Dental Applications

Vol. 2 DENTAL APPLICATIONS 171

DENTAL APPLICATIONS

Introduction

The dental industry has taken great advantage of polymer science to design ma-terials. Materials used in the oral cavity or external prostheses have very specificrequirements, ie, these materials must have physical, chemical, biological, andaesthetic requirements, not always fulfilled by currently available materials. Ahost of requirements must often be met, including adequate strength, resilience,wear or abrasion resistance, dimensional stability for both fabrication and use,translucency or transparency to provide a match of the natural tissue replaced,good color stability, resistance to the oral environment, show tissue tolerance andlow toxicity, and exhibit ease of fabrication into a needed dental device. Sincefew resins fulfill all the mentioned requirements, the search for improved dentalmaterials has been limited to a few classes of polymeric materials. The varioustypes of monomers and polymers used in dentistry are discussed, along with briefattention given to new areas of promise for preparing better materials.

The largest volume of polymeric materials used in dentistry is in prostheticapplications. Polymeric materials are also important in operative dentistry, beingused to produce composite resins, dental cements, adhesives, cavity liners, and asa protective sealant for pits and fissures. Elastomers are employed as impressionmaterials. Resilient prosthetic devices are often fabricated to restore external soft-tissue defects. Mouth protectors are fabricated to prevent injury to teeth, as well asprevent head and neck injuries. Other polymer applications include fabricatingpatterns for metal castings and partial denture frameworks, impression trays,orthodontic and periodontal devices, space maintainers, bite plates, cleft palateobdurators, and oral implants. Polymeric materials may also be used to fabricatean artificial tongue, when disease results in its loss.

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

172 DENTAL APPLICATIONS Vol. 2

The discovery of vulcanized rubber in 1839 gave rise to the first polymericdentures, followed by celluloid dentures about 1870, and subsequent use of vinylchloride copolymers, phenol–formaldehyde resins, and polystyrene. Acrylic-basedresins gained rapid acceptance after 1937, and are being greatly used today. Theirmain disadvantages are related to shrinkage because of the free-radical polymer-ization of methyl methacrylate, poor abrasion resistance, and fracture toughness.Other polymers, such as epoxy resins, polystyrene, polyesters, polycarbonates,polysulfones, vinyls, silicones, polysulfides, and polyethers, have been explored orused to some degree.

The various dental polymers discussed in this article include impressionmaterials, dentures and denture liners, artificial teeth, crown and bridge mate-rials, mouth protectors, maxillofacial materials, restoratives (consisting of glasspolyalkenoates or glass-ionomers and composites), adhesives, and sealants. Spec-ifications and standards for dental materials are also briefly mentioned in thetext.

Impression/Duplicating Materials

Impressions must accurately show the dimensions, surface detail, and relation-ship of teeth and soft oral tissues. Materials used to accomplish this task includerigid gels of the reversible hydrogen-bonding type, irreversible alginate hydrocol-loids, and elastomers such as silicones, polysulfides, and polyethers. Duplicatesof original casts, used to fabricate partial or complete dentures, are made fromduplicating materials.

Agar (Reversible Hydrocolloids). Agar, a galactose sulfate or muco-polysaccharide (sulfonic acid ester of a galactan complex), is a long-chain polymerhaving a molecular weight of about 150,000. The material forms a colloid withwater, with the solutions being liquid at ≥70◦C and setting to a gel at 30–50◦C.Strong hydrogen bonding causes the molecule to form a helical structure, whichuncoils on heating. On cooling, the gel forms and reproduces the shape of theoral tissues. These materials can be used several times, but prolonged heatingcauses degradation. While agar is the main constituent, it is by no means themain constitutent by weight. For example, a typical formulation consists of about85.5% water, 12.5% agar, 1.7% potassium sulfate, 0.1% alkyl benzoate, and traceamount of pigments. Fillers, such as zinc oxide and a hard wax or clay, are of-ten used to modify the formulations. Borax or calcium metaborate may be used tocontrol the pH, increase viscosity, toughness, and resiliency. These materials weredeveloped to make accurate impressions, even of undercut areas. The agar usedin duplicating uses has a composition similar to the agar impression material, butwith a higher water content. ANSI/ADA Specifications No. 11 (1997) applies toagar-based impression materials.

Alginates (Irreversible Hydrocolloids). The reactive component inthese materials is the sodium or potassium salt of anhydro-o-D-mannuronic acid(alginic acid), isolated from brown seaweed (algae). A typical formulation consistsof about 18% sodium or potassium alginate, 14% calcium sulfate dihydrate, 2%sodium phosphate, 10% potassium sulfate, 56% diatomaceous earth filler, and 4%sodium silicofluoride. When mixed with controlled amounts of water, the soluble


hydrocolloid is converted to insoluble calcium alginate, ie, and an acid–base reac-tion leads to matrix cross-linking. In the sol–gel conversion, calcium cations formsalt-bridges with the carboxylate anions, forming ionic cross-linking. The materialcan only be used once. The addition of sodium tripolyphosphate or tetrasodiumpyrophosphate retards rapid precipitation of gel. Addition of a fluorosilicate com-pletes the precipitation of the insoluble alginate, after adequate working timehas elapsed. Besides diatomaceous earth filler, zinc oxide may be used to improvestrength and yield a material with a smooth, nontacky surface. Alginates, usedmost frequently to make impressions, are inexpensive, easy to manipulate, andcomfortable for the patient. But, alginates deteriorate rapidly on storage at ≥50◦Cand once set they must be kept in 100% relative humidity to prevent shrinkage. Inaddition, they are the least accurate of the impression materials. Both reversibleand irreversible hydrocolloids have been used at the same time in a laminatingtechnique. ANSI/ADA Specification No. 18 (1992) sets the requirements for fastand slow setting alginate impression materials.

Synthetic Elastomers. Synthetic elastomers, such as polysulfides, sili-cone, and polyethers, exhibit much better dimensional stability than the hydro-colloids. They are mainly used in crown and bridge applications or for impressionsof dentate patients. These polymers are used at a temperature above their glass-transition temperature (Tg). The viscosity of the materials used is influenced byboth the working temperature above Tg and the length of the polymer chains,as well as by fillers and other additives. The formulated materials are liquids orpastes at room temperature, which transform into a solid by covalent bondingbetween the long-chain molecules. The process of forming chemical bonds createsa three-dimensional organic matrix. The chemistry of the impression materials isdescribed separately, followed by a brief comparison of their properties. ANSI/ADASpecification No. 19 (1982) sets requirements for the elastomeric-type impressionmaterials.

Polysulfides. The starting materials have molecular weights of about2000–4000 and can be formulated to yield a wide range of physical and chemi-cal properties. In the two-component (paste) system, the base material is a vis-cous polysulfide liquid prepolymer having terminal and pendant mercaptan (SH)groups, such as a Thiokol LP-2 rubber, commonly called thiols (see Fig. 1). Tita-nium dioxide (TiO2), calcium sulfate, calcium carbonate, silica, or alumina is usedas fillers, to extend, reinforce, or harden the product. Dibutyl phthalate, tricresylphosphate, or tributyl citrate is added as modifiers or diluents to improve mixingand flow properties. The paste may also contain sulfur and oleic or stearic acid,which act as a retarder to control the set. The SH groups are oxidized by leaddioxide or copper hydroxide supplied as a separate part of the formulation. Whenthe accelerator (oxidizer) is added to the base (polysulfide) paste an exothermicreaction occurs, bringing about a 3–5◦C rise in temperature, depending on the

Fig. 1. Polysulfide oligomer.


Fig. 2. Polyether impression material reaction.

amount of sulfur used. The exothermic reaction is accompanied by a rapid in-crease of molecular weight and elimination of water, caused by formation of thechain-extending and cross-linking S S bonds. Besides being sticky and havinga disagreeable odor, polysulfides [R(SH)x] are difficult to mix and exhibit relativelypoor elastic recovery when stressed. In undercut areas, they may not provide ac-curate reproduction of oral structures. When properly used they exhibit good tearresistance and acceptable stability.

Polyethers. These impression materials were developed in Germanyin the 1960s. The reactive oligomer used is a polyether having cyclic imine(ethyleneimine) residues, which undergo ring opening and subsequent cross-linking via a cationic initiator, as shown in Figure 2. The two-part formulationcontains the imine functionalized polyether, silicate filler, and a glycol ether plas-ticizer as one paste, and the cationic initiator (such as an alkylester of benzenesulfonic acid), a silicate filler, and a plasticizer as the second paste (1). Addinga polyester diluent or thinner controls the rheological properties and workingtime. Mixing is moderately easy and dimensional changes in air are low (<0.1%)over several hours, as no products are released during curing. The elastic recov-ery and reproduction of detail are excellent. These materials have a much highermodulus of elasticity than silicones or polysulfides, making them more difficult toremove from the mouth. Polyether impression materials tear readily and have anequilibrium water sorption of about 14%. Dimensional stability in water is poor,necessitating storage in a dry environment.

Silicones. Two types of silicones are used in impression materials: addi-tion (Fig. 3) and condensation (hydrosilylation) (Fig. 4). Both are based on usingpoly(dimethylsiloxane), with the two having different end groups and differentcuring mechanisms. Addition siloxanes are a two-paste system, with one paste

Fig. 3. Addition silicone structure/reaction.


Fig. 4. Condensation silicone structure/reaction.

containing a low molecular weight silicone having terminal vinyl groups (Fig. 3)and reinforcing filler and the other paste consisting of the hydrogen-terminatedsilanol oligomer, filler, and chloroplatinic acid catalyst. Mixing the pastes givesa cross-linked elastomer. No volatiles are given off during polymerization—adefinite advantage—compared to the condensation type, which eliminates ethylalcohol.

The condensation-type silicone impression materials are based on hydroxyl-terminated poly(dimethylsiloxane), ie, viscous liquids of structure (Fig. 4). Col-loidal silica or micronized metal oxides, having 5–10 µm particle size, are addedto prepare a paste which is cross-linked with an alkyl silicate containing 50%ethoxy groups, such as tetraethyl orthosilicate (TEOS). Organic tin activators,such as stannous octoate or dibutyltin dilaurate (2), are used at the 1–2% level.

The chief uses of silicone impression materials are for crown and fixed partialdentures. These systems may be used in a wide range of impression techniques.They are relatively easy to mix, producing high tensile strength materials havinggood elastic properties, along with permanent-set values of about 1% at 40% strain.But, they tear at relatively low extension.

Communicable diseases have caused concern in handling of impression ma-terials, meaning impressions are not generally accepted by dental laboratoriesunless they are disinfected. However, disinfection must be accomplished with-out distorting the impression. Reversible and irreversible hydrocolloids, as wellas polyether impressions, should be avoided if they are to be disinfected, sincethey lack the required stability. A guide for disinfecting impression materials maybe found in the ADA Council on Dental Materials, Equipment and Instruments(COMEI, 1988 and 1992).

Impression Materials Merits. Elastomeric impression materials repro-duce surface details very accurately, provided low viscosity formulations are used.Since these materials are generally hydrophobic, they will not properly wet outthe surface of the tooth with saliva present. Contact angle studies of water on theelastomeric impression surfaces for polyether, polysulfide, and addition siliconesshow angles of 49.3◦, 82.1◦, and 98.2◦, respectively, indicating that polyetherswork best where saliva cannot be excluded. The setting process for polysulfidesis highly susceptible to temperature and humidity, influencing working and set-ting times. Condensation silicones may also show erratic setting, because of in-adequate mixing and possibly some hydrolysis of TEOS. The setting behavior ofaddition silicones and polyethers is the most consistent. Condensation siliconesare dimensionally less stable than polysulfides, addition silicones, or polyethers.The contraction or shrinkage of condensation silicones, upon cooling from mouthtemperature (37◦C) to room temperature, is ca 0.35%, compared to 0.20% for thepolysulfides. This shrinkage is essentially caused by release of alcohol generated


during the condensation reaction. However, condensation silicones are cleaner tohandle than polysulfides (3).

Elastomer Thermal and Mechanical Properties. Comparing stiffnessof the polysulfide (PS), condensation silicone (CS), addition silicone (AS), and thepolyether (PE) elastomeric impressions, the ranking would be as follows: PS < CS< AS < PE. The permanent set or resistance to deformation on removal would beas follows: PS > PE > CS > AS. Tear strength would be approximated as follows:PS ≫ PE > AS and CS. Polymerization shrinkage for the elastomers is of theorder PE = AS < PS < CS. The materials may thermally contract on removalfrom the oral cavity, with PE = 320, PS = 270, and AS = CS = 200 ppm/◦C.

Denture Resins/Prosthetic Materials

A variety of polymeric materials have been studied or used for preparing den-tures, including epoxy resins, cellulose nitrate, rubber or vulcanite, phenol–formaldehyde, vinyl acrylics, polystyrene, polycarbonates, and polysulfones, butacrylics have become the materials of choice. It happens that compression-molded,cross-linked acrylic dentures are as dimensionally stable and useful as the den-tures made with special resins (4).

Denture wearers demand an accurate fit and natural appearance. The fitis very important, since chewing efficiency of artificial dentures is substantiallylower than that of natural teeth. Besides being easy to fabricate, an ideal den-ture material would have high strength, stiffness, hardness, and toughness, ie, befracture resistant, have low density, good dimensional stability, show resistanceto oral fluids, have an absence of odor or taste, be resistant to bacterial growth,have good thermal conductivity, show good retention to other polymers, porcelain,and metals, be radiopaque, be easy to repair, easy to clean, have good storage life,and be inexpensive to make. Significant challenges remain to produce the idealdenture material.

To achieve needed comfort, dentures must be custom-made. To form the cus-tomized denture, in which the artificial teeth are embedded, a wax pattern is used.The wax pattern is inserted in a plaster or dental stone in a split mold flask. Afterremoval of the wax, the surface of the resulting mold cavity is painted with a sep-arating medium, usually an aqueous alginate solution, followed by the additionof the acrylic resin. The separating medium aids in removal of the cured acrylicfrom the mold.

Acrylic denture materials are made by free-radical (addition) polymerization,using methyl methacrylate (MMA) monomer. In the process, the MMA becomespoly(methyl methacrylate) (PMMA). The resins are available in either heat- orcold-cured formulations. A cross-linking monomer, such as ethylene glycol (Fig. 5)

Fig. 5. Dimethacrylate monomers/reactive diluents. EGDMA (R = CH2CH2); DEGDMA(R = CH2CH2OCH2CH2); TEGDMA (R = CH2CH2OCH2CH2OCH2CH2).


or diethylene glycol dimethacrylate (Fig. 5), is included with the MMA mixtureto improve the mechanical properties. The dimethacrylates are covalently bondedat various points along the PMMA chains, forming a cross-linked matrix. Visiblelight-cured versions have also become available, with the chemistry akin to thatof composite restoratives.

Heat-Cured Methacrylate Formulations. These resins consist of granu-lar PMMA powder blended with liquid MMA, along with a cross-linking monomer,as shown in Figure 5 (EGDMA or DEGDMA). After mixing and heating, themonomer–polymer dough forms a rigid plastic. The powder component is mostlygranules of PMMA, along with benzoyl peroxide initiator (BPO, 0.5–1%), tita-nium/zinc oxide pigments and opacifiers, dibutyl phthalate plasticizer, and acrylic-or nylon-type reinforcing fibers. The liquid component contains the inhibitedMMA, along with a cross-linker (Fig. 5, DEGDMA). The blends, normally con-sisting of about two to three parts of PMMA and one part of monomer by volume,are packed with pressure into the mold, having the properly positioned teeth.The MMA is normally inhibited with the methyl ether of hydroquinone (MEHQ)or butylated hydroxytoluene (BHT). Small amounts of other acrylic monomers,plasticizers, and 1–5% of a cross-linking agent may be employed. The polymericgranules may also have methyl acrylate in the backbone or be plasticized by ethylor butyl methacrylate or ethyl acrylate to increase solubility in the monomersyrup. Particle size and molecular weight distribution of the PMMA controls thesolubility of the polymer and the working consistency of the mixture. Traces ofpoly(acrylic acid) or soluble starch suspension agents may remain in the polymer,preventing wetting of the beads by the monomer. The residual initiator contentof the polymer beads may be at the level to obviate the need of further BPO to beadded to the mix.

Most dentures are fabricated from the heat-cured formulations with the poly-merization rate increasing directly with temperature, proportional to the squareroot of the initiator concentration. The half-life temperature (t1/2, ◦C) for BPO at72◦C is 10 h (5). The customary curing cycle of the fully mixed powder/liquid blendis about 90 min at 65◦C. Post-curing is usually done at 100◦C for 60 min so as toproduce a more fully cured denture with low porosity. After cooling, the dentureis separated from the embedding material, trimmed, and polished. Red fibrousmaterials and beads of varying translucency are added in small amounts prior tocuring so as to simulate the appearance fo natural oral-tissue.

Autopolymerizing Resins. Room temperature (RT) curing, initiated by asuitable redox (oxidizer–reducer) combination, is a simple modification of the heat-cured formulations. In such formulations tertiary aromatic amines, such as N,N-dihydroxyethyl-p-toluidine or p-N,N-dimethylaminophenethanol (6), are added atabout the 0.3–0.8% level to the monomer, which is subsequently blended with apolymer containing 2% BPO. The rate and degree of polymerization depends onboth the type and concentration of initiator (BPO) and activator (amine), as well asthe particle size of the PMMA powder. Techniques for preparing the molding arevery similar to the procedure described for heat-cured dentures. In this procedure,the freshly mixed monomer–polymer blends are more limited in use because ofthe handling characteristics of the formulations, ie, the polymerization is slightlydelayed upon mixing. The rise in temperature depends on the mass of materialand the powder/liquid (P/L) ratio used. Since polymerization occurs from inner to


outer portions of the mass, temperatures within the bulk portion of the castingare higher than those at the surface. The cured dentures are usually not veryporous, since monomer evaporation is limited. The bulk of the polymerizationtakes place within 30–45 min, but may continue for hours. The denture flask istherefore held under pressure for several hours so as to ensure complete curing.Room-temperature cured materials usually contains about 3–5% free monomer,compared with only 0.2–0.5% found in heat-cured materials. Thus, this method ofcuring is not as efficient as the heat-cured process, since the product produced hasless cross-linking density and a lower Tg than the heat-cured materials. The latterfactors also make these RT-cured materials more susceptible to creep, eventuallycreating distortion in the denture. The larger the amounts of free monomer presentin the final product, the greater the propensity for warpage to occur. Materialsgenerated by using amines in the curing have poorer color stability upon aging.Blue dye, a ultraviolet absorber (7), may be added to the formulation to mask colorshift. Production of high dimensional accuracy is one of the main advantages ofRT-cured resins, resulting from lower curing temperature leading to reductionof stresses in the matrix. Differences between thermal expansion of the dentureresin and the plaster mold may result in undesired dimensional changes when themold is subjected to a wide temperature range during processing. Dentures curedat RT have better dimensional accuracy (8,9) than heat-cured dentures. However,both are clinically acceptable.

Low Viscosity, Chemically Cured Resins. The pour and cure acrylicresins are blends of high molecular weight polymer powder mixed with monomerand other additives to achieve a pourable viscosity. The mixture is usually pouredthrough sprues into a hydrocolloid-based mold, with polymerization conductedunder pressure for about 30 min at RT (10). This procedure for preparing denturesis inferior to heat- and cold-cured acrylics. However, it is an excellent technique fordenture duplication. Polymerization shrinkage is a problem for this type of system,possibly causing posterior teeth to be displaced in the resilient mold and out of thedesired occlusal pattern (11). Methods have been developed to improve the latter,by increasing the bond of acrylics to denture teeth (12). This technique requiresgreat attention to detail in order to produce clinically acceptable prostheses.

Visible Light-cured Resins. Employing high intensity visible (blue) lightto bring about free-radical polymerization (curing) of the denture resins holdsgreat promise (13). A photopolymerizable formulation in this case could consist ofa urethane dimethacrylate–acrylic polymer combination, reinforced with a micro-fine silica filler. Thus, the materials produced have more in common with a com-posite restorative material than with the commonly used denture-based resins.The matrix produced is a highly cross-linked acrylic, having an interpenetrat-ing polymer network (IPN)-type structure. MMA is not used in the urethanedimethacrylate (Fig. 8) cross-linked IPN matrix, filled with colloidal silica andacrylic polymer beads. Akin to composite restoratives, the formulation makes useof the camphorquinone–tert-amine initiator system. With the exception of brittle-ness, the cured materials have properties as good or better as the denture mate-rials made by the methods described earlier. This technique shows good potentialfor expanded use.

Mixing/Working Properties. For denture resins certain aspects are im-portant to note. First, great care needs to be taken to use the correct P/L ratios,


usually about 2.0/1.0 wt% or 1.6:1.0 vol%. Too much powder will result in under-wetting of the beads, leading to production of a weak structure. Conversely, toomuch monomer will produce excessive shrinkage. All ingredients must be thor-oughly mixed to achieve the best results. A separating medium must be used toprevent adhesion of the resin to the mold surface. Control of porosity and preven-tion of processing stress are also two areas of concern. Polymerization shrinkageand escape of volatiles can generate porosity.

Concerning shrinkage, volume reduction of about 20% for the monomer isminimized by using polymer (PMMA) powder, cutting shrinkage to about 5–8%. Itis very important that this contraction is not translated into the high linear-typecontraction, which on the basis of volumetric shrinkage should be about 1.5–2.0%,but is actually in the range of about 0.20–0.5%. The observed change is probablydue to thermal contraction, caused by temperature changes, and not to actualpolymerization shrinkage. The resin becomes very rigid once the temperature ofthe reaction descends below the Tg of the material, at which point the curing con-traction will have essentially been completed. From this point onwards thermalcontraction contributes to dimensional changes of the denture material. Becauseof the latter, cold-cured dentures should have a better fit, since the processingtemperature is considerably lower, ca 60◦C, compared to heat-cured dentures atca 100◦C. Thus, it is very important to pack the viscous mix in the mold in suffi-cient quantity to create pressure, facilitating removal of voids and helping to cutdown on curing contraction. In all systems, polymerization generates an exotherm,which might elevate to the point of causing monomer vaporization and creatingporosity. Thus, temperatures of cure should be controlled to avoid gaseous porosityand pressure maintained to eliminate contraction porosity. Dimensional changesalso give rise to internal stress, which if allowed to relax may lead to warpage, craz-ing, and distortion of the denture. The use of acrylic teeth, rather than porcelainteeth, eliminates differential shrinkage, helping to reduce strain. Craze develop-ment is another problem that may develop in dentures as a result of strain relief,polishing, presence of alcohol, and differential contraction due to the type of teethused. Cross-linking helps reduce craze formation. A brief summary of the denturematerials’ properties is shown in Table 1.

Biocompatability. PMMA is highly biocompatible, with very few patientsshowing any type of allergic reaction. Adverse reactions are usually caused byleachable monomers. Cold-cured formulations may be a problem, since they tendto have more residual, free monomer. Post-curing of the denture helps lower freemonomer content, but may also cause some denture distortion.

Dimensional and Mechanical Properties. Even though the denture isplaced on soft tissues, there is still great need for the denture to have dimensionalstability. The denture must fit as accurately as possible to promote retention ofthe denture to the mucosa. As shown in Table 1, there is a considerable variationin mechanical properties, depending on composition, processing technique, andenvironment factors. Room-temperature cured resins have lower strength andstiffness, with about the same elastic modulus as heat-cured materials. Polymer-ization shrinkage of the monomer–polymer dough is about 6–7%. Linear shrink-age is about 0.5% across the posterior aspect, under normal denture processing.Water sorption, about 1–2 wt%, partially compensates for the shrinkage. Linearshrinkage of 0.3–0.4% is clinically insignificant, since the tissue on which the


Table 1. Physical Properties of Denture-Based Materialsa

Property Poly(methyl methacrylate) Vinyl acrylics

Compressive strength MPab 76 70–76Tensile strength, MPab 48–62 51–60Flexural strength, MPab 83–117 69–110Elastic modulus, MPab 3.8 × 103 2.3 × 103

Elongation, % 1–2 7–10Impact strength, N·mc 1050 3150Transverse strength, MPab 41–55 41–55Knoop hardness (KHN) 15–23 14–20Coefficient of linear thermal 81 × 10− 6 71 × 10− 6

expansion per ◦CHeat-distortion temp., ◦C 160–195 130–170Polymerization shrinkaged, % 6–7 6–7Water sorption, 24 h, % 0.3–0.4 0.07–0.4aPartially taken from Ref. 14.bTo convert MPa to psi, multiply by 145.cTo convert N·m to ft·lbf, multiply by 0.74.dLinear shrinkage of commercial dentures is ca. 0. 12–0.97%.

denture rests adjusts to such changes (14,15). Some cross-linked resins contain 2-hydroxyethyl methacrylate (HEMA), which promotes higher water sorption, low-ering dimensional stability. Fillers reduce the thermal expansion of the dentures,providing higher impact strength. However, they are difficult to polish, tend tostain, and may collect debris and imbibe bacteria at the surface.

It has been clearly shown that radiopaque materials are needed (16,17) forvisualization of aspirated or swallowed denture fragments. This is accomplishedwith additives such as barium sulfate, barium fluoride, barium or bismuth glasses,and halogenated organic compounds. The physical properties of the materials aresignificantly affected by large quantities of these additives. ANSI/ADA specifica-tions or requirements for radiopaque materials for denture-based polymers havebeen described (18).

While commercial denture materials are reasonably strong and show goodflexibility, improved fracture resistance and fatigue strength are still sought. Seek-ing to improve such things as fracture toughness, impact and transverse strength,tensile strength, etc, various types of fiber (glass, carbon, Kevlar, and polyethy-lene) reinforcement have been found to significantly improve many of the afore-mentioned properties (19). Glass fiber reinforced composite resins have also beenintroduced as a replacement for metal framework in crown and bridges or fixedpartial dentures as per recent clinical studies (20). Yet, research is still neededto develop improved materials, pointed toward reducing the need for denture re-pair (21). ANSI/ADA Specification No. 12 sets the requirements for denture-basedResins.

Polymeric Teeth for Dentures. Acrylic resin denture teeth were intro-duced in the late 1930s. About 60% of all preformed artificial teeth used in den-tures, at least in the United States are produced from acrylics or vinyl acrylicresins. The chemistry used is based on the well-known MMA polymerization


technology. Poor wear, crazing, blanching, fracturing, etc, found in earlier acrylicteeth, has been overcome by better methods of fabrication, improved formula-tions, and use of higher cross-linking density. The molding technique for prepar-ing the teeth must be highly controlled, with respect to particle size, molecu-lar weight, and residual initiator. Further, the mix flow properties and curingcycle must be highly controlled. Mechanical retention serves, to some degree,to anchor the teeth in the chemically activated denture-based resins. However,a combination of mechanical and chemical bonding is used to retain the teeth(22).

Polycarbonates and polyslfones have also been explored for producing moldedteeth. Compositions containing very finely dispersed spheres of pyrogenic silicaas reinforcing fillers, urethane dimethacrylate (Fig. 8) resin, highly cross-linkedIPNs, and the fabrication of layered teeth with an exterior made up of a 2,2-bis-p(2′-hydroxy-3′-methacryloxypropoxy)-phenylpropane (BisGMA)-based resinhave also been explored (23). All formulations employ pigments to provide a natu-ral appearance. Other additives/modifiers are also used to achieve a more naturalappearance.

Acrylic teeth, with compressive strength of 76 MPa (11,000 psi), abrasionresistance, elastic modulus of 2700 MPa (3.9×105 in.− 2), elastic limit of 55 MPa(8000 psi), Knoop hardness (KHN) range of 18–20 kg/mm2, and good abrasionresistance, have physical properties which are lower than those of metal alloysused for dentures and those of human enamel or dentin. For example, dentin andenamel have an ultimate compressive strength of 297 and 384 MPa, respectively,and ultimate tensile strength of 105.5 and 10.3 MPa, respectively. Furthermore,the low modulus of elasticity in plastic teeth reduces the clicking sound oftenexhibited by denture wearers. Compared to porcelain teeth, acrylic teeth haveless resistance to creep, higher water sorption, greater fracture toughness, betterresistance to thermal shock, and bond to the denture base. In contrast, porcelainteeth display better dimensional stability and increased wear resistance. A studyof plastic teeth opposite plastic teeth or opposite smooth porcelain teeth has beenpublished (24). Plastic teeth are covered by ANSI/ADA Specification No. 15.

Denture Repair Resins. Fractured dentures are readily repaired withmaterials similar to the RT- or cold-cured denture resins. Repairs are achievedwith little to no dimensional change. However, the strength of the repaired den-ture may be substantially less than the original prosthesis (25). ANSI/ADA Spec-ification No. 13 (1999) sets the requirements for cold-cured repair resins.

Denture Liner Materials. There are three groups of these materials: hardand soft liners and tissue conditioners. It is often necessary to refit the denturebecause of the changes in the denture-bearing tissue. To meet this need a hardrelining material can be employed, using an RT- or cold-cured acrylic resin at thedentist’s office, or by sending the denture to a laboratory to be repaired with aheat-cured acrylic resin. Soft liners weaken the strength of the heat-cured resins,since they can reduce the thickness of the denture base and allow diffusion ofmonomer or solvent into the base. The cold-cured formulations are basically oftwo types: a two part PMMA powder blended with MMA monomer, containingthe plasticizer di-n-butyl phthalate or poly(ethyl methacrylate), PEMA powder,blended with liquid butyl methacrylate (BMA) monomer. The BPO–amine redoxinitiator system is used for curing. The use of PEMA and BMA works best where


the patients may be sensitive to PMMA or MMA. However, the use of PEMA andBMA creates a liner with a lower Tg, which may create dimensional problems. Theliners must show good adhesion to the denture, allow recovery from deformation,provide a good cushioning effect, have good wetability, be relatively resistant tooral fluids, not support bacterial growth, not impair denture function, and beeasily cleaned. At present, a material awaits discovery for fulfilling all of theserequirements. Polyphosphazine fluoroelastomers have been formulated and curedwith peroxides (26), showing that systems may be found to eliminate some of thedeficiencies found in currently available liners.

Soft liners were developed to eliminate the use of MMA directly against softtissue. Also, there are patients who are not able to tolerate a hard relining surface,even though the denture fits well. The soft liner is more comfortable and providesa means of absorbing masticatory forces via the highly resilient material placedbetween the denture and the soft oral tissue. In addition, soft liners may be usedto reduce tissue inflammation caused by worn out or ill-fitting dentures. Polymerswith a Tg slightly above the mouth temperature exhibit a rubbery behavior in theoral cavity. A variety of materials have a Tg low enough to be useful as soft liners,including silicones and acrylics. PMMA is also useful when plasticized to obtainthe required Tg. Fortunately, acrylic monomers are available which produce poly-mers with a wide range of Tg, starting with PMMA at 105◦C, PEMA at 65◦C, andpoly(n-butyl methacrylate) (PBMA) at 20◦C. They are all useful for producing softliners. The soft liners are generally supplied as powder-liquid kits or ready-to-usesheets. Materials currently available are usually plasticized acrylics. All formu-lations contain fairly high molecular weight acrylate or methacrylate polymers orcopolymers, derived from ethyl, n-propyl, n-butyl, etc, monomers and a liquid orsolvent, such as ethyl alcohol or ethyl acetate, a plasticizer, such as dibutyl phtha-late, and a polymerizable monomer. The Tg of the material is set to ≤47◦C. Thematerials adhere well to denture-based resins. But, they have poor elasticity andharden upon aging because of the loss of the plasticizer (27–30). More hydrophilicliners have been formulated by using HEMA or other copolymers (28). However,HEMA-based resins may soften and swell excessively due to water sorption, lead-ing to undersirable functional changes.

Silicone liners are similar in composition to the previously described elas-tomeric impression materials, which are produced by condensation polymeriza-tion. Systems for the relining application may be either a one-component system,which cures in the presence of moisture or heat, or a two-component system, con-taining base and catalyst. Both types generally have poor adhesion to the denturesurface, and can readily support bacterial growth.

Tissue Conditioners. These products alleviate discomfort from soft-tissue injury or inflammation. Tissue conditioners are soft materials applied tem-porarily to the denture fitting surface, allowing better distribution of stress. Theyexhibit viscous flow under pressure, forming a very soft cushion between the harddenture and the soft tissue. In terms of softness, the material must not be too softor flow to the extent that it will be displaced from between the denture and the mu-cosa. These materials may consist of PEMA powder mixed with a solvent (such asethyl alcohol) and a plazticizer (such as n-butyl phthalate and n-butyl glycolate)(31,32). The alcohol swells the PEMA beads, rapidly promoting diffusion of theplasticizer into the polymer, yielding a plasticized gel. Alcohol and plasticizer are


slowly leached out from the applied gel, which may cause the material to becometoo rigid. The liner must be replaced every few days to retain properties, until thepatients supporting tissues return to normal state.

Crown and Bridge Temporary Resins. Materials used in this area areusually based on methyl or ethyl methacrylates and BisGMA-acrylics mixtures(20), or even an ethyleneimine-terminated monomer (33). The formulations aresupplied as a two-component paste, composed of monomers and polymerizationinitiator. Used for interim tooth coverage, these materials are not as strong asother acrylics. However, they exhibit good flow, low exotherm, and low curingshrinkage. They maintain the correct biting relationship, stop teeth drifting, andprotect the prepared tooth against fracture, while waiting for the permanent pros-thesis to be delivered.

Polymeric materials are also used for fixing veneers on crown and bridges.Polymers used for this application include acrylics, vinyl acrylics, and dimethacry-lates, as well as silica- or quartz-microfilled composites. After placing on the metal-lic substrates of the prostheses, the materials are heat or light cured. These ma-terials are easy to fabricate, and can be readily matched to the color of the toothstructure. The acrylic facings have poor adhesion to the metals, being retained onlyby curing the monomers into mechanical undercuts designed into the metal sub-strate. They have less mechanical strength, less color stability, poorer abrasion,etc, than normal dental composites, along with deforming more under bruxism.With the advent of porcelain fused to metal crowns and bridges, restoratives withpolymeric veneers are less frequently used.

Mouth Protectors. The widespread growth of contact sports has acceler-ated the use of mouth guards (34,35). Guards may be produced from natural rub-ber, poly(vinyl chloride), poly(vinyl acetate-co-ethylene), or polyurethane materi-als. Customized guards are often fabricated from poly(vinyl acetate-co-ethylene)blanks, soft acrylic dough, liquid rubber latex, polyurethane, and laminated ther-moplastic (36,37). Over the counter protectors usually fit poorly, in contrast todimensionally stable and comfortable, customized mouth protectors.

Maxillofacial Prosthetic Materials

A large number of polymers, including latexes, vinyl plastisols, silicone elastomers(heat or RT vulcanized), and polyurethanes, may be formulated into materials forfacial prostheses (38–41). The materials should be biocompatible, easy to fabricate,easy to clean, feel like real skin, translucent, stable to heat, light, and cleaning so-lutions, and sufficiently resilient to prevent tearing. It should be possible to modifythe formulation to match skin color. No material meets all these requirements.

Latexes. These materials do not have the strength and color stability to bevery useful for this application. In addition, they may cause an allergic reaction.However, a recent terpolymer derived from n-butyl acrylate, MMA, and methylmethacrylamide, can be formulated with colorants to provide a superior latex,compared to earlier materials.

Vinyl Plastisols. These materials have some utility in maxillofacial pros-theses. The formulations are viscous liquids, made up of small vinyl particlesdispersed in a plasticizer, along with colorants. They are heated to generate the


Fig. 6. Gutta-percha structure.

desired materials. The loss of plasticizer and lack of stability decreases the use ofthese materials.

Silicones. These materials have some only recently been used to producemaxillofacial prostheses. Both the RT- and heat-vulcanized materials may be used.Heat-vulcanized formulations are supplied as a semisolid or putty-like material,which requires the addition of colorants. The molded material is cured underpressure at 180◦C/30 min. The heat-cured materials exhibit better strength andcolor stability than the RT-cured materials.

Polyurethanes. This is the most recent material used in maxillofacial ap-plications. Fabrication requires accurate proportioning of the components. Theisocyanate and polyol are blended, placed in a suitable mold, and allowed to cureat room temperature. Colorants and other additives are also used in the formula-tions. Even though the fabricated prosthesis has a natural feel and appearance,the final product is still relatively unstable.

Root-Canal Sealants

A variety of materials have been used to hermetically seal the root canal, prevent-ing ingression of oral fluids into the canal. One natural material used is gutta-percha (Fig. 6), a rubber obtained from the Taban tree. This material has beenused in endodontics for over a hundred years. Rubbers are polymers of 2-methyl-1,3-butadiene (isoprene), having two possible conformations, ie, a cis and transform, with the trans form being gutta-percha. These rubbers are hardened by vul-canization, achieved by blending and heating with a few percent of sulfur, whichcauses cross-linking. A typical formulation used in a root canal has about 19–22%gutta-percha, 59–75% zinc oxide filler, 1–17% heavy metal salts, and 14% waxplasticizer. The material softens at about 60–65◦C and melts in the vicinity of100◦C, providing a temperature range to soften, deform, and condense the mate-rial into the prepared root-canal space. Other materials used for endodonticallytreated, fractured teeth are based on zinc oxide-eugenol, epoxy resin, polyvinylresin, calcium hydroxide resin based formulations, and glass-ionomers formula-tions.

Other Uses

Patterns for gold-inlay castings can be prepared from acrylics. Castings madethis way are not superior to castings produced from a wax pattern, accountingfor the lack of interest in this technique. Some dental laboratories use epoxy die


materials for fabrication of casts. Autopolymerizing resin formulations are used tomake custom impression trays, with such resins containing substantial quantitiesof fillers to increase the rigidity of the materials. PMMA or other thermoplasticsare used to prepare occlusal night guards. Other areas of dentistry make extensiveuse of polymers in retainers, splints, temporary space maintainers, and bite plates.

Restorative Materials

Filling Materials. Resins used to formulate dental composites/restorativeswere initially derived from the free-radical polymerization of doughs made fromPMMA beads blended with MMA monomer (42–46). Many studies have focusedon improving these materials, by addition of cross-linking monomers and rein-forcing fillers (45), with much of the early work done at the National Bureau ofStandards. Earlier, the polymer industry had previously shown that epoxy resins,based on bisphenol A, were excellent matrix resins for a variety of composites.However, epoxies exhibited poor ambient polymerization characteristics underconditions useful in the oral cavity. Knowing the latter, as well as the need to havea new monomer which would be both free-radical polymerizable with less shrink-age in composites, Bowen (47,48) made the discovery that the diepoxide derivedfrom bisphenol A and epichlorohydrin could be treated with methacrylic acid toform a unique hybrid monomer, well known today as BisGMA (45,47,48). Alter-natively, the BisGMA monomer may also be produced by reaction of bisphenol Awith 2 mol of glycidyl methacrylate. The synthetic path to the bulky, thermosettingdimethacrylate, BisGMA, launched a new era for development of composites usedin dentistry. BisGMA (Fig. 7) has two chiral carbon atoms (denoted by an asterisk)with the dimethacrylate diastereomers helping to make the BisGMA mixture aviscous syrup. Compared with the PMMA–MMA doughs, the BisGMA-based for-mulations, combined with inorganic reinforcing fillers and suitable polymerizationinitiators, made available composite fillings with lower polymerization shrink-age, improved mechanical properties, reduced water sorption, a more attractivecoefficient of thermal expansion, better esthetics, and more acceptable biocom-patibility (44,45). A good review of dental composite resins has been published(49).

Dental Composite Restoratives. Polymeric restoratives have three ma-jor components: an organic resin matrix, an inorganic filler modified with a cou-pling agent, and a suitable polymerization initiator system. The formulation usedto produce the organic matrix, or continuous phase, is made up of free-radicalpolymerizable monomers. The monomer mostly used in the formulations for bothanterior and posterior resins is BisGMA (Fig. 7), or alternatively formulated with

Fig. 7. BisGMA structure(s).


Fig. 8. Monomers structures in urethane dimethacrylate mixture.

a urethane dimethacrylate monomer (Fig. 8). A variety of fillers may be used tomake up the reinforcing phase. Fillers may consist of silanized quartz, glass orvarious ceramics, with silanization affording the coupling mechanism. A prepoly-merized resin containing pyrogenic silica may also be used to reduce polymeriza-tion shrinkage. Various polymerization initiators or activator–initiator combina-tions are known to be useful to achieve curing. Inhibitors are required to preventpolymerization during storage. Sealants used in dentistry have similar composi-tions, but are more lightly filled or possibly unfilled. Composites used in dentistrycan come in various forms, ie, a one- or two-part formulation, a uniform paste, apowder–liquid, a paste–liquid, or a paste–paste formulation.

Polymerization. Methacrylate-based restoratives are free radical not rad-icals polymerized by redox systems or photochemically by visible light, usingphotoinitiators or photoinitiator–photosensitizer combinations with only the vis-ible light range (400–500 nm) allowed for curing. Redox initiation systems mayconsist of such things as BPO/tert-amines, hydroperoxides-thioureas, peroxideswith ascorbic acid or derivatives, and BPO/tert-amine with synergistic promot-ers, eg, polythiols (4,50–52). A commonly used combination for redox use is theBPO/N,N-dihydroxyethoxy-p-toluidine system. The combination of α-diketones,such as camphorquinone (CQ), with a tertiary amine, such as N,N-dimethyl-aminoethyl methacrylate (DMAEMA) (52), are particularly useful for visible light-curing (VLC) (53,54). With light activation, CQ and DMAEMA form an “exciplex”with the “exciplex” subsequently producing a DMAEMA-based radical, which ini-tiates the polymerization. VLC resins provide the dentist a significant control overthe restorative hardening process. Further, the porosity and surface tackiness, ow-ing to air inhibition, are lower in the VLC-cured materials compared with that ofthe redox formulations. But, properly mixed redox formulations polymerize muchmore uniformly, especially in areas of great bulk, compared to the VLC formula-tions. When using VLC systems to fill deep cavities, a layering technique must beused to ensure adequate polymerization.

Monomers or Matrix Phase. Although BisGMA is widely used in com-mercial dental composites, other types of dimethacrylates have also been stud-ied or developed for use as alternatives to BisGMA (55–76). Because of the highviscosity of BisGMA, a variety of dimethacrylates have been found to be usefulas reactive diluents or polymerizable modifiers to provide suitable viscosities forhigh filler loading, with diethylene glycol and triethylene glycol dimethacrylates(Fig. 5) being good examples. Studies on the kinetics and the nature of the network


formation in the dimethacrylate polymerizations have been given significant at-tention (77–83), with the types of monomers used and the degree of carbon–carbondouble bond conversion achieved determining the Tg obtained (49). There is grow-ing interest in varying the polymerization stages, ie, using a “soft start” stage, asa way to improve the resin properties.

Resins have also been designed to utilize multifunctional methacrylateprepolymers, derived from the chain-extending reaction of BisGMA with a di-isocyanate, such as 1,6-hexamethylene diisocyanate, combined with a diluentmonomer (64,65). Incorporating urethane residues in the matrix, which en-hances toughness, gained great acceptance in Europe, with an example beingthe monomer derived from the reaction of HEMA with a branched aliphatic diiso-cyanate (Fig. 8).

Many experimental dimethacrylates have been explored for reduction ofshrinkage and water sorption in composites, seeking to improve such things as di-mensional stability, creep resistance, fracture toughness, wear resistance (66,84–95). The goal is to formulate a better alternative to the commonly used amal-gam. Some of these have been prepared from nonhydroxylated homologues ofBisGMA, having lower viscosity (66). In one case, a multimethacrylate preparedfrom esterification of a low molecular weight styrene–allyl alcohol copolymer wasshown useful in reducing shrinkage or polymerization contraction stress in Bis-GMA/TEGDMA neat resin blends (96,97).

A variety of fluorine-containing monomers and polymers have been preparedand examined to produce hydrophobic dental composites with low water sorption,high contact angles, reduced margin leakage, and resistance to surface staining(67–73). One such formulation consisted of 75 wt% octafluoro-1,1,5-trihydropentylmethacrylate and 25 wt% of a nonhydroxylated homologue of BisGMA (66–68).But, the polymerization shrinkage and strength deficiencies were not improved.Also, the volatility of the nonbulky fluorinated-monomer component was a prob-lem. In contrast, a stronger hydrophobic composite was prepared from a lowsurface-energy matrix resin consisting mainly of a highly fluorinated multifunc-tional methacrylate prepolymer, ie, PFMA (70–73,98). The viscosity of this for-mulation was comparable to that of BisGMA, along with solubility in a varietyof diluent monomers. The dimethacrylate derivative of a fluorinated triethyleneglycol, having low viscosity, was recently examined as a reactive diluent, showingsome promise to improve the performance properties of an ethoxylated bisphenolA dimethacrylate (BisEMA)-based composite (99–101).

Elimination of polymerization shrinkage has been under study for some time(74,75). In the early work, a blend of a BisGMA-based resin and a solid spiroortho-carbonate (SOC), which also polymerizes by a free-radical, ring-opening mecha-nism, was cured to give thermosets with very low volume shrinkage (75). Followingthe latter study, a great variety of compositions have been designed for ring open-ing, expanding polymerizations, focused on eliminating or reducing the shrinkageas well as improving the physical and mechanical properties of composites. Thefocus has mainly been on SOC and SOC-type monomers with epoxy resins (102–113). This concept has not, to date, produced marketable dental restoratives.

More recently, the possible use of liquid crystalline monomers (114), hy-perbranched polymers (115,116), inorganic–organic hybrid monomers (117–120),and sol–gel technology (121) has started to be examined for preparing improved


composites. The use of multimethacrylate substituted hyperbranched polymerslooks to be a particular attractive path to explore for generating composites withsignificantly lower shrinkage, since low viscosity, high molecular weight entitiescan be formulated into the monomer–polymer mixture. Also, the in situ gener-ation of a nanocomposite, where the inorganic component is truly distributedat the molecular level in the cured composite, may satisfy the great need forpreparing restoratives with both low shrinkage and significantly improved wearresistance.

Reinforcement Phases. After introduction of quartz filler in the late1950s, many types of reinforcing fillers have been studied or used in dentalcomposites. In addition to quartz, fused or colloidal silica, and other inorganicfillers, finely divided, organic prepolymenized composite particles are also usefulas a dispersed phase. This is especially true for microfilled and hybrid compos-ites (122–124). Fillers serve to increase hardness and compressive strength, andsignificantly reduce shrinkage of the restoratives. Filler technology has broughtmany improvements to currently used composites. But, in order for the compos-ites to have acceptable mechanical properties, it is critical for the resin matrix andfiller to have a strong interfacial bond. A breakdown at the interface, followed byload application, will not allow the stresses developed to be effectively distributedthroughout the material. Bonding is achieved by the use of coupling agents, in-corporated into the formulation at the surface of the filler. Vigorous efforts havebeen focused on maximizing filler volume or obtaining highly loaded composites,which exhibit much better stress-bearing capability.

Physical Properties. Table 2 shows some physical properties of unfilled(neat) resins and filled composites. Microfilled composites generally have inferiorproperties compared to conventional or the more recent hybrid restoratives, with

Table 2. Physical Properties of Resin Restoratives

Conventional andUnfilled Microfilled filled hybrid

Property PMMA composite composite

Inorganic contenta, wt% 33–51 70–80Compressive strength, MPab 55–78 225–350 250–400Knoop hardness (KHN) 15 5–30 50–60Tensile strength, MPab 14–28 30–55 50–90Modulus of elasticity, GPac 1.9–2.3 3.2–6.0 8.0–20.0Linear coefficient of thermal 92 50–70 20–40

expansion, 10− 6 ◦CWater sorption, mg/cm2 1.70–2.03 0.94–2.20 0.50–0.70Polymerization shrinkage, vol% 5.2–8.0 1.0–1.8 1.2–5.3Thermal conductivityd,e W/(m·K) 2.4 6.3–8.4 1.5–4.0aThe volume (weight) of the filled systems varies from about 30 to 77% (50–85%). Traditional, smallparticle filled, micro-filled, and hybrid composites have fillers ranging in size, respectively, from 8–12,1–5, 0.04–0.4, to 0.6–1.0 µm.bTo convert MPa to psi, multiply by 145.cTo convert GPa to psi, multiply by 145,000.dThermal conductivity of enamel and dentin, respectively, is 0.87 and 0.59 W/(m·K).eComposite restoratives are good insulators.


the exception of polishability. By definition, hybrids (blends) have a combinationof colloidal and fine particles as filler, at approximately 60–65 vol% with the fillersranging in size from 8–12, 1–5, 0.04–0.4, to 0.6–1.0 µm. The higher volume ratioof polymer to inorganic filler, ie, low filler content, of the microfilled resins leads tolower modulus of elasticity, greater thermal dimensional changes, less resistanceto indentation, and higher water sorption. Requirements for dental compositesare covered by ANSI/ADA Specification No. 27.

Dental Cements. Polymeric matrices used in formulating cements aresimilar to those used in methacrylate-based composites and sealants. BisGMA orsome other dimethacrylate is blended with monomers such as MMA, along withfillers and other additives, to make the formulated adhesive useful in the oralcavity. They may be of the VLC type and/or chemically cured type. They may alsocontain additives such as inorganic fluoride salts, which may result in reducingrecurrent decay.

Polyelectrolyte-Based Restoratives. Alginates, previously describedunder Impression Materials, were the first ionic polymers to be employed in dentalapplications. But, polyelectrolytes derived from poly(acrylic acid), various co- andterpolymers of acrylic acid, and other alkenoic acids (ie, itaconic acid and maleicacid) are used in aqueous solutions for formulating polyelectrolyte-based cements.These materials are also known as glass polyalkenoates or more commonly glass-ionomers (GIs) (125–135). Several articles present the chemistry associated withGIs (136–139). Cements based on glass ionomers adhere well to enamel but onlyweakly to dentin.

Glass Polyalkenoate Cements. Polyalkenoate cements, which evolvedfrom the zinc phosphate cements, were formulated with an organic polyacid andsubsequently called zinc polyacrylate cements (127). Zinc oxide was the basic pow-der component, with smaller amounts of magnesium or tin oxide and, optionally,small quantities of silica, calcium hydroxide, stannous fluoride, and other salts.The acid component was an aqueous solution of poly(acrylic acid), having about30–50% solids, with the polymer having an average molecular weight of 30,000–50,000. Copolymers of acrylic and other alkenoic acids, eg, itaconic, maleic, andaconitic acids, were subsequently used to make shelf stable aqueous solutionswith lower viscosity, providing a cement with better properties. A series of acid–base reactions brings about hardening or setting of the cements, with ionic-typecross-linking achieved by the ionized carboxylate groups forming a complex (salt-bridge) with the divalent metal cations, producing a highly, ionically cross-linked,hydrophilic matrix. Free powder embedded in the stiff polymeric hydrogel or ma-trix acts as a normal reinforcing agent. Other types of additives are incorporatedinto the formulations to improve certain properties. Cements of this type, whichhave an opaque appearance due to the high concentration of zinc oxide, adherewell to tooth structure and base metals, enabling them to be used as bases un-der permanent fillings and to cement prefabricated restorations and orthodonticappliances. The properties of various cements are given in Table 2. A family of non-aqueous polycarboxylate cements may also be formulated, with such systems dis-playing excellent hydrophobic and energy-absorbing properties. Chain-extending,acid-base reactions of dimer and trimer acids with various basic powders maybe used (140). The water-based cements are covered by ANSI/ADA SpecificationNo. 96 (1994).


Table 3. Physical Properties of Resin Restoratives

Conventional andUnfilled Microfilled filled hybrid

Property PMMA composite composite

Inorganic content, wt% 33.2–50.9 70.4–80.2Compressive strength, MPaa 55–76 221–330 127–350Tensile strength, MPaa 14–28 28–56 28–63Modulus of elasticity, GPab 2.3 3.2–5.0 7.1–16.2Linear coefficient of thermal 92 46–70 25–40

expansion, 10− 6 ◦CWater sorption, mg/cm2 2.03 0.94–2.20 0.13–0.74Polymerization shrinkage, vol% 5.2–8.0 1.9–5.8 1.2–5.3Thermal conductivity, W/(m·K) 2.4 6.3–8.4 10.5–13.8aTo convert MPa to psi, multiply by 145.bTo convert GPa to psi, multiply by 145,000.

More translucent glass polyalkenoate- or polyelectrolyte-based cements(GI) were developed by using similar aqueous solutions of polyacids, such aspoly(acrylic acid), poly(acrylic acid-co-itaconic acid), poly(acrylic acid-co-maleicacid), etc, and ion-leachable calcium fluoro aluminosilicate type glass powders(126,128,135). The matrix in this conventional-type GI is formed by acid-base re-actions involving the di- (Ca2+) and trivalent (A13+) cations, binding carboxylategroups in ionic cross-links (salt-bridges), producing a stiff hydrogel into whichpartially reacted glass particles are embedded (136–138). Glass-ionomers, simi-lar to the zinc polycarboxylate cements, exhibit good adhesion properties. Glass-ionomer cements may be used as a cosmetic filling materials and for the repairof cervical erosions because of good adhesion and semitranslucent nature. In thearea of mechanical strength, GI-based cements are superior to zinc phosphateand zinc polycarboxylate, approaching the dental silicate cements in compres-sive strength and modulus of elasticity (Table 3). Other polyelectrolytes have alsobeen explored as polymeric acids for formulating GIs, with poly(vinyl phosphonicacid) being an example (141,142). The vinyl phosphonic acid polymer formula-tions were too acidic to allow suitable working and setting times (142). Experi-mental cements have also been described for taking advantage of the reaction ofan aqueous solution of phytic acid and myoinositol hexakisphosphate with zincoxide or alumino-silicate glass powders (143). In such a formulation, a polyelec-trolyte network matrix is formed by ionic chain extension and cross-linking re-actions involving leachable multivalent cations and the organophosphoric acidgroups.

Acrylic acid copolymers were recently modified with N-acryloyl- or N-methacryl-oylamino acids, such as N-methacryloyl-glutamic acid (MGA), provid-ing a possible path to improved, conventional GIs (144–148). The copolymers stud-ied had the carboxylic acid groups tethered at various distance off the copolymerbackbone, with the acid groups having a range of pKa or dissociation constants(149,150). The AA IA MGA copolymer having an 8:1:1 monomers ratio lookedparticularly attractive to use in formulating conventional GIs, as shown by sta-tistically designed experiments. The study showed that an 8:1:1 (AA:IA:MGA)


copolymer gave the highest compressive strength (148). Raman spectroscopystudies showed that tethering the acid groups various distances off the copoly-mer backbone influences the level of salt-bridge formation (149). This type ofmodification was also shown to enhance the adhesion of the GI to tooth structure,improve fracture toughness, and enhance fluoride release. All the improvementswere attributed to greater availability of the carboxylic acid groups to ionize andform salt-bridges. The copolymers with pendant amino acid residues have alsobeen developed for preparing VLC formulations (146,150).

The monomer N-vinylpyrrolidinone (NVP), 1-vinyl-2pyrrolidinone, has beenexplored for modification of poly(acrylic acid-co-itaconic acid), providing a path tonew polyelectrolytes for formulating GIs (151–155). For these copolymers, the opti-mum monomer ratios and molecular weights, to give good mechanical properties,were determined by statistical design of experiments. Formulations containingNVP residues were also developed for VLC applications. Clearly, new polyelec-trolytes can be prepared for formulating improved GIs.

VLC Glass-Ionomers. Presently, the GI formulations used in the UnitedStates are either the free-radical, VLC, redox initiated, or a combination of VLCand redox initiation types (128,136–139). These are water-soluble polylectrolyteshaving both pendant carboxylic acid groups and free-radical polymerizable moi-eties, such as methacrylate residues. These hybrid-type formulations, which aremore composite-like, are also referred to as resin modified glass-ionomers (RMGIs)(138,156–158). The two-component formulations, with one being an aqueous solu-tion of the polyelectrolyte (acidic polymer), initiators, and reactive monomers andthe second the basic glass powder consisting of calcium fluoro-aluminosilicate, aremixed, placed in the cavity, and exposed to visible light. Hardening occurs in twoways: formation of a free-radically cross-linked organic matrix and salt-bridge for-mation. A combined VLC, redox, and salt-bridge reaction in the same formulationappears to work best (157,158). The VLC GIs are less technique sensitive, cure inshorter time, may be finished at the time of placement, are more plastic in nature,have better adhesion to tooth structure, and reduce microleakage better than theconventional GI formulations (159). All in all, the RMGI modifications made theGI family of restoratives significantly stronger than the conventional GIs, leadingto recommendations that these materials could be used in Class V and Classes Iand II restorations in primary teeth.

Compomers. Glass-ionomers have evolved along with composite resintechnology to produce a new family of materials for restoratives, now commonlycalled compomers (160–166). To some degree, compomers offer the advantagesof both GI and composites in one material. Starting from the first resin modi-fied cements (160,161) work, compomers were first introduced to the market in1995. The term polyacid modified composite more readily describes the chemistryof compomers. These formulations consist of an organic and an inorganic phase,with the inorganic phase having some level of fluoride-containing, basic or reac-tive glass particles. The organic phase also contains carboxylic acid functionalizedmonomers and initiators. On exposure to light, free-radical polymerization occursto give the cured resins. Water sorption of the cured organic matrix facilitatesan acid-base reaction, bringing about further hardening due to formation of someionic-type cross-linking. Water diffusion also facilitates the release of fluoride ionsto the tooth structure. The physico-mechanical properties of GIs, compomers, and


composite restoratives have recently been compared (167), with this and otherefforts showing that the mechanical properties of compomers, as well as their re-sistance to wear, are significantly better than both conventional- and VLC-typeGIs. The compressive strength and diametral tensile strength for compomers is inthe range 280–460 MPa and 52–62 MPa, respectively, with polymerization shrink-age in the range of 2.0–3.0%, and the coefficient of thermal expansion (ppm/◦C)being in the range 12–41. The reported coefficient of thermal expansion for coro-nal tooth structure is about 11 ppm/◦C. Clearly, research on compomers suggeststhat there are still others ways to be discovered for improving the handling char-acteristics and compensating for polymerization shrinkage in the design of dentalrestoratives.

Sealants

A variety of sealants have been explored or developed (168), with many peoplehaving one or more applications of a sealant. Sealants are vital for promotion ofadhesion, which significantly reduces caries formation (169–177). Pit and fissuresealants are covered under the American Dental Association (ADA) AcceptanceProgram. These materials are used to seal high caries-susceptible pits and fissuresof the deciduous and permanent molars, and also to seal microspaces betweenthe tooth and restorative materials, enabling these materials to adhere firmlyboth to prepared cavity walls and to other restoratives. They provide dental pulpprotection and protection from secondary caries formation.

Most dental sealants are resinous materials derived from free-radical poly-merizable monomers, but GI dental cements (discussed earlier) also have some useas sealants. Sealing with resinous materials or GIs is part of modern preventivetechnology, where the sealants used for this purpose are called preventive dentalsealants (PDS). Dental caries that occur around restorations are called secondarycaries. Sealing the microspaces with adhesive resinous materials is effective incontrolling secondary caries; here we call these adhesive materials the restorativedental sealants.

A brief history and state-of-the-art ion enamel and dentin bonding was pub-lished in 1995 (178). Recent advancements in synthetic chemistry and polymerscience of dental sealants are now briefly reviewed or discussed, including thosedesigned for various clinical uses, directed toward prolonging the lifetime serviceof natural teeth.

Preventive dental sealants, used to seal the susceptible areas of teeth, areclassified into pit and fissure sealants and smooth surface sealants. From a ma-terial science perspective, pit and fissure sealants can be further classified intoresin sealants and GI cements. Preventive dental sealants are usually placedonto molar teeth of young children who are at high risk for caries develop-ment.

Pit and Fissure Sealants. Resin sealants consist of a free-radical poly-merizable monomer mixture, having a viscosity low enough to penetrate easilyinto narrow pits and fissures, capable of being cured to a hard and durable sealingmaterial. BisGMA, urethane dimethacrylate, and other methacrylates are verypopular as monomers for resin sealants, along with other monomers, to lower


mixture viscosity to enhance penetration ability, which is interesting to note.The highly active, anerobically cured cyanoacrylate monomers have also beenevaluated for sealant applications (179). Etchants, such as phosphoric acid, citricacid, or other acidic solutions, are applied to the enamel pit or fissure area priorto application of the sealant, to provide the required tooth bonding to the nonad-hesive resin (179–181). This so-called acid etching changes the enamel surface toa microrugged structure, which fosters better penetration of the sealant into thefissure and results in stronger adhesion between the sealant and the tooth surface(182). Acidic monomers have been developed for use in the sealant formulations tofurther enhance the penetration and adhesion. Polymerization of the sealants isinitiated chemically with BPO/amine or sulfinate (redox combinations) or visible(blue) light irradiation, where the formulation contains CQ as the photosensitizer.These sealants may be one- or two-component formulations. One-component, pho-topolymerizable sealants can be cured very rapidly (<60s) by exposure to visiblelight, with two-component, self-curing sealants taking significantly longer time toharden or cure after mixing (183).

Glass-ionomer cements, where the chemistry was described earlier in thetext, are widely used as sealants. Uses of this type of hydrogel (184) vary with thedental treatment, where low viscosity pastes, which exhibit fast setting, charac-terize the cements for pit and fissure sealing. In this process a glass powder andan aqueous polymer solution are mixed in given proportions and packed into pitsand fissures without prior acid etching of the tooth surfaces. The cement pastecures within a short time through the salt-bridge formation between poly(acrylicacid) (185), or acrylic acid copolymers with maleic and itaconic acids (6), and Ca2+

and Al3+ cations leaching out from the glass powder. Poly(acrylic acid) under-goes ionic interaction with calcium ions on the enamel surface, producing bondstrengths of 2–4 MPa. Fluoride ion released from these compositions helps pre-vent caries formation, improving the acid resistance of surrounding dental enamel(186).

Smooth surface sealants are used to prevent bacteria, staining, and phys-ical damage to susceptible teeth. Cured sealants are directly exposed to theoral conditions, and therefore strong adhesion to enamel and dentin, goodphysical properties, good wear resistance, chemical stability, biological sta-bility, and thin-film formation are required. Glass-ionomer cements fail tomeet these requirements because of insufficient physical properties. Instead,highly cross-linked resin sealants, reinforced with ultrafine silica particles, areused.

Shear bond strengths of composite restoratives on enamel, etched with 30–40% phosphoric acid solutions are typically in the range of 18–25 MPa (182). Incomparison, bonding to dentin is a much tougher challenge because of its organicconstituents, fluid-filled tubules, and intrinsic compositional variation (187). How-ever, substantial progress has been made to facilitate minimal invasive restora-tive therapy, by design of new bonding techniques/agents. First, it was discov-ered that resin penetration into the acid etched (conditioned) dentin, which iscritical to achieve good bonding, is influenced by dentinal conditions (moist ordry) and by the type of solution used (188,189). There is great correlation of thebond strength received with the permeability and dentin penetration depth (189–191). The elusive goal, not yet totally reached, is to have a simplified application


method for a dentin bonding system which will produce a very durable bond withtotal elimination of future microleakage (192–194). Composite resin polymeriza-tion shrinkage is the main reason for marginal gaps and stress at interfaces,leading to microleakage (192,195). Thermo- and load-cycling studies are essen-tial for determining or predicting the dentin-restoration bond longevity (196,197).Apart from using simple acid etching, the evolutionary development of surface-active comonomers with both hydrophilic and hydrophobic moieties, as part ofthe primer system, has made a very significant impact on improving bonding todentin (198–201). The work on self-etching primers, to improve the understand-ing of what it takes to bond well to dentin, also played a very important rolein the design of improved dentin bonding systems (202–206). For specific infor-mation on some of the most recent, commercial enamel and dentin bonding sys-tems, see literature on Excite (Ivoclar AG or Vivident), Single Bond (3M DentalProducts), One Step (BISCO), Prime Bond (Dentsply/DeTrey), and Optibond Solo(Kerr Sybron).

Dental Restorative Sealants. Dental amalgams are mixed and con-densed into molar teeth cavities, hardened by the amalgamation process. Theamalgam has no chemical interaction with the cavity wall, and irregular mi-crospaces are produced along the interface between them. Amalgam sealant andamalgam bonding agents are used to seal the microspace and control the inci-dence of secondary caries around the amalgam restoration. Chemical compositionof this sealant or bonding agent may be as follows: a monomer such as diethyleneglycol dimethacrylate (Fig. 5) or a comonomer such as methacryloxyethyl phtha-late (MEP). Other formulation additives may be tert-butyl hydroperoxide initiator,o-sulfobenzimide accelerator, and hydroquinone inhibitor.

This sealant is composed of only one liquid, which remains uncured beforeapplication, but cures quickly once applied to amalgam. Because of tert-butyl hy-droperoxide and o-sulfobenzimide, polymerization occurs anaerobically when it iscut off from air and it contacts the copper in the amalgam. When placed betweentwo glass plates at 25◦C, it takes more than 10 min to cure. However curing occurswithin 2 min between a glass plate and the amalgam. The MEP (207) monomeris used to enhance penetration of the sealant into micro spaces and achieve adhe-sion to both arnalgam and tooth surface. This sealant achieves amalgam–enamelbonding by about 1.0 MPa and bonds amalgam–dentin by about 2.6 MPa withoutacid etching. Examples of two amalgam bonding agents illustrate the latter, briefdiscussion.

Amalgam Restorative Sealants. These adhesives are of two types: adhe-sive resin cements and bonding agents originally developed for composite restora-tions. One commonly used adhesive resin cement is Amalgambond (Sun MedicalCo., Japan), which consists of PMMA powder and MMA monomer, containing4-methacryloxyethyl trimellitate anhydride (4-META) and tributylborane oxide(TBB-O) as a catalyst. TBB-O is activated by the water on the surface of adher-ents and promotes graft polymerization of MMA onto collagen in dentin, promotingstrong adhesion (201,207,208).

Panavia 21 (Kuraray Co., Japan) is an adhesive cement, characterized bythe use of 10-methacryloyloxydecyl dihydrogen phosphate (MDP). This cementconsists of a primer and a composite paste. The primer is a monomer solutioncontaining MDP, which develops strong adhesion to enamel and dentin. The


cement paste is a mixture of polymerizable monomers, including MDP, andinorganic filler, with polymerization/curing caused by the catalyst BPO/text-amine/sulfinate. Strong adhesion to tooth substances and amalgam is achieved.Panavia has a shearbond strength, in MPa, of 20 to dentin, 37 to enamel, 15 toamalgam, and 47 to Ni–Cr alloy.

Composite Restoration Sealants. Systems used in this case consist es-sentially of dimethacrylate mononers, such as BisGMA, homologues of BisGMA,urethane dimethacrylate, and silanated inorganic fillers, as discussed in the sec-tion on Composite Resins. The sealants used for restorations (called bondingagents) are applied onto the cavity walls prior to the placement of compositesand bond the two substances tightly.

Bonding Agent Chemistry. Composite bonding agents are a mixture ofpolymerizable monomers or a solution of monomers in volatile solvents, whichcures through free-radical polymerization on the cavity walls or between cav-ity walls and composite restorations. Liquid monomers, such as MMA, HEMA(207,209,210), other erythritol methacrylates, and ethylene glycol dimethacry-late, exhibit strong adhesion to dry teeth. The well-known Gluma adhesive sys-tem is made up of water, HEMA, and glutaraldehyde (211). When the teeth cav-ity walls are wet, application of acidic monomers is common for getting reliableadhesion, even when using a water-free adhesive primer (189). Monomer com-position is selected for a balance of hydrophilicity and hydrophobicity, physicalproperties of the cured polymers, adhesion to composite restorations, curing time,and so on. Polymerization catalysts used are similar to those for pit and fissuresealants.

Pretreatment of cavity walls with bonding agents is a must for compositerestorations. A typical procedure for restoration with composites, using a light-cured bonding agent and a light-cured composite is as follows: (1) cavity prepara-tion, (2) washing and air drying, (3) acid etching of the cavity wall, (4) applicationof the bonding agent and visible light irradiation, (5) composite restoration fillingand visible light irradiation, and (6) finishing and polishing. Researchers haveimproved this procedure by using acidic monomer in the bonding agent as theacid for etching. Combining the etching and bonding agents reduces the numberof steps and gives higher bonding strength to the dentin.

Dental Adhesives (For Alloys and Ceramics)

Various materials such as polymers, alloys, ceramics, and composites are used torestore or replace tooth structure. It is vitally important for these materials tobe bonded securely to the tooth surface. Lack of bonding between the materialsand tooth will induce secondary dental caries or restoration detachments. Recentdental adhesive techniques are based on chemo-mechanical bonding between theadhesive and adherend.

Surface conditioning before applying adhesives effectively increases the sur-face bonding area and enhances the strength of the bonding agent. The enamel iscommonly etched with an aqueous solution of phosphoric acid or other acids. Mostcurrent enamel bonding systems employ phosphoric acid in various concentrationsand viscosities as an etchant (169,170).


Casting alloys in dentistry are categorized into base metals and noble metals.Base metals include cobalt–chromium (Co–Cr), nickel–chromium (Ni–Cr), and ti-tanium (Ti) alloys, while noble metals include gold (Au), silver (Ag), and palladium(Pd) alloys. Mechanical methods, such as air-abrading with 50–200 µm alumina,are the most common way to prepare the alloy surface. Electrolytic etching createsmicro-mechanical retention on the alloy surface. Other types of surface modifica-tion include electroplating, silica coating, and ion-coating.

Dental ceramics consist mainly of metal oxides of silicon, aluminum, potas-sium, and sodium. Etching with an aqueous solution of hydrofluoric acid roughensthe ceramic surface.

The roughened, adherend surface is treated with a primer for chemicalbonding. Because the composition of dental adhesives is based on methacrylatemonomers, the primers for chemical bonding also contain functional monomers.The functional monomer usually consists of a polymerizable methacrylate or vinylgroup, a hydrophobic intermediate spacer, and a functional group capable of bond-ing to the adherend surface. Functional groups effective for bonding base metalalloys are the acid anhydride, carboxylic acid, and phosphoric acid groups. Thiolderivatives are used to prime noble metal alloys. The dental ceramic surface isprimed with silane couplers activated in acids, such as carboxylic acid monomers,ferric chloride, and/or a tri-n-butylborane derivative (TBB) (212,213). Silane cou-plers are also used for surface modification of filler particles included in dentalcomposites (214).

After surface preparation, the restorative material is bonded to the toothstructure with auto-polymerizing adhesive resin. One prerequisite for dental ad-hesives is an ability to cure in the mouth; that is why methacrylate-based polymermaterials are used in dentistry. Initiation systems are BPO–t amine or TBB (201).Monomers used are MMA or bifunctional methacrylate monomers. The neededoptical opacity (215,216) is achieved by various inorganic and metal compoundsincorporated into the formulations.

Acid anhydride or acidic monomers, such as 4-methacryloxyethyl trimel-litate anhydride (4-META), 4-methacryloxyethyl trimellate (4-MET), 4-(3-methacryloyloxypropoxy carbonyl) phthalic anhydride (MPRPA), and 4-(4-methacryloyloxybutoxycarbonyl) phthalic anhydride (MBPA), prepared bycondensing hydroxyalkyl methacrylates and trimellitic anhydride acid chloride,are very important coupling agents. The phosphoric acid derivative, MDP issynthesized from methacrylic acid, 1,10-decanediol, and phosphorus oxychloride.These very useful, functional (polymerizable) monomers are added to the primeror liquid part of the adhesive.

The MMA-based adhesive system, 4-META/MMA–TBB resin formulation,has three components: initiator, monomer liquid, and powder. The initiator is par-tially oxidized TBB (TBB-O). The monomer liquid contains 5% 4-META in MMA.The powder is finely pulverized PMMA, with a number-average molecular weightof about 100,000. The original MMA–TBB resin without 4-META is a transparent,unfilled resin. Bonding to enamel and base metal alloys is improved by addition of4-META monomers to the liquid part. A primer containing 0.5% 6-(4-vinylbenzyl-n-propyl)amino-1,3,5-triazine-2,4-dithol (VBATDT) in acetone is used to bond no-ble metal alloys. Mercaptan (SH) groups on the VBATDT monomer bond to noblemetals and copper on the alloy surface. The ceramic surface is treated with a


two-part liquid primer before applying 4-META resin. One liquid contains 4% γ -methacryloxypropyl trimethoxysilane (γ -MPTS), while the other liquid contains5% 4-META (or 4-MET) in MMA or 0.5% ferric chloride in ethanol. The γ -MPTSis activated by mixing with 4-META (or 4-MET) or ferric chloride to silanate thedental ceramic surface. The use of MMA polymer (PMMA)-coated titanium oxide(217) makes 4-META/MMA–TBB-O resin optically opaque.

Shear bond or tensile tests determine the dental adhesive bond strength,with thermocycling in water effectively evaluating the bond durability. Themonomer VBATDT is effective for bonding Ag–Pd alloy, while 4-META is suit-able for Ni–Cr alloy and titanium. For Ni–Cr alloys, the chromium content influ-ences bond durability. Ferric chloride combined with a silane coupler significantlyimproves the bond strength of 4-META/MMA–TBB-O resin joined to dental porce-lain.

Ceramic restoratives are often bonded directly to the tooth surface with ad-hesive resins, reducing the amount of healthy tooth reduction needed in clinicaltreatment. Resin-bonded fixed partial denture or resin-bonded ceramic restorationmethods are often employed. Adhesive resins are also used for bonding orthodonticbrackets, amalgam restoratives, and many other metal and ceramic materials.

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GENERAL REFERENCES

K. J. Anusavice, Phillips Science of Dental Materials, 10th ed., The Saunders Press,Philadelphia, Pa., 1996.R. van Noort, Dental Materials, 1st ed., C. V. Mosby Co., St. Louis, Mo., 1989.W. J. O’Brien, Dental Materials Properties and Selection, Quintessence Publishing Co.,Chicago, 1997.R. G. Craig, ed., Restorative Dental Materials, 10th ed., C. V. Mosby Co., St. Louis, Mo.,1997.

Vol. 2 DIACETYLENE AND TRIACETYLENE POLYMERS 203

C. M. Sturdevant, The Art and Science of Operative Dentistry, C. V. Mosby Co., St. Louis,Mo, 1997.A. D. Wilson and J. W. McLean, Glass-Ionomer Cement, Quint essence Publishing Co.,Chicago, 1988.A. D. Wilson and J. W. Nicholson, Acid–Base Cements: Their Biomedical and IndustrialApplications, Cambridge University, Cambridge, 1993.

BILL M. CULBERTSON

RONALD E. KERBY

The Ohio State University

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