glasses containing barium, strontium, or zinc; lithium-aluminum silicate; and, more recently, a synthetically produced porous zirconia silica filler. The filler imparts significant physical properties to the material, such as increased strength and modulus of elasticity, and reduces the polymerization shrinkage, the coefficient of the thermal expansion, the heat of polymerization, and the water sorption. Another advantage of most glass fillers is that they contribute radiopacity to the restorative material, facilitating detection of secondary caries as well as marginal overhangs and voids. These glasses are also softer than quartz, resulting in materials easier to polish. Prior to 1977, all composite resin materials were filled with inorganic particles between 1 and 100 um in diameter (15). These materials suffered from particle sedimentation, poor finishing, and low wear resistance. A marked improvement in the polishability, surface texture, and even distribution of particles was achieved by the use of microfine amorphous colloidal silica particles (31) with a mean particle size of approximately 0.04 um. Because of the small size of these particles, they have a marked effect on the viscosity of the resin during manufacture of the composite (13). Therefore, only a relatively small amount of the filler can be incorporated into the resin. To solve this problem, the manufacturers of microfilled composite material incorporate the colloidal silica particles into more dilute bis-GMA resins or use less viscous monomer systems. The filler monomer mixture is polymerized and crushed into relatively coarse organic particles containing the microfill particles (30). These are mixed with a monomer that also contains a filler fraction of collodial silica particles to produce the final product. The filler loading achieved by this process is slightly over 50% by weight. A further development has been the blending of conventional particle-sized fillers 370
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with colloidal silica, producing "hybrid" materials. The surface properties of these materials, however, do not compare with those of the microfilled materials and are less suitable for anterior use; however, they can offer radiopacity, whereas the microfine composite materials are not radiopaque. In recent years, manufacturers have concentrated efforts to reduce the particle size and size distribution of the conventional fillers as well as the introduction of new synthetic fillers that can be produced in relatively small particle size. Materials with average particle size of 0.1-5 (xm are considered small-particle composite materials. These tend to have a considerably higher inorganic loading and, thus, provide superior physical and mechanical properties coupled with surface finish superior to that of the conventional larger particle materials. Of equal importance to the reduction in particle size itself is the particle size distribution within the composite, which when optimal permits the composite to be compacted so that a much thinner interparticle resin layer is produced (17;37). The thinner resin layer provides greater wear resistance, which is of particular significance when these materials are used for posterior restorations. FILLER-RESIN COUPLING
Two possible mechanisms are available to achieve the bond between the resin matrix and the filler particles (47). A filler particle surface may be produced that resin can bond to mechanically (23;24;25), or a chemical bond may be achieved (6). Mechanical bonding can be performed either by sintering the glass (23) or by etching the glass (6). In both of these techniques, a porous structure is obtained into which the resin may penetrate and on polymerization mechanically bond the filler to the matrix. The chemical bonding method is the most frequently used method for bonding fillers to the matrix. This technique forms covalent bonds at the filler's surface by the use of coupling agents such as silane (4). Silane treatment may be done by aqueous solution or by dry-blending (41). More recently, synthetic zirconia silicate sintered fillers have been produced (16). As these filled particles are silane treated, the advantage of both mechanical and chemical bonding to the resin matrix is achieved. The bonding of the filler to the matrix has considerable clinical significance, as evidence suggests that debonding caused by hydrolytic degradation or slow crack growth is a major factor in wear of composite materials. POLYMERIZATION SHRINKAGE AND WATER SORPTION
The matrix resin of all composite restorative materials shrink volumetrically approximately 10% on polymerization (27). This shrinkage is markedly reduced by the incorporation of filler particles, and, therefore, the higher the filler loading, the less shrinkage should take place. The clinical significance of polymerization shrinkage is its effect on the enamel-composite resin interface and the formation of a marginal gap. This gap at the tooth restorative material interface may produce microleakage with resultant postoperative sensitivity and subsequent recurrent decay. "Contraction gaps" of up to 17 urn in commercially available composite materials have been demonstrated in laboratory studies (20). The use of the acid-etch technique, where enamel surrounds the preparation, will minimize this gap formation clinically. In areas where cementum or dentin interfaces are present, the careful use of dentin adhesives will reduce the incidence of microleakage. All composite resins absorb water from the oral environment and undergo INTL. J. OF TECHNOLOGY ASSESSMENT IN HEALTH CARE
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hygroscopic expansion (15). Linear expression, caused by water uptake ranging from 0.09-0.72%, has been reported (7). It has been shown experimentally that the marginal gap can in fact be closed by hygroscopic expansion of the composite resin material (1). However, most of these experiments on the marginal gap have been done without using the acid-etch technique to bond to the enamels surrounding the restoration. BIOCOMPATIBILITY OF COMPOSITE RESIN MATERIALS
Considerable histological evidence of pulp reactions to composite resin restorative materials is seen in the literature (10;27;32;33;48;49;50). The use of a liner or base on the dentin prior to the application of the composite is strongly recommended by all authors. Acid etching the enamel markedly reduces microleakage and the possibility of the invasion of bacteria. The secondary invasion of bacteria into these spaces is suggested by many authors (11;12;35) to be of primary importance in considering the biocompatibility of these materials. Incremental placement of composite materials and the careful use of dentin adhesives in areas where enamel is not present also helps to reduce the incidence of microleakage and secondary bacterial invasion. The most severe pulp reactions to the composite material constituents occur in inadequately polymerized materials. Unpolymerized monomer can penetrate through the dentinal tubules into the pulpal tissue with subsequent inflammatory response. CLINICAL USE OF COMPOSITE RESTORATIVE MATERIALS
Composite resin restorative materials were introduced in the United States approximately 25 years ago. Considerable progress has been made in the improvement of these materials particularly over the past 10 years. A greater understanding of the role of the various components that make up the composite materials has been achieved particularly in relation to the filler materials, where the greatest improvements have come. As an anterior restorative material, the composite materials are the restorative materials of choice in most cases. Together with the development of the acid-etch technique, these materials are used in a wide range of restorative procedures from Class III or V restorations to complex tetracycline masking and diastema closing. The microfill composites provide a restorative material of considerable esthetic quality. The range of hue, value, and chroma of these materials can be selected to match closely those of enamel and dentin, as required of a material that is needed to replace both tissues in a restoration (15). The color stability of these materials, although by no means perfect, has improved considerably over the earlier conventional composites. This is true particularly of the microfill materials that are polymerized by visible light (20). The clinical performance of conventional composite resins used as a restorative material in the posterior teeth has been extremely disappointing, particularly because of the excessive wear that occurs on the occlusal surface (22;39). In recent years, three different approaches have been taken in an attempt to improve the wear characteristics of the material. First, a variety of filler materials has been developed in an attempt to minimize occlusal wear. The second approach was the development of improved polymerization systems that include the use of UV and visible light photoinitiators. The third method used to improve the mechanical properties of the composite restorative materials has been reduction in the amount of resin exposed to the abrasive and erosive forces. This was accomplished by increasing the amount of filler loading, reducing the average particle size and optimizing the particle size distribution and particle filler surface so as to minimize the amount of matrix resin exposed to the abrading 372
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forces. More recent laboratory studies have shown that the new posterior composite materials that have made use of these improvements in composite technology are approaching the wear characteristic of amalgam (C. L. Davidson, personal communication; W. D. Douglas, personal communication). According to sales statistics of the American Dental Trade Association, over 40% of practicing general dentists in the United States are using composite materials developed primarily for posterior use. Because these materials are also used for "core" buildup and other nonrestorative use, it is estimated that these dentists are placing posterior composites in 15-25% of the posterior teeth that they restore. With few exceptions, therefore, there does not appear to be a widespread indiscriminate use of these materials in North America, but rather a judicious, selective use in the vast majority of practices that use these materials. Posterior composite materials are comparatively new, and techniques for their use are just being developed. They are also less forgiving as a restorative material than amalgam and require more exacting techniques for manipulation. Consequently, an acceptable restoration takes more time to complete. DENTIN BONDING
The bonding of a restorative material to dentin has been a goal of dental material scientists for many years. The advantages of such a material are numerous and include the preservation of dental tissue in that nonretentive cavities can be prepared leading to elimination of microleakage and its sequelae. A number of dentin bonding agents have the morphological features of the "smear" layer. The removal of the smear layer by acid treatment is a prerequisite for successful bonding of clearfill resin (Kuraray) (28). Pashley and others (9;36;38) showed that removal of the smear layer increases the permeability of dentin in vivo. A possible consequence of this is moisture contamination of the dentin surface from fluid moving out of the dentinal tubules. The recommended use of Scotchbond (3M Co.), however, requires that the smear layer remains intact. The adhesive resin infiltrates the surface of the smear layer and chelates to the liberated calcium. Gluma (Bayer U.K. Ltd.), a mixture of gluteraldehyde and 2hydroxyethylmethacrylate (HEMA) (36), fixes the smear layer with the gluteraldehyde prior to the application of the adhesive resin. Finally, adhesive such as Scotchbond 2 (3M Co.) requires the removal of the smear layer with a primer composed of a mixture of carboxylrc acids and the precipitation of HEMA onto the dentin surface to which the adhesive is then able to bond. Other adhesive systems require pretreating surfaces with acidified ferric oxylate solutions, then /V-phenylglycine, followed by PMDM (7). It appears, however, that the forces exerted by the polymerization shrinkage are considerably greater than the bonding forces of the dentin adhesive materials (18). Cavity shape and incremental layering of the restorative composite material can minimize the polymerization shrinkage (19). McLean et al. (34) proposed an alternative means of achieving bonding to dentin: the use of the glass-ionomer cement as a base to the composite resin. The advent of visible light cure made possible glass-ionomer materials with higher bond strengths to dentin than those obtained with conventional glass ionomer materials. This should make a very suitable technique for clinical use, as the bonding of the glass ionomer to the composite material is readily achieved. FUTURE PROSPECTS AND DEVELOPMENTS
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materials. Two major deficiencies remain in the use of composite restorative material for posterior use. These are polymerization shrinkage, which leads to marginal gap formation and microleakage, and the lack of resistance to wear. Both of these phenomena are primarily due to the matrix resin. Alternative reactive monomers may replace the bis-GMA and urethane dimethacrylates that are predominantly used today. Cook (15) suggested the use of fluorinated dimethacrylates because of their hydrophobic nature that may reduce marginal gap formation and microleakage. He further suggested departure from conventional resins by using novel monomers such as the unsaturated spiro-orthocarbonates that expand during cure, thus eliminating the marginal gap. Newer and superior dentin adhesive materials are in the developmental stage and should make bonding to dentin and cementum a practical realization. At this stage, the techniques for posterior composite placement are exacting and time-consuming; however, the potential for the development of materials that can be considerably more compatible to the oral environment and easier to manipulate is well advanced. FISSURE SEALANTS The pit and fissure sealant resins are basically identical to those used in the preparation of composite restorative materials. Both bis-GMA and urethane dimethacrylate diluted with the appropriate dimethacrylate resins are used for pit and fissure sealing. Chemically cured as well as visible-light-cured materials are widely used, and both types appear to have adequate retention over prolonged periods. Recently, glass ionomer pit and fissure sealants have been introduced. Their proposed advantage is the possibility of the transfer of fluoride ion from the fluoride glass present in the filler of the ionomer. Unfortunately, early results show the retention of this material in pits and fissures to be extremely low. However, high levels of residual fluoride may be retained for some period in the fissures, although this has not been clearly demonstrated. Of marked significance was the National Institute of Health Consensus Development Conference held on "Dental Sealants in the Prevention of Tooth Decay" in December 1984. The conference focused on seven questions. The answers to the more significant questions are summarized below. The panel concluded that pit and fissure sealants are needed because dental caries today is largely a disease of pits and fissures since the widespread use of fluorides has markedly decreased smooth surface caries. Sealants were found to be highly effective in preventing pit and fissure caries, with 100% protection in pits and fissures that remain completely sealed. The risks associated with the use of sealants were found to be minimal, and sealants are safe when properly placed with the state-of-the-art materials and procedures. The current status of sealant research has established the safety of the method with a high degree of confidence. Clinical investigators have shown that sealants are a highly effective means of preventing pit and fissure caries. The panel suggested that the following objectives should be a basis for future research: • improving current sealant technology, such as improved acid-etch methods and new materials, possibly with a fluoride releasing ability; • studying the effects of sealants on cariogenic bacteria; • collecting more data on cost-effectiveness; • developing new technologies for bonding sealants to enamel that would not require acid etching of the enamel or the strict avoidance of contamination of the enamel surface; • further understanding of the reasons for sealant underutilization; and • developing low-cost screening methods to identify children at high risk of developing pit and fissure caries. 374
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Experimental compositions of sealants withfluoride-releasingability are already being tested both in vitro as well as in pilot clinical studies. Early results with these materials are extremely promising (29) and may, in addition to occluding the pits and fissures, be a novel means of delivering aflowof fluoride ion into the oral environment. REFERENCES
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