Composite and Sealant Resins -- Past, Present, and Future

Composite and sealant resins -- past, present, and future

R. L. Bowen, DDS

Abstract they were very adhesive to most solid substances, Composite dental filling materiMs were developed in and became physically strong and chemically inert response to the shortcomings of silicate cements and polymers. At the time, it seemed reasonable that unfilied resins (based on .methyl methacrylate monomer these resins could be used as adhesive binders for par- and its polymer). A hybrid monomer, which came to be ticles of porcelain, fused quartz, or other appropriate known as "BIS-GMA"in the dental literature, was inorganic filler materials. It was thought that such synthesized; this molecule resembles an epoxy resin except that the epoxy groups are replaced by a mixture might be placed into a dental cavity methacrylate groups. BIS-GMAformulations can preparation where the maximally filled epoxy resin polymerize rapidly under oral conch’tions, and they have would harden and adhesively bond the particles polymerization shrinkage less than that of methyl together and the "silica-resin" material to the cav- methacrylate. BIS-GMAresins are used as binders for ity walls, thereby forming an esthetic, durable glass, porcelain, or quartz particles to form relatively restorative material. The major flaw in this scheme durable direct esthetic filling materials. In combination was that such materials did not harden quickly with the acid-etch technique, developed elsewhere, BI~ enoughfor use as direct filling materials in dentistry. GMAformulations are used in the repair of fractured This limitation called for the synthesis of a new incisor teetlz The combination is also useful to bind monomer which would resemble the epoxy resin so orthodontic brackets directly to teeth and for surgical as to have relatively low hardening shrinkage, yet ex- procedures in which teeth are not properly placed or hibit a rapid polymerization and hardening reaction. ah’gnedfor eruption. This resin without filler is also used to prevent decay by the filling of developmental Methyl methacrylate polymerized rapidly and was pits and fissures in teeth which would otherwise have a used, together with particles of its polymeras a filler, high susceptibility to caries. Improvementsin the glass for direct dental fillings. The methyl methacrylate filler for composite resins maylead to greater direct filling resins were flawed primarily because of durability in their chnical uses. Recent developments in their large polymerization shrinkage, low stiffness, adhesive bonding to teeth wili also widen the utility of high coefficient of thermal expansion., and other composites. lesser5.6 faults. A hybrid monomer, "BIS-GMA," was then This is an informal essay that will give a broad synthesized. 7 It was a large molecule that resembled perspective and set the stage for more specific discus- an epoxy resin except that the epoxy ,groups were sions in articles following. replaced with methacrylate groups. 8 Therefore, it could polymerize rapidly under oral conditions, and Past History yet its polymerization shrinkage was only about one- Composite dental filling materials were developed third as great as that of methyl methacrylate. This in response to the severe shortcomings of silicate viscous liquid resin {BIS-GMA}could be used as a cements.~ Silicate cement restorations were subject to binder for glass, porcelain, or quartz particles to form acidic decays and were useful for only four to five years a stiff, strong and relatively durable direct esthetic on3 the average. filling material. This resin came to be knownas "BIS- Epoxy resins were being used in industrial applica- GMA",an acronym that is more convenient than the tions, and their intriguing properties suggested that long chemical name of this molecule. they might have useful dental applications. 4 The Dr. Michael Buonocore, Eastman Dental Center, liquid epoxy resins could be mixed with a liquid at the University of Rochester School of Medicine hardener whereupon they would solidify at ordinary and Dentistry, had discovered that acid etching of temperature with very little hardening shrinkage; dental enamel made its surface slightly rough and

10 COMPOSITEAND SEALANT RESINS: Bowen porous -- thereby receptive to a micromechanical There has been a trend, unfortunately, in many of bonding of polymerizable monomers.9 In his at- the commercial composite formulations toward the tempts to prevent decay from forming in use of lower viscosity liquids and lower volume developmental pits and fissures, he experimented percentages of reinforcing filler materials. This may with various resins as sealants. When BIS-GMA facilitate mixing and placement by the use of became available, he found it to work best. syringes; however, the physical properties and values Somewhatlater, the acid-etching of enamel in and of the resulting restorations suffer from increased around cavity preparations was found to be hardening shrinkage, reduced stiffness, decreased beneficial. color stability, and other factors. Thick mixes (that Soon after Buonocore published his early is, a minimumamount of slightly viscous monomers findings, 9,’° Regenos and other research-oriented with a maximum amount of inorganic reinforcing clinicians ’’-’3 began using etching of enamel with filler) mixed thoroughly and placed immediately will phosphoric acid solutions as a means of attachment give the best results. The material hardens best if and retention of direct filling resins. They discovered protected from air, which inhibits surface hardening. that fractured incisors could be repaired and restored The maximumfeasible time should be allowed for promptly and esthetically without cutting the den- polymerization before it is trimmed or finished. tin. Methods were also developed to use acid etching After a composite restoration is finished to con- and resins to bond orthodontic brackets directly to tour, it should be "etched" so as to remove debris teeth, a procedure with advantages in many cases from surface air bubbles and to provide "extension over the cementing of bands. Because the bonding for prevention" of the resin gIaze that is subsequently to acid-etched enamel was effective with both the un- applied. Currently it is difficult for the practitioner filled resins and composite materials containing in- to apply a thin glaze or veneer of transparent and inorganic reinforcing fillers, there was a gradual tran- visible resin to etched enamel surfaces because of air sition toward the use of composites with most of inhibition of surface polymerization. If the inhibited these therapeutic procedures. layer is comparable in thickness to the desired glaze In surgical orthodontics, bonding to acid-etched (polymerized layer), currently available materials are enamel allowed the more convenient and effective at- difficult to use for this purpose; the glaze must be tachment of orthodontic buttons or pads with eyelets applied as two layers, applied extra thickly, or to unerupted teeth that are not properly placed or covered with something that will exclude at- aligned for normal eruption, or that have failed to mospheric oxygen during polymerization. erupt. Composite materials were designed as a replace- ment for silicate cements, and not much thought was Present Practices given during their development as to their use in Although there is not uniform agreement about the posterior restorations. The poor durability of com- best agent for etching enamel for adhesive resin ap- posite materials on the occlusal surfaces of posterior plications, there are both experimentaP 5 and teeth contraindicates their use on the occlusal sur- theoretical TM reasons for using phosphoric acid at a con- faces of permanent dentition, unless esthetics is an centration of about 30% as an etching agent. The overriding consideration. In may cases occlusal com- dentin surfaces should be protected from the applica- posite restorations will remain in fairly good condi- tion of an acid like this, because there is increased tion for a couple of years, thereafter showing increas- pulp irritation -- not only from the acid treatment ing loss of surface material. In the case of deciduous itself, but also from subsequent response to com- teeth, the question of the suitability of composite posite restorations if a calcium hydroxide type of materials for occlusal restorations involves a number liner is not used. of variable factors including the estimated length of time until the tooth is shed and replaced, the dura- There is experimental evidence on both sides of the bility of the particular composite material under controversy regarding the use of an unfilled resin these conditions, esthetics, ease of placement, cost layer applied to acid~tched enamel before the ap- of the materials, and other factors. Present practices plication of a composite material. For most photo- and associated rationale are more fully described initiated systems, there is adequate free monomer elsewhere in this journal. available from the composite mixture to penetrate the small volume of the pores in the acid-etched enamel surface. At the other extreme, with a very dry Future Possibilities mix of chemically activated composite (especially if It is justifiable to speak of future dental practice the mixing and placement is not quick) it is quite because it takes many years for nascent technology possible that the prior application of an unfilled resin, emerging from scientific research laboratories to be used sparingly, might give more reliable bonding. developed and made commercially available for den-

PEDIATRICDENTISTRY: Volume 4, Number1 11 tists’ use. Therefore, some of the current research in the oral environment. An acid-etch treatment of successes represent future improvements in patient the restoration surface as well as the adjacent enamel care. surface could give interpenetrative bonding to a resin The durability, repairability, and quality of the sur- used as a glaze. This same mechanism may allow for face texture of composites are matters that are bonding of repair increments. receiving considerable research. The weakest link in The X-ray opacity of such restorations is expected the physical integrity of composite materials is prob- to be intermediate between that of dental amalgam, ably the bonding between the organic resin and the which is almost totally opaque, and quartz-filled com- surfaces of the inorganic filler particles. Anything posites which are practically radiolucent. With this which improves the strength and durability of this intermediate radiopacity, voids from trapped air interaction could lead to restorations that are under restorations, air bubbles within the restora- stronger and more durable. tions that inevitably occur during the mxing and placement of such materials, and secondary or One experimental approach toward getting a bet- underlying decay associated with the restorations ter interaction between the organic resin and the in- become visible. Voids have been present in radiolu- organic reinforcement of a composite restorative cent and radiopaque restorations but; have gone material is the use of a three-dimensional glass fiber undetected because of the extreme radiolucency of network. ’7 This can be obtained by heating a cotton- the one and the total radiopacity of the other. like form of superfine glass fibers under pressure. To the extent that new composite material can be This results in a sintering, or melting together of the bonded to older restorations and to tooth surfaces, glass fibers, where they contact one another, thereby it might be unnecessary to remove all of a restora- forming a dense network with microscopic pores. tion to fill a void needing a remedy. This three-dimensional glass network can then be If the restorative materials continue to change for broken into "particles" of suitable size. treated with the better, it is quite probable that the techniques an appropriate silane coupling agent, and combined for their proper use will also change. These changes with a hardenable liquid resin so as to form a com- can be expected to lead to modified cavity prepara- posite restorative material. The hardening of the tions. For example, a classical cavity preparation for resin should form an interlocking composite of con- a gold foil restoration cannot be expected to be ideal tinuous organic and inorganic phases; it is hoped that for a future durable and adhesive composite resin this will lead to improvements in properties such as material. polymerization shrinkage, modulus of elasticity {stiff- One feature that will probably change the least is ness}, and resistance to wear. It will be of con- the access form of the cavity. It will continue to be siderable interest to follow the developments in this necessary to remove softened or discolored enamel research. or dentin, and access to do this will doubtless be The development and use of "semiporous" glass necessary. filler particles is a somewhat different experimental approach toward the objective of improving physical A novel method for discriminating between properties of composites by increasing the bonding carious dentin that should be removed and under- between the inorganic and organic phases. In this lying dentin that should be retained under a restora- case, only the surface of glass filler particles is made tion is that of staining or dyeing the carious dentin porous. This is done by acid-etching the glass par- with a colored solution. For example, a 1% solution ticles lanalogous to the acid etching of enamel} to get of Acid Red in propylene glycol has been proposed superficial porosity into which the liquid resin can by Fusayama2’ as an objective way of indicating that flow and polymerize. To obtain a glass that would part of the carious layer of dentin that needs to be have the necessary X-ray opacity, the appropriate removed. refractive index, and would be capable of giving a The ideal retention form and resistance form of porous surface when acid etched, a new kind of glass prepared cavities will depend upon the degree and was developed. ’82° Experimental quantities of this reliability of adhesive bonding between the glass have been prepared by glass manufacturers, restorative material and the hard tooth tissues, and and samples have been made available to dental on the physical properties of the completed manufacturers for their appraisal research and prod- restoration. uct development. For instance, it is currently recommended that The improvements in composite properties made most composite restorations utilize acid etching of by one of these or other research activities will the enamel and that the cavosurface should be a possibly include improved surface texture of fin- beveled, rounded, or chamfered configuration and not ished composites with significantly greater durability a ninety-degree angle. This extends the enamel sur-

12 COMPOSITEAND SEALANT RESINS: Bowen face which, after acid etching, allows additional reten- just recently been explained by Asmussen and tion and sealing of the restoration by penetration of J~rgensen. 24 This emptying requires the high surface the liquid resin into the microscopic pores etched in- tension of water, a small capillary {pore} diameter, to the ends of the enamel rods cut in making the bevel and a wetting of the capillary walls by the water. The or chamfer. Since the tensile strength of the com- diameters of the pores in acid-etched enamel are small posite material is greater than the tensile strength enough and water readily wets their surfaces; the of human enamel,22 the tapering resin at the margin dentist need not be concerned with these two factors. resists the stresses built up during hardening However, the surface tension of water can very easi- shrinkage and increases subsequent margin integri- ly be lowered by contamination with any of a large ty if there is good penetration of the resin into the number of things, and this is where the dentist’s at- etched enamel. tention needs to be focused. If the water is clean, the ~-~ compressed air stream over the surface will remove I’rom an/n vitro study, there has been a research the superficial water and allow the automatic ejec- report that the use of pyruvic acid might have cer- tion of the water, as vapor, from the depths of the tain advantages over phosphoric acid as an etchant pores. This happens because the surface tension in for dental enamel. 23 Although a number of studies such small pores lowers the pressure in the water so 24 have been made comparing the relative merits of dif- much that it boils out. ferent concentrations of phosphoric acid, there have Resins that are sufficiently liquid to be pourable been relatively few studies exploring the merits of at room temperature will readily flow into these emp- other kinds of acids {having various dissociation con- tied pores by capillary action, but the resin cannot stants and pKa values} wherein the optimum concen- go into the pores if the water has not come out of tration of each acid is determined. It would not be them first. Lowering the viscosity of the resin below surprising if phosphoric acid were not only the only that of a viscous but pourable liquid will undesirably acid found suitable for the acid etching of enamel. increase its polymerization shrinkage. In acid etching enamel, the importance of washing Adhesive bonding to dentin is more difficult. Even with clean water and the prevention of even the so, recent experimental research efforts give good slightest contamination before, during, or after dry- reason to predict that future clinical dentistry will ing with compressed air has not been adequately em- be able to utilize materials and methods for signifi- phasized either in the dental literature or in the in- cant adhesive bonding to dentin as well as to enamel. structions supplied by dental manufacturers. The term "rinse," often used, is ambiguous in that it can The scanning electron microscope has been be interpreted to include the swishing of water especially valuable to researchers in showing that the around in the mouth by the patient after taking water surface of dentin that has been cut by rotary or hand from a cup. Many sealants and acid-etched enamel instruments is covered by a smeared or disturbed sur- applications have failed because the washing of the face layer. ~ Some workers removed this with the acid-etched enamel surface was not done ade- strong acids used to etch enamel, but there is quately and exclusively with clean water. Water- evidence that such treatment can cause pulp irrita- soluble crystals form on the surface while the acid tion, especially if followed with a composite material is etching the enamel. These must be completely in the absence of a protective calcium hydroxide dissolved and removed during the washing step. liner. ~ Others have found that the smearedlayer can be Furthermore, even the slightest trace of saliva, removed without significant enlargement of the den- blood, or even tooth debris might negate the bonding. tinal tubular openings, 27 or pulp irritation? ~ by the It was once thought that gross contamination with brief application of isotonic concentrations of acids saliva or other material, sufficient to clog the open- of intermediate strength. ings of the pores, was at fault. Now, it seems that In the research laboratory, dentin surfaces can be even invisible traces of saliva or other soluble treated with a mordant~ {a metallic salt solution} materials can reduce bonding. This is probably due which chemically modifies the dentin surface and to a lowering of the surface tension of the water on makes it more receptive to adhesive materials. Other the enamel when an attempt is made to dry it with solutions are then applied which lay down coupling compressed air. Most semi-soluble materials, which agents~ that provide a basis for adhesion with a subse- would certainly include salivary solutes, traces of quently applied composite resin. 31 Figures 1-3 show blood, or other materials that might be found in the the fractured surface of an adhesive bond to the den- mouth, can significantly lower the surface tension of tin of an extracted tooth. After storage in water for water. two days, breaking the bond required 1,910 pounds The mechanism by which the water in the pores per square inch {13.2 MPa~in tension. of etched enamel is removed by an air stream has It is quite possible that when these adhesive

PEDIATRICDENTISTRY: Volume 4, Number1 13 Figure 1. Scanning electromicrograph Figure 2. Higher magnification of an Figure 3. The surface in the upper part of the dentin surface after breaking the area of Figure 1. The circular features of this picture is the same fractured adhesive bond. The bond strength was are regions in the composite contain- surface shown in Figures 1 and 2. 1,910 psi (13.2 MPa), and the fracture ing air bubbles when the material was However, in this case the dentin occurred partly at the interface mixed and placed on the surface. Air specimen was fractured again to (adhesive failure) and partly through bubbles represent sites of relative observe the internal aspects of the den- the composite resin (cohesive failure) weakness, and the fracture tends to tin underlying the adherent composite leaving remnants of the composite leave the interface and involve these material. Filler particles and resin material adhering to the dentin sur- voids. Small depressions representing overlie an altered dentin region con- face. Original magnification 47x. the openings of dentinal tubules where taining enlarged and mostly-filled den- the fracture involves the altered layer tinal tubule openings. With this bond- of surface dentin can be seen in other ing method, however, the dentinal areas. Original magnification 470x. tubules are neither enlarged nor filled with resin "tags" to any significant depth beneath the adhesive interface. Original magnification 600x. materials become available to the dentist, the linked polymeric coatings might remain intact for materials, to be successful, will be accompanied by long periods of time, protecting these areas from low a very detailed description of the steps that must be pH of plaque and nutrient stagnation. Preventive followed. The future might offer a tradeoff: a re- measures of this kind may not now be considered duced amount of cutting of sound enamel and den- cost-effective, at least as a public health measure; tin (with the maintenance of greater strength and in- nonetheless, fluoridation, antibacterial dentrifices, tegrity of the tooth crown due to a reduction, or and mouth rinses, improved levels of oral home care, possibly an elimination, of classical retention form and lessened busyness in dental practices may raise and resistance form); an elimination, of microleakage the feasibility of some of these forms of dental treat- and marginal staining; and a lessened need for ment in many practices. anesthetics to maintain patient comfort. These Composite and sealant resins will probably have benefits may well require precise adherence to exact- a future. If research to improve these materials is not ing procedures in the application of the adhesive adequately supported, if dental manufacturers fail to restorative materials. transfer technology from the laboratory to the With the advent of effective adhesive bonding, it marketplace, if dental practitioners hold them in a is quite possible that dentists can routinely apply low esteem (thinking them to be a quick and easy way esthetic or invisible protective coatings for entire to provide second class dentistry), then the future of tooth crowns as a logical extension of sealants and composites and sealants may be dim — if not dismal. glazes. These can be used as protection against white The more probable course is that of continued ad- spots and smooth-surface carious lesions for newly- vancements in the scientific laboratories, ready ac- erupted (or perhaps not-so-newly-erupted) teeth. Such ceptance and promotion by manufacturers, and thin veneers will doubtlessly wear away in areas sub- careful use by conscientious dentists who constitute ject to mastication, brushing, interproximal contact, the majority of our colleagues. The placement of the and vigorous flossing; these areas, however, are less best possible composite restoration (including the use prone to the development of lesions. In regions that of materials yet to be developed) might be as demand- are not self-cleansing (that is, tooth surfaces least ac- ing and give as much cause for pride, because of the cessible to natural and artificial cleansing), cross- merits of its serviceability, as have been the place-

14 COMPOSITE AND SEALANT RESINS: Bowen ment of top-rate gold foil, porcelain inlay, silver 15. Silverstone, L. M. Fissure sealants: laboratory studies. Caries amalgam, or other traditional restorations. Research 8:2, 1974. 16.Chow,L. C. and BrownW. E. Phosphoric acid conditioning of Future improvements in the level of oral hygiene, teeth for pit and fissure sealants. J Dent Res 52:1158, 1973. the use of fluorides, antiseptics, sealants, and pro- 17.Ehrnford, L. A method for reinforcing dental composite tective coatings to prevent decay, together with im- restorative materials. OdontRevy 27:51, 1976. provements in composites to repair traumatically 18.Bowen,R. L. and Reed, L. E. Semiporousreinforcing fillers for damaged or malformed teeth, can lead to better composite resins I. Preparation of provisional glass formulations. J Dent Res 55:738, 1976. general health because of better oral health. 19.Bowen,R. L. and Reed, L. E. Semiporousreinforcing fillers for compositeresins II. Heat treatment and etching characteristics. Dr. Bowenis associate director of the AmericanDental Associa- J Dent Res 55:748, 1976. tion Health Foundation, National Bureau of Standards. Reprint 20.Bowen,R. L. Compositedental material. U.S. Patent Office, requests should be sent to him at: National Bureau of Standards, 4,215,033, 1980. Washington, D.C. 20234. 21.Fusayama, T. NewConcepts in Operative Dentistry. Chicago, Quintessence Pub. Co., Inc., 1980, pp 45-59. 1. Paffenbarger, G. C., Schoonover,I. C., and Souder, W. Dental 22.Bowen, R. L. and Rodriguez, M. S. Tensile strength and silicate cements; physical and chemical properties and a modulusof elasticity of tooth structure and several restorative specification. JADAand The Dent Cosmos25:32, 1938. materials. JADA64:378, 1962. 2. Henschel, C. J. Observations concerning/~ yiyo disintegration 23.Oshawa,T. Studies on solubility and adhesion of the enamel of silicate cement restorations. J Dent Res 28:528, 1949. in pretreatment for caries preventive sealing. Bull TokyoDent 3.Bowen, R. L., Paffenbarger, G. C., and Mullineaux, A. L. A Col 13:65, 1972. laboratory and clinical comparisonof silicate cements and a 24.Asmussen,E. and J~rgensen, D. K. The stability of water in direct-filling resin: a progressreport. J Pros Dent20:426, 1968. the pores of acid etched humanenamel. Acta Odont Scand 4.Bowen,R. L. Use of epoxy resins in restorative materials. J 36:43, 1978. Dent Res 35:360, 1956. 25.Eick, J. D., Wilke, R. A., Anderson,C. H., and Sorensen, S. 5. Paffenbarger, G. C., Nelsen, R. J., and Sweeney,W. T. Direct E. Scanningelectron microscopyof cut tooth surfaces and iden- and indirect filling resins: a review of some physical and tification of debris by use of the electron microprobe. J Dent chemical properties. JADA47:516, 1953. Res 49:1359, 1970. 6.Coy, H. D. Direct filling resins. JADA47:532, 1953. 26.Aida, S., Matsui, K., Hirai, Y., and Ishikawa, T. A clinico- 7. Bowen,R. L. Properties of a silica-reinforced polymerfor den- pathological study of pulpal reaction to acid etching with tal restorations. JADA66:57, 1963. phosphoric acid solution at various concentrations. Bull Tokyo 8. Bowen,R. L. Dental filling materials comprisingvinyl silane- Dent Coll 21:163, 1980. treated fused silica and a binder consisting of the reaction prod- 27.Bowen,R. L. Adhesive bonding of various materials to hard uct of bisphenol and glycidyl methacrylate. U.S. Patent Office tooth tissues XIX.Solubility of dentinal smearlayer in dilute 3,066,012, 1962. acid buffers. Intl Dent J 28:97, June 1978. 9. Buonocore, M. G. Simple method of increasing the adhesion 28.MjSr, I. A. Henste~Pettersen, A., and Bowen,R. L. Biological of acrylic filling materials to enamelsurfaces. J DentRes 34:849, assessments of experimental cavity cleaners. (Manuscript in 1955. preparation.) 10.Buonocore, M. G., Wileman, W., and Brudevold, F. A report 29.Bowen,R. L. Adhesive bonding of various materials to hard on a resin compositioncapable of bondingto humandental sur- tooth tissues VII. Metal salts as mordantsfor coupling agents. faces. J Dent Res 35:846, 1956. in Dental Adhesive Materials, Moskowitz,H. D., Ward, G. T., ll.Laswell, H. R., Welk, D. A., and Regenos, J. W. Attachment and Woolridge, E. D. (eds}. NewYork City, Prestige Graphic of resin restorations to acid pretreated enamel. JADA82:558, Services, 1974, pp 205-221. 1971. 30.Misra, D. N. and Bowen, R. L. Adhesive bonding of various 12.Doyle, W. A. Pedodontic operative procedures, in Current materials to hard tooth tissue XII. Adsorption of Therapy in Dentistry, ed. McDonald,R. St. Louis, C. V. Mosby N-(2-hydroxy-3-methacryloxpropyl)-N-phenylglycine INPG- Co., 1968. Vol. 3, Chapt. 38. GMA}on hydroxyapatite. J Coll Interface Sci 61:14, 1977. 13.Doyle, W. A. Acid etching in pedodontics, in Dent Clinics of 31.Bowen,R. L., Cobb, E. N., and Rapson, J. E. Adhesive bond- North America. Philadelphia, W. B. Saunders Co., 1973, pp ing of various materials to hard tooth tissues XXV.Improve- 93-104. ment in bond strength to dentin. (Manuscript in preparation.} 14.Newman,G. V., Snyder, W. H., and Wilson, C. E., Jr. Acrylic adhesives for bondingattachments to tooth surfaces. Angle Or- thodont 38:12, 1968.

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