Chemistry Everyday for Everyone edited by
Products of Chemistry
George B. Kauffman
The Chemistry of Modern Dental Filling Materials
California State University Fresno, CA 93740
John W. Nicholson King’s Dental Institute, University of London, Denmark Hill, London, UK;
[email protected] H. Mary Anstice Biomaterials Deparment, Eastman Dental Institute, University of London, London WC1X 8LD, UK
The materials that dentists use to restore our teeth after the decay has been removed have come under discussion because of the controversy surrounding the use of dental amalgams. But the question is, what materials are there apart from amalgam? In fact, there have been many developments polymer chemistry applied to the field of dental fillings and these have led to the availability of a variety of tooth-colored restorations. As we show in this article, dental materials science is an interdisciplinary field in which chemistry plays a major part. Ideally, a dental restorative material should be perfectly compatible with the oral environment and should fulfill the criteria set out below: easily mixed and placed as an unset paste short working and setting times rapid buildup in mechanical properties on setting match of thermal and expansion properties with the tooth high resistance to erosion and degradation by oral fluids/saliva, brushing, and flossing biologically inert or bioactive achieves a hermetic seal with the surrounding tooth tissue color and translucency to match the tooth high strength (tensile and compressive) inexpensive
Of course, the best material of all is the natural tooth, which is a mixture of hydroxyapatite and collagen. Its major weakness, though, is that it is destroyed by the action of lactic acid generated as the result of metabolism of bacteria, mainly Streptococcus mutans, in the mouth. This condition is known to dentists as caries, but to most of us as tooth decay. Modern repair materials must, as far as possible, mimic the mechanical behavior of the natural tooth while resisting chemical attack by lactic acid or by any other component of the saliva or of foodstuffs. A variety of materials have been developed for use in repairing teeth, and though none is perfect, there are materials available which, in different parts of the mouth, can give good service for many years. In the rest of this article, we consider the different types of material available, their chemical composition (see Table 1), and their properties. Composite Resin Although in materials science generally, the term composite is given to any material consisting of two different phases, in dentistry this term has been applied to a particular group of materials. These are based on bulky methacrylate monomers, which set by a free-radical polymerization and
are heavily filled with a finely divided ceramic of some sort. The organic resin matrix is bonded to the inorganic filler by means of coupling agents, which are typically organosilane compounds (1). Large dimethacrylates, for example bisGMA and urethane dimethacrylate, are used so that the polymerization shrinkage of the material is minimized. The disadvantage is that these monomers tend to have very high viscosity, making incorporation of the filler difficult. To overcome this, smaller monomers of lower molecular weight, for example EGDMA (ethylene glycol dimethacrylate) or TEGDMA (triethylene glycol dimethacrylate), are included as diluents in the resin formulation. These substances reduce the viscosity of the paste, thereby allowing higher levels of filler to be incorporated. On the other hand, they have the disadvantage that they have a relatively larger polymerization shrinkage. Blending a composite resin of this type for clinical use thus involves a tradeoff between viscosity, ease of incorporation of filler, and polymerization shrinkage. The structures of the monomers commonly used in commercial composite resins are shown in Figure 1. The filler is a vital component of composite resins because its nature, particle size, and concentration profoundly influence the properties of the resulting formulation. Composite resins are often characterized by their filler type and tend to be classified as “conventional”, “microfilled”, or “hybrid”. In the socalled conventional composite resins the filler is quartz or a glass (e.g., aluminosilicate or borosilicate), with particle size in the range 1–50 µ m, although usually at the lower end of this range (2). The total filler loading lies in the range 60– 80% by volume. Microfilled composites use colloidal silica as filler, with particles 0.01–0.1 µm. The small particle size Table 1. Composition of Dental Cements Material
Components
Composite resin
methacrylate monomers nonreactive glass and/or silica filler initiator system
Glass-ionomer
acid-degradable glass polymeric acid water
Resin-modified glass-ionomer
acid-degradable glass polymeric acid (often modified with pendant unsaturated groups) water various methacrylate monomers initiator system
Acid-modified composite resin methacrylate monomer acidic monomer acid-degradable glass initiator system
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Chemistry Everyday for Everyone O
O
O
R' O
O
O
N H
O
R" N H
O
O
R' N H
N H
O
O
O
Urethane demethacrylate (R' and R" are different organic functional groups)
O
O
O
O
OH
O
O
OH
BisGMA
O O
O
OH O
O
O O
O
O
2-hydroxyethyl methacrylate (HEMA)
EGDMA (Ethylene glycol demethacrylate)
O O O
TEGDMA (Triethylene glycol dimethacrylate)
Figure 1. Organic monomers used in dental composite resin filling materials.
ensures that a restoration can be polished to a good surface finish, but the high surface area of the filler increases the viscosity of the paste and makes it difficult to achieve high filler loadings. To overcome this problem the filler is mixed with the resin and polymerized to form a highly loaded material. Blocks of this material are then broken into particles sized between 10 and 40 µm (3) and combined with more resin to form the cement paste. This procedure leads to a cement with a filler loading of 30–60% by weight. The most recent development is the hybrid composite resin, which combines conventional and microfine particle sizes and seeks to achieve a balance in the properties of both. These materials are predominantly filled with large-particle-sized glass (ca. 75% by weight) but also contain ca. 10% by weight of colloidal silica (4 ). The variation in particle size allows efficient packing so that high filler loadings can be achieved. This results in high strength in the finished material. At the same time, the material can be polished readily, has an excellent appearance, and experiences minimal wear, especially in load-bearing areas (i.e., the molar teeth toward the back of the mouth). Dental composite resins are supplied to the dentist as either one- or two-paste systems and the polymerization reaction is either initiated by light (one-paste) or chemically, by mixing components (two-paste). The light source used is a high-intensity visible light (blue light, 470 nm), and the initiation system consists of an α-diketone plus amine reducing agent. The setting reaction is a photochemical polymerization initiated by the radicals formed on exposure of the initiator to the light of the particular wavelength and intensity. The two-paste systems, on the other hand, tend to undergo selfcure upon mixing because the initiator is a two-component substance formed, for example, from a peroxide initiator in one paste with a tertiary amine activator in the other paste. Polymerization of a composite resin does not go to completion, but instead the set material still contains unreacted 1498
double bonds, which may range from 5 to 45% of the original concentration (5). The unpolymerized methacrylate groups exist as residual monomer or, alternatively, as pendant side chains. In either form they can act as plasticizers, causing a reduction in mechanical strength of the material. The extent of the polymerization in a light-activated material depends upon exactly how the cure was carried out—that is, at what intensity and for how long. The intensity of the curing light within the material falls exponentially with distance so that, if a deep restoration is being cured, care must be taken to ensure that the maximum depth of viable light-penetration is not exceeded. The depth of cure decreases as the filler loading gets higher and the particle size gets finer, owing to attenuation of the light beam as it crosses numerous resin/glass and glass/ resin boundaries. All these factors mean that clinically, deep restorations have to be cured incrementally: that is, a relatively thin layer is placed and cured, a further thin layer is placed on top and cured, etc. Composite resins have the advantages of good esthetics, good mechanical strength, and quite good wear resistance, but because of the nature of their setting reaction they have the disadvantages of setting shrinkage, water uptake, and monomer leaching. The setting shrinkage can lead to the formation of gaps, allowing leakage around the restoration. Leakage around a filling allows the ingress of food debris and bacteria and can lead to further decay of the remaining tooth. A gap of 5 µm, for example, would allow substantial numbers of bacteria to colonize the floor of the cavity. Since these materials do not adhere to tooth structure, special adhesives or other techniques must be used. To bond only to the outer, hard enamel layer of the tooth, the technique of acid etching is employed. Here the tooth surface is treated with moderately concentrated phosphoric acid (37%) for 1–2 minutes. This causes a frosty appearance to develop as the mineral phase of the tooth is etched away and the surface becomes roughened. Etching is followed by placement of a clear layer of dimethacrylate monomer, which is polymerized and locks into the roughened surface. Composite resin is then placed onto this unfilled resin layer, to which it adheres with a reasonably high bond strength. For bonding to the softer dentin component of the tooth, special adhesives are required. These are typically based on molecules, such as 2-hydroxyethylmethacrylate, that have a polar head and a nonpolar tail and are thought to orient themselves predominantly with the polar head towards the hydrophilic tooth surface, thus creating a hydrophobic surface to which the monomer molecules of the composite resin will happily attach (6 ). The whole topic of bonding to the vital biological surface of dentin is complicated, and the subject of much current research. Glass-Ionomer Cement (GIC) The glass-ionomer cement was developed in the early 1970s as a new translucent filling material. It consists of a water-soluble organic polymer such as poly(acrylic acid) and a solid powdered base made from special reactive aluminosilicate glasses. Setting involves an acid–base reaction, and the set cement consists of metal polyacrylates formed by calcium and aluminum ions derived from the glass (7). Recent research has shown that there is also an inorganic structure in the
Journal of Chemical Education • Vol. 76 No. 11 November 1999 • JChemEd.chem.wisc.edu
Chemistry Everyday for Everyone
Figure 2. Transmission electron micrograph of a set GIC. Crosssection (2.5 µm square) through a GIC showing glass particles surrounded by haloes of silica-rich material set in the complex polyacrylate/silicate matrix. (Used with the kind permission of P. Hatton and I. Brook, University of Sheffield, UK.)
Figure 3. Glass-ionomer cement restoration of maxillary first molar, 9 years after placement.
matrix, formed by reaction of ion-depleted species from the glass (8). The overall cement consists of this complex matrix, in which unreacted glass particles are embedded. The setting reaction begins as soon as the components of the cement are mixed. As the reaction proceeds the acid groups are gradually ionized, causing the polymer chains to become extended, which leads to an increase in viscosity and the cement paste becomes stiff. The initial reaction product is calcium polyacrylate but later, aluminum polyacrylate is distinguishable; and the inorganic network continues to develop for many months following initial mixing of the cement. The structure of the cement can be seen in Figure 2. When used clinically, glass ionomers need to be handled with special care, since they are vulnerable to moisture contamination the moment they are placed and must be protected with a layer of varnish or unfilled resin for at least the first hour in the mouth. This is because, prior to the formation
of the inorganic network, the matrix-forming ions can be washed out. The cement is also sensitive to desiccation, and since water is essential for the neutralization reaction, its loss leads to an inferior cement. In the mouth the glass-ionomer cement is quite translucent, has a high compressive strength, and is reasonably resistant to acid and aqueous attack. It also has two major clinical advantages, namely, adhesion to the tooth and fluoride release (10). The nature of the adhesive bond formed to the tooth is not fully understood, but it is probably due initially to the formation of salt bridges to the mineral constituent of tooth. A permanent but dynamic bond is formed quickly. Adhesion of the filling is an advantage on two counts: it allows for minimal cavity excavation (no retentive undercuts are required, so all sound tooth material is retained), and it prevents leakage around the filling. The other great advantage of the glass ionomer is its release of fluoride. Fluoride is included in the glass and may be leached in clinically useful amounts for a considerable time after placement, perhaps up to five years (11). Fluoride ions are known to inhibit dental decay, although the mechanism is still subject to debate. Possible explanations include (i) that fluoride ions replace the hydroxyl ions in the mineral phase of the tooth, making it more resistant to acid attack, and (ii) that when demineralization does occur, the presence of fluoride on the surface of the tooth favors the formation of the fluoridated salts rather than dissolution of the tooth. Whatever the mechanism, it has been found clinically that fluoride protects the teeth and that teeth restored with glass ionomer are less likely to suffer from further decay. Fluoride is thought to be released by a rapid surface elution followed by a slower bulk diffusion process, probably based on an ion-exchange mechanism. Clinically, the fluoride release may continue indefinitely because glass-ionomer cements have been shown to take up fluoride from toothpaste and other fluoride-containing products. This suggests that glassionomer cements may act as a kind of fluoride store, taking it up in the presence of high concentrations and releasing it gradually later. Simple glass-ionomer cements are not suitable for loadbearing applications in the mouth (i.e., on biting or chewing surfaces. However, metal-reinforced glass ionomers have been developed, and these have higher strength and wear properties. They have been used for restoring chewing surfaces (Fig. 3) and have even been suggested as suitable replacements for amalgam (12). Metal reinforcement is achieved by one of two methods. The first is by the simple inclusion of finely divided alloy particles in the cement mixture; the other, by a more complicated approach of fusing the glass with silver or gold prior to mixing (13). The metal–ceramic hybrid is then used in the cement-forming reaction. These approaches are the subject of continuing research as dental materials scientists strive to find a suitable replacement for the traditional amalgam. Resin-Modified Glass-Ionomer Cements The most recent development in this area is the introduction of the resin-modified glass-ionomer cement. The rationale behind the development of these materials was to overcome the problems associated with the conventional glass ionomer, notably their tendency toward brittle failure. At the
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same time, it was hoped to retain the advantages of the adhesion to tooth and fluoride release (14). In the attempt to achieve this, water-miscible monomers such as 2-hydroxyethyl methacrylate were added to the parent cement, yielding materials that have two setting reactions: a neutralization and a polymerization. In the first clinical materials the polymerization reaction was photoinitiated, giving a command set facility to the dentist and causing the initial set of the material. More recently, resin-modified glass ionomers have been formulated as non-light-cured versions in which the polymerization reaction is initiated in the same way as in two-paste composite resins, that is, by a chemically induced generation of free radicals brought about by mixing a peroxide initiator with an amine accelerator. The concept of the resin-modified glass ionomer has been further developed in systems in which the polyacid itself has been modified. This modification involves the replacement of some of the carboxylic acid groups by methacrylate species so that the polymer also has unsaturated groups pendant from its backbone (15). In these compounds the polyacid is involved in both the neutralization and the polymerization reactions. Despite the modification of the acid, these formulations still include some monomer, usually a methacrylate, to ensure an effective cross-linking reaction. When a resin-modified glass-ionomer cement is mixed, the neutralization reaction starts immediately, though the rate of reaction is reduced by the presence of organic species. Polymerization complements this neutralization, and both processes are necessary for the cement to develop optimum strength. It is currently thought that the set resin-modified
COOH
COO–
COOH H2O
+ H+ COO–
COOH
1
H+ + ion-leachable glass
Al3+, AlF2+, AlF2+
2
COO–
COO– + Al3+, AlF2+, AlF2+
COO–
COO
COO
COO
COO
COO
Al COO
COO COO
hν
Al
COO
COO
COO
COO
Al
COO
Al COO
3
Figure 4. Setting reaction of Vitrebond.
1500
COO–
HEMA
glass-ionomer cement consists of two interpenetrating matrices, an ionic matrix from the neutralization reaction and an organic one from the polymerization reaction (17). A simplified version of the setting reaction is shown in Figure 4. The resin-modified glass ionomers are esthetically superior to unmodified cements so they are preferred for use where the appearance of the restoration is important. They have the advantages of conventional glass ionomers, namely, fluoride release and adhesion (though their adhesion tends to be reduced slightly by the presence of the organic monomers). Therefore they are usually used in conjunction with the same kind of adhesive systems that are used to bond composite resins to dentin. They are tougher than conventional glass ionomers, though not necessarily stronger. They also have some disadvantages compared to conventional glass ionomers. For example, they undergo polymerization shrinkage and exhibit a setting exotherm (17, 20). “Compomers” (Acid-Modified Composite Resins) Within the last few years, a new system, known as acidmodified composites or, more informally, compomers, has been introduced to the dental profession. It aims to combine properties of composite resins and glass-ionomer cement. Compomers contain many of the components of composite resins, namely, inert filler, bulky methacrylate monomers, and photoinitiators. In addition, they contain a lightly carboxylated monomer and basic glass powder of the type used in glass ionomers. The carboxylated resins are not water soluble, unlike the acid in a glass ionomer, hence neutralization is not part of the setting process in these materials. Instead, like true composite resins, they set entirely by addition polymerization. Later, once they are set, they are capable of taking in water from saliva. This activates the acidity of the carboxylic acid groups in the new monomer and leads to reaction with the basic glass filler. This results in the development of a small amount of salt matrix, from which fluoride ions can be released. Acid-modified composite resins have so far proved very popular with dentists. They have good esthetic qualities, and early reports on their clinical performance are good. However, there were concerns that their in vitro wear characteristics were poor, and subsequent versions of these materials have been formulated with additional cross-linker and fillers of smaller particle size, both approaches designed to reduce the loss of material due to abrasive wear. Bonding to the tooth requires bonding agents and techniques similar to those for conventional composite resins, though the presence of some more polar monomers within these materials does seem to improve their strenght of bonding to hydrophilic tooth surfaces. Conclusions As we have shown, the chemistry of modern dental fillings is complicated and draws on a range of basic subjects, such as polymer science and physical chemistry. A proper understanding of these materials also requires a good knowledge of biomechanics and the physics of fatigue. Although often overlooked, dental materials science is really a subject that makes a vital contribution to the welfare of society. As the general population is increasingly concerned with good oral health, and as patients increasingly retain their teeth well into
Journal of Chemical Education • Vol. 76 No. 11 November 1999 • JChemEd.chem.wisc.edu
Chemistry Everyday for Everyone
old age, so the ability of dentists to provide functional tooth repair is becoming more and more important. This, in turn, relies heavily on the contribution of chemists, often working as part of interdisciplinary teams, and chemistry is likely to remain central to continuing progress in this branch of health care well into the future. Literature Cited 1. McCabe, J. F. Applied Dental Materials; Blackwell Scientific: London, 1990; p 145. 2. Coombe, E. C. Notes on Dental Materials; Churchill Livingstone: Edinburgh, 1986; p 89. 3. Van Noort, R. Introduction to Dental Materials; Mosby: London, 1994; p 89. 4. Willems, G.; Lambrechts, P.; Braem, M.; Vanherle, G. Quintessence Int. 1993, 24, 641–658. 5. Ferracane, J. L.; Greener, E. H. J. Dent. Res. 1984, 63, 1093–1095. 6. Nakabayashi, N.; Ashizawa, M.; Nakamura, M. Quintessence Int. 1992, 23, 135–139.
7. Wilson, A. D.; Nicholson, J. W. Acid–Base Cements: Their Biological and Other Applications; Cambridge University Press: Cambridge; 1993; p 37. 8. Watts, D. C. In Materials Science and Technology—A Comprehensive Treatise, Vol. 14; Williams, D. F., Ed.; VCH: Weinheim, 1992; Chapter 6. 9. Wilson, A. D.; McLean, J. W. Glass Ionomer Cement; Quintessence: Chicago, 1988; pp 83, 126. 10. Wasson, E. A. Clin. Mater. 1992, 12, 181–190. 11. Mathis, R.; Ferracane, J. L. J. Dent. Res. 1987, 66, 113 (Abstr. 51). 12. Antonucci, J. M.; McKinney, J. E.; Stansbury, J. W. Resin-Modified Glass Ionomer Dental Cements; U.S. Patent 7,160,856, 1988. 13. Mitra, S. M. Photocurable Ionomer Cement Systems; European Patent 0,323,120, 1988. 14. Wilson, A. D. Int. J. Prosthodent. 1990, 3, 425–429. 15. Anstice, H. M. Studies in Light-Cured Dental Cements; Ph.D. Thesis, Brunel University, 1993. 16. Kanchanavasita, W.; Pearson, G. J.; Anstice, H. M. Biomaterials 1995, 16, 921–929. 17. Kanchanavasita, W.; Pearson, G. J.; Anstice, H. M. Biomaterials 1995, 16, 1261–1265.
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