Building on findings concerning adhesion to enamel, R. L. Bowen and his colleagues at the Paffenbarger Research Center, National Institute of Standards and Technology began addressing and solving problems associated with (1) silicate cements and unfilled resins, (2) bonding in an aqueous environment, and (3) the development of an adhesion system for both dentin and enamel that could withstand various stresses. This article reviews the development of an adhesion system for bonding dental composites to dentin and enamel.
In the Broadway musical "South Pacific," Bloody Mary asked rhetorically, "If you don't have a dream, how you gonna have that dream come true?" Although reality almost always seems to fall short of conceptualization, a variety of dental researchers have, in their separate ways, brought a long-standing dream to fruition -- that of providing the profession with adhesives that can effectively bond dental composites to dentin and enamel simultaneously. Perfection has not been achieved by any of them and much hard work remains as they seek to improve their various systems.
There are names in the lexicon of dental adhesion that those of us in the field should always remember to acknowledge, because it was on their shoulders that we stood as we grappled with out own research problems. Dr. Michael Buonocore is certainly one of the best-known pioneers in adhesive bonding of resins to teeth. He found that lightly etching enamel created a microporous surface into which direct-filling liquid resins could flow, polymerize, and cake a micromechanical attachment (Buonocore, 1955). He thereby achieved his primary objective of providing a conservative means of sealing developmental pits and fissures. As a result, innumerable carious lesions have been prevented on the occlusal surfaces of posterior teeth. He tried many kinds of resins (a generic term for monomers or their polymers), most of which were not satisfactory, and finally settled on the kind now used in composites.
One of Dr. Buonocore's contemporaries, Dr. George Newman, developed similar methods to bond orthodontic brackets directly to the enamel of teeth (Newman, Snyder & Wilson, 1968). The acid-etch technique was later used for a variety of other dental procedures, but was found to be ineffective for the adhesive bonding of restorative materials to dentin (Lasswell, Welk & Regenos, 1971; Doyle, 1968 & 1973). However, there are many instances in which it would be very beneficial to bond to dentin. Examples include: the restoration of teeth with root caries; placement of porcelain veneers, inlays, onlays, crowns, and resin-retained bridges; and in treatment of amelogenesis imperfecta and some cases of enamel hypoplasia.
Adhesive bonding to both enamel and dentin can prevent marginal leakage and its resulting problems. It will be even more effective when polymerization shrinkage of dental composites is sufficiently reduced. Another advantage of dentin and enamel adhesives is that the removal of healthy dentin for the mechanical retention of composite restorations, a process that is painful without an anesthetic, is unnecessary when effective adhesive bonding materials and techniques are successfully employed.
Dentin, composed mostly of hydrophilic calcium phosphate crystals and collagen, contains a significant amount of water. Volumetrically, about half of the dentin is composed of hydrated organic material (Erikson, 1989). The water content is thought to vary within the dentin, increasing with proximity to the pulp (Pashley, 1990). It is a formidable challenge to find appropriate adhesion system components that can adequately compete with water for tooth surfaces, particularly dentin (Swartz & Phillips, 1955; Schouboe, Paffenbarger & Sweeney, 1956; Bowen, 1956; Buonocore, Wileman & Brundevold, 1956). Moreover, in addition to being adhesive to dentin and enamel in an aqueous environment, a successful bonding system must be able to withstand the stresses of composite shrinkage during hardening (Bowen, 1967), the stresses accompanying thermal changes of the filling material (Turner, 1946), the stresses of mastication, and other stresses (Smith & Schoonover, 1953).
In the early 1950s, there were two kinds of aesthetic direct restorative materials available: silicate cements and unfilled methyl methacrylate resins. Both had important shortcomings. Silicate cements (Paffenbarger, Schoonover & Souder, 1938) were subject to acidic degradation (Henschel, 1949) and were useful for an average of only about four to five years before requiring replacement (Bowen, Paffenbarger & Mullineaux, 1968). Problems inherent to the unfilled resins, based on methyl methacrylate, were profound shrinkage during hardening, insufficient stiffness, and an excessive coefficient of thermal expansion compared to tooth structure (Tylman & Peyton, 1946; Paffenbarger, Nelsen & Sweeney, 1953; Coy, 1953).
Epoxy resins, which were relatively new in the early 1950s, had some intriguing characteristics that made them candidates for use in restorative dentistry. They were known to harden by a ring-opening mechanism at room temperature with very little shrinkage and to produce an insoluble polymer with rather good physical properties. They were also very adhesive to most surfaces, including the kind of inorganic materials that might be used for reinforcing fillers. Preliminary experiments with an aggregate of porcelain or fused quartz particles bound by epoxy resin were encouraging (Bowen, 1956). But epoxies were not very adhesive to tooth surfaces after immersion in water, and the amine-cured epoxies didn't harden rapidly enough for direct clinical techniques. The quick-hardening epoxies, which used acidic catalysts for their polymerization, were not effective because tooth surfaces buffered the acid and yielded incomplete polymerization.
Research to find curing agents with epoxy resins that would give the quick hardening necessary for clinical use as a direct filling material proved fruitless. However, a monomer was conceived and synthesized that resembles an epoxy but had methacrylate groups instead of epoxy groups (Bowen, 1962b). Methacrylate ester groups polymerize rapidly under oral conditions by a free radical chain reaction. This monomer was called "BIS-GMA" and became the hardening binder for a variety of inorganic fillers (Bowen & Rodriguez, 1962) of different sizes and distributions (Bowen, 1964) with varying degrees of radiopacity (Chandler & others, 1971; Bowen & Cleek, 1972; Bowen & Reed, 1976a, b; Bowen 1980) that collectively compromise the class of restorative materials known as dental composites. They replaced silicate cements and unfilled methyl methacrylate resins as dentists' direct tooth-colored restorative material of choice.
Although an improvement over the materials they replaced, dental composites have some troublesome properties. Composites with a higher proportion of filler and less resin tend to have better physical properties; but even with a maximum proportion of inorganic filler particles, composites exhibit some hardening shrinkage, a stiffness lower than that of the tooth, and a coefficient of thermal expansion higher than that of the tooth crown. In spite of these problems, there has been some success in solving the problem of bonding dental composite resins to dentin and enamel (Bowen, Cobb & Rapson, 1982a; Bowen, Cobb & Setz, 1982b; Bowen & Cobb, 1983; Bowen, Cobb & Misra, 1984a; Bowen, Cobb & Setz, 1984b; Bowen & others, 1984c; Bowen, 1985; Bowen, 1986; Bowen, 1987). BIS-GMA resins have no specific chemical groups or mechanisms for competing with water that interacts strongly with tooth structures. Therefore, chemical coupling agents (surface-active comonomers) are needed. These have groups that can compete with water for the tooth surfaces and also groups that can interact by copolymerization with dental resins such as BIS-GMA. Surface activity tests (Bowen, 1962a) showed that there were groups that could compete with water for dentin or enamel surfaces.
Many attempts have been made to synthesize different coupling agents for tooth surfaces. One of the earliest successful compounds tested was NPG-GMA, the reaction product of N-phenylglycine and glycidyl methacrylate (Bowen, 1965a). The use of this surface-active comonomer alone improved the water-resistant bonding between resins and enamel and dentin to a degree that was statistically but not clinically significant (Bowen, 1965b, c, d & e).
Removal of the structurally weak smeared layer, pellicle, or other superficial layers of the tooth surface by use of acidic (Fusayama & others, 1979, Fusayama, 1980) or chelating agents might reduce the availability of calcium ions on dentin surfaces for interaction with a chelating surface-active comonomer like NPG-GMA or other coupling agents with, preferably, multiple-bonding ligand groups. To supplement calcium ion sites for improved bonding, certain appropriate metal cations were evaluated for use on tooth surfaces. Experiments indicated that the most effective agent might be ferric oxalate, primarily because of the iron ion's high tendency to be bound strongly by dentin and enamel and its high chelate stability constants with molecules that have ligand groups similar to those of NPG-GMA (Misra, Bowen & Wallace, 1975; Misra & Bowen, 1978; Bowen, 1978; Bowen, McLendon & Gills, 1978). Furthermore, the oxalate would form an insoluble precipitate with calcium ions, which, together with insoluble ferric phosphate, would seal the dentinal tubules to provide pulp protection and desensitization.
It was then discovered that the additional use of a relatively hydrophilic monomer containing two free carboxyl groups in addition to two polymerizable groups on each molecule dramatically improved bond strengths to levels of clinical significance (Bowen & others, 1982a). This monomer was called "PMDM" (the reaction products of pyromellitic dianhydride and hydroxyethylmethacrylate). There was a synergistic interaction between the NPG-GMA and the PMDM (Bowen & others, 1984a).
The original adhesive system developed was a sequential application of aqueous acidic ferric oxalate, followed by an acetone solution of NPG-GMA or NTG-GMA (the reaction product of N-p-tolyglycine and glycidyl methacrylate), and then an acetone solution of PMDM. This system was effective only if placed in the described sequential order utilizing all three components. The acidic ferric oxalate solution was removing the original smear layer, the disturbed surface layer caused by mechanical abrasion in preparing a restoration site (Bowen & others, 1984c), and laying down a layer of precipitation product that was plugging up the lumina of dentinal tubules. The latter function significantly reduced tooth sensitivity to the subsequent procedure. The NTG-GMA was necessary to induce polymerization of the PMDM, but the exact mechanism of this free radical initiation is still not clear. During subsequent experimentation, it was discovered that the smear-removing capabilities of ferric oxalate were due primarily to the presence of small amounts of nitric acid left over from the synthesis of the oxalate (Cobb & others, 1989). Controlled additions of nitric acid to the aqueous oxalate solution were made to determine the optimum acid concentration for this solution (Blosser & Bowen, 1988). A small increase in the concentration of nitric acid to about 2.5% HNO3 by weight also improved the simultaneous etching of instrumented enamel.
However, an adverse side effect of the application of the ferric oxalate solution was discovered: the occasional appearance of black staining at the adhesive interface in early animal trials (Stanley, Bowen & Cobb, 1988). This could be reproduced in the laboratory by applying a sodium sulfide solution to ferric oxalate-treated dentin. The cause of this staining in vivo is probably (although not proven to be) the reduction of ferric to ferrous ions by sulfide-forming anaerobic microorganisms, resulting in the formation of black ferrous sulfide pigments.
To eliminate this, acidic aluminum oxalate was substituted, and it produced no staining on dentin (Bowen & others, 1987). Aqueous solutions of aluminum oxalate and nitric acid were then applied, and no staining problems occurred in animal trials (Blosser & others, 1989). There was some evidence in vitro that aluminum oxalate did not produce as much of the reaction products plugging the dentin tubules as had ferric oxalate. Eventually, the first successful transfer of the adhesion technology developed by scientists at the ADA Health Foundation's Paffenbarger Research Center involved the development of a product that incorporated aluminum oxalate in a conditioning solution.
In the continuing research, it was found that the aluminum oxalate could be eliminated entirely from the experimental system without loss in adhesion, if the dilute nitric acid solution was retained. None of the other acids evaluated, in a wide range of concentrations, were as good or better than the dilute nitric acid (which should be distinguished from concentrated nitric acid, a strong oxidizing agent). It was then surprisingly discovered that NPG (N-phenylglycine) could be substituted for NPG-GMA or NTG-GMA. The experimental system was then reduced to the three components of (1) dilute nitric acid, (2) an NPG-acetone solution, and (3) a PMDM-acetone solution.
The three components still had to be applied individually in sequence to achieve adhesion, and efforts were concentrated on ways to simplify application. It was then suspected and verified that NPG would be soluble in the dilute aqueous nitric acid solution. This permitted a simplification of the procedure to the application of two solutions: (1) an acidic NPG solution and (2) a PMDM-acetone solution.
However, preparation and storage of the first solution was difficult because of the reactivity of the NPG molecule to atmospheric oxygen. Storage times were very short if the acid-NPG solution was exposed to air. If the solution was used shortly after mixing, adhesion was effective. Methods were then developed for preparing the solution under an inert atmosphere and protecting it from subsequent oxygen exposure. These protected solutions were stable under normal storage conditions. Some commercial products are currently based on this two-solution system.
Current experimentation with the system is focusing on optimizing the individual components. Nitric acid concentrations will continue to be refined to yield optimal treatment of both dentin and enamel. Different analogues of the NPG molecule are being synthesized toward improving effectiveness, storage, stability, and ease of synthesis (Johnston, Asmussen & Bowen, 1989). PMDM is being investigated to isolate more effective linking agents between tooth surfaces and the overlying restorative resins (Johnston, Farahani, & Bowen, 1990).
In addition to the ADA Health Foundation's system(s) being developed by research associates at the National Institute of Standards and Technology, there are, of course, a number of other methods and different materials being used now for dental adhesive bonding. Most of them have certain features in common (Bowen & others, 1989). Symposia such as this, where the various technologies are described, will help point toward further improvements. One of the problems with current systems for bonding with dentin is technique sensitivity, which probably accounts for the disparity between reports of the relative merits of different systems. Researchers involved in developing a given technique and those experienced in using it probably utilize, unconsciously, procedural details or measures that are not described in sufficient detail in the methods for use for others to exactly match their results. Consequently, other researchers using nominally the same materials and protocols may get poorer results because of the details that are left out of the descriptions for use. That is probably one reason why the ranking of bond strengths differs between laboratories. Another obvious reason is differences in test methods, which have not yet been standardized.
Many years of experience in etching enamel with phosphoric acid have shown bonding by this method to be most reliable clinically. However, it is noteworthy that the use of the chemically functional and more-hydrophilic dentin bonding agents, such as those described here, significantly increases bond strengths to acid-etched enamel, at least in laboratory tests. A number of dentin (and enamel) bonding systems already available to practitioners are beneficial for increased versatility toward improving the performance of restorative materials. And, given the high tensile strength of dentin (Bowen & Rodriguez, 1962), the progress made in the last decade, and the currently recognized need for dentin as well as enamel bonding, it is reasonable to expect that before the end of this decade the intensive and extensive research efforts will succeed in providing clinicians with completely satisfactory materials and methods for preventive and restorative dentistry by way of adhesive bonding to both dentin and enamel.
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