DENTAL COMPOSITES/GLASS IONOMERS: THE MATERIALS RAFAEL L. BOWEN WILLIAM A. MARJENHOFF

American Dental Association Health Foundation Paffenbarger Research Center National Institute of Standards and Technology Gaithersburg, Maryland 20899 Adv Dent Res 6:44-49, September, 1992

Abstract—Most commercial dental composites contain liquid dimethacrylate monomers (including BIS-GMA or variations of it) and silica-containing compositions as inorganic reinforcing filler particles coated with methacrylate-functional silane coupling agents to bond the resin to the filler. They also contain initiators, accelerators, photo-initiators, photosensitizers, polymerization inhibitors, and UV absorbers. Durability is a major problem with posterior composites. The typical life-span of posterior composites is from three to 10 years, with large fillings usually fewer than five years. Polymerization shrinkage and inadequate adhesion to cavity walls are remaining problems. Some pulp irritation can occur if deep restorations are not placed over a protective film. Some have advocated the use of glass-ionomer cement as a lining under resin composite restorations in dentin. The concept of glass-ionomer cements (GICs) was introduced to the dental profession in the early 1970's. Current GICs may contain poly(acrylic acid) or a copolymer. Highermolecular-weight copolymers may also be used to improve the physical properties of some GICs. Stronger and less-brittle hybrid materials have been produced by the addition of watersoluble compatible polymers to form light-curing GIC formulations. The ion-leachable aluminosilicate glass powder, in an aqueous solution of a polymer or copolymer of acrylic acid, is attacked by the hydrated protons of the acid, causing the release of aluminum and calcium ions. Salt bridges are formed, and a gel matrix surrounds the unreacted glass particles. The matrix is adhesive to mineralized tissues. Provisions must be made for maintenance of the water balance of restorations for the first 24 hours. A varnish to seal newly-placed restorations is provided by most manufacturers. The introduction of metal powder to GICs significantly improved abrasion resistance.

This manuscript is published as part of the proceedings of the NIH Technology Assessment Conference on Effects and Sideeffects of Dental Restorative Materials, August 26-28,1991, National Institutes of Health, Bethesda, Maryland, and did not undergo the customary journal peer-review process.

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esthetic dental composites were introduced in the mid1960's as an alternative to silicate cements and unreinforced methyl methacrylate direct filling resins forthe restoration of anteriorteeth (Bowen, 1962,1965a). There were serious shortcomings in the durability and physical properties of silicate cements (Paffenbarger et al., 1938; Henschel, 1949; Bowen et al., 1968) and the "acrylic" resins (Tylman and Peyton, 1946; Paffenbarger et al., 1953; Coy, 1953). It had been found that when inert fillers were incorporated into poly (methyl methacrylate), there was a reduction in the coefficient of thermal expansion and in water sorption in proportion to the concentration of fillers (Rose et al., 1955a). But these resins lacked the adhesive qualities that were sought in a direct filling material. Adhesiveness was one of several intriguing characteristics of epoxy resins that led to experimentation with them in conjunction with various hardening systems and inorganic fillers (Rose et al., 1955b; Bowen, 1956). Although epoxy resins were initially adhesive to hard tooth tissues, bond strengths dropped off appreciably after prolonged exposure to water.

COMPOSITES A compromise between epoxy and methacrylate resins was conceived in 1956. The reaction sites (oxirane rings) of the epoxy molecule might be replaced by methacrylate groups, and this might yield a hybrid molecule that polymerized through methacrylate groups. An experimental dimethacrylate monomer was synthesized by the reaction of bisphenol A and glycidyl methacrylate and was later also produced by the reaction of the diglycidyl ether of bisphenol A and methacrylic acid (Bowen, 1965b). The acronym given to the new monomer was BIS-GMA, considerably easier to say than the proper chemical name of 2,2-bis[p-(2'-hydroxy-3'-methacryloxypropoxy)phenyl]-propane. It was suitable for use as a binder for reinforcing fillers, because it was nonvolatile, had relatively low polymerization shrinkage (about a third that of methyl methacrylate), and hardened rapidly under oral conditions when suitably formulated with an appropriate initiator system. Commercial dental composites contain BIS-GMA and/or other liquid organic monomers, inorganic reinforcing filler particles, and additives that cause the liquid to solidify (polymerization initiators). Other additives are used to stabilize the liquid until its hardening is desired (polymerization inhibitors), to improve adhesive bonding between the resin and the filler particle surfaces (silane coupling agents), and to diminish discoloration of the resin during subsequent aging (stabilizers). Other compounds may be intentionally or inadvertently present in composite restorative materials. Composites have lower polymerization shrinkage, a lower coefficient of thermal expansion, higher compressive strength

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and stiffness, and (depending on the filler materials used) higher radiopacity than do unreinforced resins. Compared with silicate cements, composites have lower solubility, higher tensile strength, and comparable compressive strength. However, composite restorative materials also have notable shortcomings. Restorations of incompletely polymerized resin composites are more likely to degrade than more fully polymerized materials. Not all of the methacrylate groups of the dimethacry late monomers in most resin composites are reacted in the polymerization process (Ruyter and Svendsen, 1978); unreacted methacrylate groups can amount to 30-50% in the crosslinked polymer (Ruyter, 1984). Also, the outer surface of a composite restoration and the surfaces of entrapped air bubbles inside the filling may be poorly polymerized because of the inhibiting effect of oxygen. Decomposition products, constituents incompletely reacted, oxidation products such as formaldehyde (Bergman, 1990), and initial impurities from the sources of the ingredients can present some potential for sideeffects. Since dental composites are not intrinsically adhesive to hard tooth tissues, it has been desirable for special coupling agents, adhesion systems, and techniques to be developed to effect bonding between composite restorations and dentin and enamel.

Posterior Composites Composites were originally designed as a replacement for silicate cements and unfilled resins, so during the development of composites, not much thought was given to their use in posterior restorations. In the years following their introduction, however, the mechanical and physical properties of resin composites, fillers, coupling agents, and bonding agents were improved to the point that some dental researchers and dental manufacturers began proposing composites as posterior filling materials. Because they were esthetic, posterior composites were especially popular with actors, singers, and others who were in the public eye or particularly sensitive to matters of appearance. In time, posterior composites became more popular with dental consumers at large. In 1984, the American Dental Association Council on Dental Materials, Instruments and Equipment (CDMIE) provisionally approved the use of composites for primary molars, and in 1986 the CDMIE developed an acceptance program for posterior composites to be used in minimal stressrelated areas. Current CDMIE guidelines provide for provisional acceptance of posterior composites based on a minimum of three years' durability, and full acceptance by the Council is based on a five-year minimum. Durability is a major problem with large posterior composite restorations. The typical life-span of posterior composites is three to ten years, with life-spans of large fillings usually being fewer than five years. Although the durability of posterior composites may be enhanced by improvement in the structural homogeneity of the material during manufacture (Watts et a/., 1990), by maximal finishing (Goldstein, 1989), by the use of sealants (Mertz-Fairhurst etal, 1991; Dickinson etal, 1990), and by other techniques, a host of investigators have determined

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the inadequate resistance to wear (loss of anatomic form) of composites under masticatory attrition. Some advances have been made in strengthening the bond between the reinforcing fillers and the resin binder of composites and toward developing stronger binding materials themselves, but esthetic posterior composites still fall short of what is required for load-bearing restorations.

Marginal Leakage Polymerization shrinkage and inadequate adhesion to cavity walls with the deleterious effects of microleakage are also problems associated with composites. Although well-placed modern dental composites have been shown to cause little or no pulpal reaction (Mjor and Wennberg, 1985), some irritation of the dental pulp can occur if deep composite restorations are not placed over a calcium hydroxide liner or other protective film. Thermal stress has been shown to increase marginal leakage around composite restorations (Momoi et al., 1990), as has the use of composites with higher viscosity and lower water-sorption values (Crim, 1989). There is somewhat less microleakage associated with composite inlays that have been post-cured with heat and light (Wendt, 1991; Shortall et a/., 1989; Biederman, 1989), but even with inlays, and despite the introduction of adhesion systems that strongly bond composites to dentin and enamel simultaneously, the problems associated with marginal gaps have not been completely solved (Cheung, 1990). Microleakage can be diminished in composite restorations by a number of techniques, most notably the use of glass-ceramic inserts, or "megafillers" (Donly et ai, 1989), and by the application of unfilled resin to the margins of the restoration (Wilson et aL, 1990; Penning and van Amerongen, 1990).

Improvements Needed The hydrolytic stability of composites is also in need of improvement. Color stability, water sorption, and other properties can be significantly influenced by chemical corrosion or disintegration of the filler-resin interface and of the resin matrix itself. Restorations that have, with time, undergone occlusal wear, color change (especially darkening), and surface staining generally require replacement or repair, usually by resurfacing of the old composite (Crumpler et al., 1989). Composite materials with a radiopacity similar to that of enamel are perhaps best for the detection of recurrent caries (Goshima and Goshima, 1989). However, the radiopacity of composite brands and shades has been shown to vary widely. In one study of 28 shades of 18 brands, five materials had radiopacity lower than that of dentin, and two had radiopacity between that of enamel and dentin (Van Dijken et ai, 1989). "Sandwich" Technique

Some dental researchers and clinicians have advocated the use of glass-ionomer cement as a lining under resin composite restorations, particularly where the cavo-surface margin is in dentin. Adhesion between glass-ionomer cement and dentin is accepted as being a long-term union, and when the surface of the cement is etched, a degree of bonding is possible between resin composite and glass-ionomer cement. This laminate or

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"sandwich" technique has been suggested primarily for lessening microleakage. The strength of the union between the glass-ionomer cement and resin composite is highly dependent on the materials used and the methods of handling, however (Mount, 1989; Reeves etai, 1990), and the technique does not entirely stop microleakage (Saunders et aL, 1990a,b; Wilson and Kent, 1971).

GLASS-IONOMER CEMENTS (GIC) Polyacrylic acid as a replacement for phosphoric acid in zinc phosphate cements resulted in the development of polycarboxylate cements (Smith, 1968). Analysis of silicate cements and an understanding of the fundamental acid-base chemistry of that cement led to the development of glassionomer cements (Wilson and Kent, 1972; Wilson and Prosser, 1984). The aqueous solution of phosphoric acid was replaced with an aqueous solution of polycarboxylic acid and, based on knowledge in glass chemistry, a more reactive glass was developed by replacement of silicon atoms with aluminum atoms. The concept was to combine the strength, rigidity, and fluoride release properties of a silicate glass powder with the biocompatibility and adhesive qualities of a poly (acrylic acid) liquid (Wilson and McLean, 1988; Atkinson and Pearson, 1985). The original composition of the silicate glass powders used in GICs was based on the formulation SiO2-Al2O3-CaF2-A1PO4Na3AlF6. GIC glass contains a high percentage of aluminum as different aluminum salts (Crisp et al., 1976). Although some researchers have expressed concerns about a possible toxic role of aluminum (King et aL, 1981), most GICs studied have released no detectable amounts of aluminum (Meryon and Jakeman, 1987). Some current commercial products contain fluorite or corundum to enhance opacity, and/or a variety of components to achieve radiopacity, including strontium, barium, lanthanum, zinc oxide, or zirconium oxide. Glasscermet cements blend or fuse the glass with metal powders such as silver, silver alloy, gold, platinum, or palladium (McLean andGasser, 1985a). Some current GICs may contain a copolymer of acrylic acid with a dicarboxylic or tricarboxylic acid to control the viscosity of the polycarboxylic acid, where it is used in high concentrations (Tezuka and Karasawa, 1978; Schmitt et al, 1985; Suzuki, 1976). Some of these acids can also provide greater reactivity. Higher-molecular-weight copolymers may also be used to improve the physical properties of some GICs, and problems associated with high viscosity may be addressed by the incorporation of powders of poly (aery lie acid) or a copolymer with the glass to produce a blended powder that sets when it is mixed with water or a tartaric acid solution (Smith, 1971; Wilson et al., 1977; Prosser et al.9 1986). Stronger and less brittle hybrid materials have been produced by the addition of water-soluble or compatible monomers such as hydroxyethyl methacrylate (HEMA), capable of free radical polymerization (e.g., via light-curing) to GIC formulations (McKinney and Antonucci, 1986; Antonucci, 1987; Antonucci and Stansbury, 1989; Mitra, 1991a,b). GICs were not introduced commercially until about a decade after dental composites and enamel-bonding materials

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had come to market. Composites, with their potential for producing direct esthetic restorations, had become dentists' esthetic direct filling material of choice.

Setting Mechanism The original commercial GIC consisted of an aluminosilicate glass powder in an aqueous solution of a copolymer of acrylic acid containing tartaric acid. On addition of the liquid to the powder, hardening occurs due to attack of the glass surface by hydronium ions (hydrated protons of the acid), causing the release of calcium and aluminum ions. The ions form salt bridges between carboxyl groups of the polyacid, resulting in a gelled matrix surrounding the intact glass particles (Nicholson et al.9 1988). The matrix is adhesive to tooth surfaces because of multiple interactions of carboxylate groups of the polyacid chains. The maturation of GIC restorations is relatively slow. In the initial stages of the setting reaction, calcium ions are rapidly released and form primarily calcium salt bridges between poly-acrylate chains within the cement. At this stage, both water uptake and water loss can occur, with the attendant clinical problems of water contamination and dehydration. Provisions must be made to maintain the water balance of restorations for the first 24 hours. In later stages of the setting reaction, cross-linking by aluminum ions gives greater stability to the matrix structure. A varnish, preferably light-cured, to seal newly placed restorations is provided by manufacturers, but these varnishes are not completely waterproof (Blagojevic and Mount, 1989). GICs may not be completely resistant to water loss for two weeks following placement (Sulong and Aziz, 1990). Clinical Applications

The original GIC had a number of clinical drawbacks that limited its acceptance. Clinical failings were apparent in manipulation, setting sequence, early moisture sensitivity, esthetics, and surface texture. Although some GIC properties have been significantly improved over the years (Hunt, 1990), GICs are still not as widely used as filling materials as are composites. GICs as a restorative material have remained in the dental armamentarium, however, and have won many converts for several important reasons. They have good bonding facility, and they have the capacity to release fluoride. Patient response to GICs is usually excellent, because the placement technique can be "microconservative" (Arnold, 1990), placement is usually quick and painless, and the resulting restoration is esthetic. In some cases, where a GIC has been used as a temporary measure as in a split-cusp repair, the temporary repair has proved to be as effective for longevity as a pincomposite or amalgam (Mount, 1990a). But while the use of GICs in many anterior applications and in primary teeth appears to be satisfactory, they continue to have limitations for use in permanent posterior teeth, particularly with regard to large restorations. These limitations include low tensile strength and low impact and fracture resistance (brittleness). Developments in the formulation of GICs have made them

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useful as cavity lining materials and for cementation and preventive applications, as well as for their original intended use as a direct filling material. As a filling material, GICs are perhaps best used for the restoration of deciduous teeth and for the restoration of gingival erosion and abrasion cavities and fissures in Class V and Class III lesions. GICs are not, however, recommended for restorations where toughness and resistance to wear from enamel are major considerations (McLean, 1980). It has generally been recognized that GIC wear resistance to occlusal contacts is inadequate. Clinical studies have shown that gradual loss of contour can be expected because of surface wear and chemical degradation (Smales and Joyce, 1978). One study of a commercial GIC, with a commercial resin composite as a control, found that the GIC abraded about three times more rapidly by volume than did the composite (McLean, 1984). In the early to mid-1980' s, it was found that the introduction of metal fibers or powder in the glass-ionomer system (glasscermet cements) significantly improved abrasion resistance (Simmons, 1990). The addition of silver alloy powder to GIC, in particular, results in a number of physical property improvements (Croll, 1990). The silver cermet material has a light gray color and has a major disadvantage in that it has a low fracture toughness, making it of limited value in regions subject to occlusal stresses (McLean and Gasser, 1985b). The use of glass-cermet cements is generally indicated only in lowstress-bearing areas, because they have lower mechanical strength than either amalgam alloys or posterior resin composites (Mount, 1990b). Silver released from cermets can diffuse into the surrounding tooth structure and cause discoloration (Sarkar et ai, 1988; Croll and Phillips, 1986). GICs, including cermets, are technique-sensitive (Knibbs and Plant, 1989; Smales andGerke, 1990; Smales etaL, 1990; Watson, 1990). Poor-quality GIC restorations are not uncommon, although technique sensitivity is not a problem that often finds its way into the dental literature. It has been suggested that because dispensing systems vary, predispensed materials mixed mechanically may give better and more uniform performance, since their component ratios are more favorable and more consistent (Wong and Bryant, 1985). Although GICs exhibit significantly less polymerization shrinkage than do composites, some curing contraction does generally occur, leading to marginal gap formation (Feilzer et ai, 1988; Saunders etal, 1990b). Marginal leakage associated with GICs can be reduced further if the restoration is covered with a thin layer of resin composite (Guelmann et a/., 1989). Poor finishability and lack of translucency are also characteristic of GICs (Suzuki and Jordan, 1990). Even with the most skillful placement technique, successful GIC restorations may hinge on the composition of commercial GIC materials, which may vary widely depending on their source and intended use (Smith, 1990).

REFERENCES Antonucci JM (1987). Formulation and evaluation of resin modified glass ionomer cements. Trans 13th Ann Meeting: 225. Antonucci JM, Stansbury JW (1989). Polymer modified glass

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ionomer cements (abstract). J Dent Res 68:555. Arnold J (1990). Glass ionomer vements (letter). AustDentJ 35:185. Atkinson AS, Pearson GJ (1985). The evolution of glassionomer cements. BrDentJ 159:335-337. Bergman M (1990). Side-effects of amalgam and its alternatives: local, systemic and environmental. IntDentJ 40:4-10. BiedermanJD(1989). Direct composite resin inlay. JProsthet Dent 62:249-253. Blagojevic B, Mount GJ (1989). A laboratory study of glass ionomer cement in relation to clinical dentistry. Aust Dent 7 33:320. BowenRL(1956). Use ofepoxy resins in restorative materials. J Dent Res 35:360-369. Bowen RL, inventor (1962). Dental filling material comprising vinyl silane treated fused silica and a binder consisting of the reaction product of bis phenol and glycidyl aery late. US patent 3,066,112. Bowen RL, inventor (1965a). Silica-resin direct filling material and method of preparation. US patents 3,194,783 and 3,194,784. Bowen RL, inventor (1965b). Method of preparing a monomer having phenoxy and methacrylate groups linked by hydroxy glyceryl groups, US patent 3,179,623. Bowen RL, Paffenbarger GC, Mullineaux AL (1968). A laboratory and clinical comparison of silicate cements and a direct-filling resin: a progress report. / Prosthet Dent 20:426. Cheung GS (1990). Reducing marginal leakage of posterior composite resin restorations: areview of clinical techniques. J Prosthet Dent 63:286-288. CoyHF(1953). Direct resin fillings. J Am DentAssoc 47:532. Crim GA (1989). Influence of bonding agents and composites on microleakage. J Prosthet Dent 61:571 -574. Crisp S, Lewis BG, Wilson AD (1976). Glass ionomer cements: chemistry of erosion. J Dent Res 55:1032-1041. Croll TP (1990). Glass ionomers for infants, children, and adolescents. J Am Dent Assoc 120:65-68. Croll TP, Phillips RW (1986). Glass-ionomer silver cermet restorations for primary teeth. Quint Int 17:607. Crumpler DC, Bayne SC, Sockwell S, Brunson D, Roberson TM (1989). Bonding to resurfaced posterior composites. Dent Mater 5:417-424. Dickinson GL, Leinfelder KF, Mazer RB, Russell CM (1990). Effect of surface penetrating sealant on wear rate of posterior composite resins. J Am Dent Assoc 121:251-255. Donly KJ, Wild TW, Bowen RL, Jensen ME (1989). An in vitro investigation of the effect of glass inserts on the effective composite resin polymerization shrinkage. JDent Res 68:1234-1237. Feilzer AJ, De Gee AJ, Davidson CL (1988). Curing contraction of composites and glass-ionomer cements. JProsthet Dent 59:297-300. Goldstein RE (1989). Finishing of composites and laminates. Dent Clin North Am 33:210-219, 305-318. Goshima KT, Goshima T (1989). The optimum level of radiopacity in posterior composite resins. Dentomaxillofac

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chemical properties. J Am Dent Assoc 47:516. Paffenbarger GC, Schoonover IC, Souder W (1938). Dental silicate cements: physical and chemical properties and a specification. J Am Dent Assoc 25:32. Penning C, Van Amerongen JP (1990). Microleakage of extended and nonextended class I composite resin and sealant restorations. / Prosthet Dent 64:131 -134. Prosser HJ, Powis DR, Wilson AD (1986). Glass-ionomer cements of improved flexural strength. J Dent Res 65:146148. Reeves GW, Fitchie JG, Scarbrough AR, Hembree JH (1990). Microleakage of gluma bond, scotchbond 2 and a glass ionomer/composite sandwich technique. Am JDent 3:195198. Rose EE, Lai J, Green R, Cornell J (1955a). Direct resin filling materials: coefficient of thermal expansion and water sorption of polymethyl methacrylate. / Dent Res 34:589596. Rose EE, Lai J, Williams NB, Falcetti JP (1955b). The screening of material s for adhesion to human tooth structure. JDent Res 34:577-588. Ruyter E (1984). Kompositte tannfyllingsmaterialer. Tandlakartidningen 76:1207. Ruyter E, Svendsen SA (1978). Remaining methacrylate groups in composite restorative materials. Acta Odontol Scand 36:75. Sarkar NK, El Mallakh B, Graves R (1988). Silver release from metal-reinforced glass ionomers. Dent Mater 4:103. Saunders WP, Grieve AR, Russell EM, Alani AH (1990a). The effects of dentine bonding agents on marginal leakage of composite restorations. J Oral Rehabil 17:519-527. Saunders WP, Strang R, Ahmad I (1990b). In vitro assessment of the microleakage around preventive resin (laminate) restorations. / Dent Child 57:433-436. Schmitt WP, Purrmann R, Jochum P, Gasser O, inventors (1985). Mixing component for dental glass ionomer cements. US patent 4,360,605. Shortall AC, Baylis RL, Baylis MA, Grundy JR (1989). Marginal seal comparisons between resin-bonded class II porcelain inlays, posterior composite restorations, and direct composite resin inlays. Int J Prosthodont 2:217-223. Simmons JJ (1990). Silver-alloy powder and glass ionomer cement. J Am Dent Assoc 120:49-52. Smales RJ, Gerke DC (1990). The use of glass ionomer cements for restoring occlusal tooth surfaces. AustDent J 35:181-182. Smales RJ, Gerke DC, White IL (1990). Clinical evaluation of occlusal glass ionomer, resin, and amalgam restorations. / Dent 18:243-249. Smales RJ, Joyce K (1978). Finished surface texture, abrasion resistance, and porosity of ASPA glass-ionomer cement. / Prosthet Dent 40:549-553. Smith DC (1968). A new dental cement. BrDentJ 125:381384. Smith DC (1971). A review of the zinc polycarboxylate cements. / Can Dent Assoc 37:22-30. Smith DC (1990). Composition and characteristics of glass ionomer cements. J Am Dent Assoc 120:20-22.

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Sulong MZ, Aziz RA (1990). Wear of materials used in dentistry: a review of the literature. / Prosthet Dent 63:342-349. Suzuki M, inventor (1976). Dental cement composition. US patent 3,962,267. Suzuki M, Jordan RE (1990). Glass ionomer-composite sandwich technique. J Am Dent Assoc 120:55-57. Tezuka C, Karasawa S, inventors (1978). Setting solution for dental glass ionomer cements. US patent 4,089,830. Tylman SC, Peyton FA (1946). Acrylic and other synthetic resins used in dentistry. Philadelphia (PA): JB Lippincott. Van Dijken JW, Wing KR, Ruyter IE (1989). An evaluation of the radiopacity of composite restorative materials used in class I and class II cavities. Acta Odontol Scand 47:401407. Watson TF (1990). A confocal microscopic study of some factors affecting the adaptation of a light-cured glass ionomer to tooth tissue. J Dent Res 69:1531-1538. Watts DC, Wilson NH, Omer DE (1990). Radiographic inhomogeneity of posterior composites. / Oral Rehabil

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17:151-155. Wendt, SL Jr (1991). Microleakage and cusp fracture resistance of heat-treated composite resin inlays. Am J Dent 4:10-14. Wilson AD, Crisp S, Abel G (1977). Characterization of glass ionomer cements. 4. Effect of molecular weight on physical properties. J Dent 5:117-120. Wilson AD, Kent BE (1971). The glass-ionomer cement. A new translucent dental filling material. / Appl Chem Biotech 21:313. Wilson AD, Kent BE (1972). A new translucent cement for dentistry. BrDentJ 132:133-135. Wilson AD, McLean JW (1988). Glass ionomer cement. London (England): Quintessence. Wilson AD, Prosser HJ (1984). A survey of inorganic and polyelectrolytecements. BrDentJ 157:449-454. Wilson EG, Mandadjieff M, Brindock T (1990). Controversies in posterior composite resin restorations. Dent Clin North Am 34:27-44. Wong TCC, Bryant RW (1985). Glass ionomer cements: dispensing and strength. AustJ Dent 30:336-340.

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glass ionomers: the materials.

Most commercial dental composites contain liquid dimethacrylate monomers (including BIS-GMA or variations of it) and silica-containing compositions as...
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