Сraig. Dental Materials

Most commercial liquids are supplied as a 32% to 42% solution of polyacrylic acid, having a molecular weight of 25,000 to 50,000.The manufacturer controls the viscosity of the cement liquid by varying the molecular weight of the polymer or by adjusting the pH by adding sodium hydroxide. Itaconic and tartaric acids may be present to stabilize the liquid, which can gel on extended storage.

The cement powder is essentially zinc oxide and magnesium oxide that have been sintered and ground to reduce the reactivity of the zinc oxide. The cement powder that is mixed with water contains 15% to 18% polyacrylic acid coated on the oxide particles.

SETTING REACTION

The set cement is a zinc polyacrylate ionic gel matrix that unites unreacted zinc oxide particles. The gel is bound to the polyanion chains by electrostatic interactions rather than by stronger specific ion binding. The matrix appears to be amorphous. The setting reaction can be retarded by a cool environment or accelerated by a warm environment.

MANIPULATION

The cements supplied with the polyacrylic acid in the liquid are usually mixed at a powder/liquid ratio of 1:lto 2:l. One cement mixed with water has a powder/liquid ratio of 5:l for cementation consistency. The consistency of the mixes is creamy compared with that of zinc phosphate cements. The mixed cement ispseudoplastic; that is, the viscosity decreases as the shear rate increases, or, in other terms, the flow increases as spatulation increases or as force is placed on the material. The correct consistency is found in a mix that is viscous but that will flow back under its own weight when drawn up with a spatula.

Dispense the liquid immediately before mixing to prevent evaporation of water and subsequent thickening. A nonabsorptive surface, such as a glass slab or treated paper, will keep all the

liquid available for the reaction and facilitate spatulation. Mix polyacrylate cements within 30 to 60 seconds, with half to all of the powder incorporated at once to provide the maximum length of working time (typically 2.5 to 6 minutes). Extend the working time to 10 to 15 minutes by mixing on a glass slab chilled to 4" C. The strength of the mixed cement is not compromised by this technique. Some manufacturers supply the cement as a capsulated powder-liquid system for mixing in a mechanical mixer.

Polyacrylate cements have been used to cement metal inlays and crowns and to make bases. Apply the cement to clean cavity walls that are well isolated in a dry field. Use the mixed cement only as long as it appears glossy on the surface. Once the surface becomes dull, the cement develops stringiness and the film thickness becomes too great to seat a casting completely.

PROPERTIES

ANSI/ADA Specification No. 96 This specification establishes maximum values of setting time, film thickness, acid erosion, and arsenic and lead content, and a minimum value of compressive strength for zinc polyacrylate cement. A summary of these requirements is given in Table 20-2.

Viscosity The effect of temperature on the initial viscosity of zinc polyacrylate cement and the viscosity 2 minutes after mixing is shown in Fig. 20-3. The initial viscosity was essentially unaffected by the temperature increase from 18' to 25" C, although the viscosity was higher than the initial viscosity for comparable mixes of zinc phosphate cement. The viscosity of zinc polyacrylate cement 2 minutes after mixing increased modestly at all three temperatures; however, the increases in viscosity were substantially less than those for comparable zinc phosphate cement mixes. Thus the initial viscosity of zinc polyacrylate cements is higher than zinc phosphate cements, and a delay of 2 minutes in cementation reverses the situation.

Setting Time The setting-time test measures the time at which the cement is sufficiently hard to resist indentation by a standard indenter. The net setting time should occur within 2.5 to 8 minutes so final finishing procedures associated with the restoration can occur. As shown in Table 20-4, setting of the zinc polyacrylate cements usually occurs within 7 to 9 minutes from the start of mixing.

Film Thickness The film thickness test excludes materials that might have excessively large powder particles or a short working time, because complete seating of a casting with zinc polyacrylate cement might not occur. The film thickness of the polyacrylate cements is slightly higher than that of zinc phosphate cements but is well within clinical limits, as shown in Table 20-4.

Strength The compressive strength test excludes materials that have a compressive strength of less than 7 0 MPa. Clinical studies have shown that cements of this strength or greater will satisfactorily retain castings with a good fit.

The 24-hour compressive strength of polyacrylate cements for luting is lower than that of zinc phosphate cements, 98 to 133 MPa compared with 57 to 99 MPa; however, the tensile strength of polyacrylate cements is about 40% higher than that of zinc phosphate cements. The higher tensile strength may be influenced by the test method. In the diametral compression test, zinc polyacrylate cement specimens deform somewhat before breaking. The deformation results in a higher load before fracture is recorded than would occur if brittle fracture occurred. The modulus of elasticity of the zinc polyacrylate cements is about one third that of the zinc phosphate cements mixed to a luting consistency.

Bond Strength An interesting feature of polyacrylate cement is its bonding to enamel and dentin, which is attributed to the ability of the carboxylate groups in the polymer molecule to chelate to calcium. The bond strength to enamel

has been reported to be from 3.4 to 13 MPa, and the bond strength to dentin has been found to be 2.1 MPa. Optimum bonding, however, requires cleaned tooth surfaces. The bonding of the polyacrylate cements to gold casting alloy is likewise highly dependent on surface preparation. Sandblasting or electrolytic etching of the gold alloy surface is necessary to achieve optimum bonding. Clinical studies have not demonstrated improved retention of crowns and bridges as a result of cementation with polyacrylate cements, however.

Because of the adhesion of polyacrylate cements to enamel, the cements were used at one time for direct bonding of orthodontic brackets to teeth. Presently, direct bonding is accomplished with composite cements.

Solubility and Disintegration The solubility and disintegration test excludes materials that are excessively soluble in distilled water. Solubility in distilled water does not always correlate with solubility in vivo. Solubility in water at 1 day varies from 0.12% to 0.25% for typical zinc polyacrylate cements. One cement tested increased in solubility from 0.25% at 1 day to 0.60% at 1 month in water. Other cements were not affected by longer-term storage. ANSI/ ADA Specification No. 96 specifies the maximum rate of acid erosion of zinc polyacrylate cements as 2.0 mm/hr.

Dimensional Stability The zinc polyacrylate cements show a linear contraction when setting at 37" C. The amount of contraction varies from 1% for a wet specimen at 1 day to 6% for a dry specimen at 14 days. These contractions are more pronounced than those observed for zinc phosphate cements and start earlier.

Acidity Zinc polyacrylate cements are slightly more acidic than zinc phosphate cements when first mixed, but the acid is only weakly dissociated, and penetration of the highmolecular weight polymer molecules toward pulpal tissue is minimal. Histological reactions to

polyacrylate cements appear similar to those of ZOE cements, although the production of reparative dentin under the polyacrylate cements is more evident.

APPLICATIONS

Zinc polyacrylate cements are used primarily for luting permanent alloy restorations and as bases. These cements have also been used in orthodontics for cementation of bands.

COMPOSITION

Glass ionomer cements are supplied as a powder and a liquid or as a powder that is mixed with water. Several products are encapsulated. The liquid typically is a 47.5% solution of 2:l polyacrylic acidhtaconic acid copolymer (average molecular weight 10,000)in water. The itaconic acid reduces the viscosity of the liquid and inhibits gelation caused by intermolecular hydrogen bonding; D(+) tartaric acid (5%, the optically active isomer) in the liquid serves as an accelerator by facilitating the extraction of ions from the glass powder.

The powder of glass ionomer cement is a calcium fluoroaluminosilicate glass with a formula of

The nominal composition of the glass is listed in Table 20-11. The maximum grain size of the powder appears to be between 13 and 19 pm. The powder is described as an ion-leachable glass that is susceptible to acid attack when the Si/A1 atomic ratio is less than 2:1.Barium glass or zinc oxide may be added to some powders to provide radiopacity.

In some products the polyacrylic acid is coated on the powder. The liquids of these products may be water or a dilute solution of tartaric acid in water.

Adapted from Prosser HJ, Richards CP, Wilson AD:

J Biomed Mater Res 16:431,1982.

SETTING REACTION

The setting reaction is an acid-base reaction between the acidic polyelectrolyte and the aluminosilicate glass, as diagrammed below.

The polyacid attacks the glass to release cations and fluoride ions. These ions, probably metal fluoride complexes, react with the polyanions to form a salt gel matrix. The ~1~~ ions appear to be site bound, resulting in a matrix resistant to flow, unlike the zinc polyacrylate matrix. During the initial setting reaction in the first 3 hours, calcium ions react with the polycarboxylate chains. Subsequently, the trivalent aluminum ions react for at least 48 hours. Between 20% and 30% of the glass is decomposed by the proton attack. The fluoride and phosphate ions form insoluble salts and complexes. The sodium ions form a silica gel. The structure of the fully set

cement is a composite of glass particles surrounded by silica gel in a matrix of polyanions cross-linked by ionic bridges. Within the matrix are small particles of silica gel containing fluorite crystallites.

Glass ionomer cements bond chemically to enamel and dentin during the setting process. The mechanism of bonding appears to involve an ionic interaction with calcium and/or phosphate ions from the surface of the enamel or dentin. Bonding is more effective with a cleaned surface provided the cleansing process does not remove an excessive amount of calcium ions. Treating dentin with an acidic conditioner followed by a dilute solution of ferric chloride improves the bonding. The cleansing agent removes the smear layer of dentin while the ~ e ions are deposited and increase the ionic interaction between the cement and dentin.

MANIPULATION

Glass ionomer cements mixed with the more viscous carboxylic acid liquids have a powder/ liquid ratio of 1.3:l to 1.35:1, whereas those mixed with water or a liquid with a consistency like that of water have a powder/liquid ratio of 3.3:l to 3.4:l. The powder and liquid are dispensed onto a paper or glass slab. The powder is divided into two equal portions. The first portion is incorporated into the liquid with a stiff spatula before the second portion is added. The mixing time is 30 to 60 seconds. Encapsulated products are typically mixed for 10 seconds in a mechanical mixer and dispensed directly onto the tooth and restoration. The cement must be used immediately because the working time after mixing is about 2 minutes at room temperature (23" Cj. An extension of the working time to 9 minutes can be achieved by mixing on a cold slab (3" C), but because a reduction in compressive strength and in the modulus of elasticity is observed, this technique is not recommended. Do not use the cement once a "skin" forms on the surface or when the viscosity increases noticeably.

Glass ionomer cements are very sensitive to contact with water during setting. The field must

be isolated completely. Once the cement has achieved its initial set (about 7 minutes), coat the cement margins with the coating agent supplied with the cement.

PROPERTIES

ANSI/ADA Specification No. 96 (IS0

9917) Requirements of this specification for glass ionomer cements used as cements, bases, and restorative materials are given in Table 20-2.

Film Thickness The film thickness of glass ionomer cements is similar to or less than that of zinc phosphate cement (see Table 20-4)

~and+ is suitable for cementation.

Setting Time Glass ionomer cements set within 6 to 8 minutes from the start of mixing. The setting can be slowed when the cement is mixed on a cold slab, but this technique has an adverse effect on the strength.

Strength The 24-hour compressive strength of glass ionomer cements ranges from 90 to 230 MPa and is greater than that of zinc phosphate cement. Values of tensile strength are similar to those of zinc phosphate cement. Unlike zinc polyacrylate cements, glass ionomer cements demonstrate brittle failure in the diametral compression test. The elastic modulus of glass ionomer cements is less than that of zinc phosphate cement, but more than that of zinc polyacrylate cement. The rigidity of glass ionomer cement is improved by the glass particles and the ionic nature of the bonding between polymer chains.

The compressive strength of glass ionomer cements increases between 24 hours and 1 year, unlike that of zinc polyacrylate cement. A glass ionomer cement formulated as a filling material showed an increase from 160 to 280 MPa over this period. The strength of glass ionomer cements improves more rapidly when the cement is isolated from moisture during its early life.

Bond Strength Glass ionomer cements bond to dentin with values of tensile bond strength reported between 1 and 3 MPa. The bond strength of glass ionomer cements to dentin is somewhat lower than that of zinc polyacrylate cements, perhaps because of the sensitivity of glass ionomer cements to moisture during setting. The bond strength has been improved by treating the dentin with an acidic conditioner followed by an application of a dilute aqueous solution of FeC1,. Glass ionomer cements bond well to enamel, stainless steel, tin oxide-plated platinum, and gold alloy.

Solubility Values of solubility of glass ionomer cements as measured in water are substantially higher than those measured for other cements (see Table 20-4). However, when these cements are tested in acid (0.001 N lactic acid), the values are quite low compared with values for zinc phosphate and zinc polyacrylate cements. The rankings determined by solubility tests in acid correlate well with clinical evaluations.

ANSVADA Specification No. 96 specifies the maximum acid erosion rate as 0.0j mm/hr. This specification also sets limits on the acid-soluble arsenic content and lead content (see Table 20-2).

Biological Properties Biological evaluations of glass ionomer cements have been done by tissue culture and animal tests. The culture cells showed a weaker reaction to glass ionomer cement than to ZOE or zinc polyacrylate cements. Pulp tissue reactions of monkeys tested in vivo showed no difference between glass ionomer and ZOE cements. These reactions have been described as mild.

Glass ionomer luting cements may cause prolonged hypersensitivity, varying from mild to severe, Microleakage has been suggested as an explanation, but a recent study showed no increase in bacterial counts 56 days after cementation of crowns with a glass ionomer cement. These cements may be bacteriostatic or bactericidal, however, because of fluoride release.

Good isolation appears essential when glass ionomer cements are used. The use of the proper powder/liquid ratio and the application of a calcium hydroxide base in areas closest to the pulp are recommended.

APPLICATIONS

Glass ionomer cements are used primarily for permanent cement, as a base, and as a Class 5 filling material (see Chapter 8). The cement has been evaluated as a pit and fissure sealant and as an endodontic sealer. The sensitivity of the cement to moisture and desiccation may minimize its use in these latter applications. Glass ionomer cements are being used clinically for cementation of orthodontic bands because of their ability to minimize decalcification of enamel by means of fluoride release during orthodontic treatment.

Self-cured and light-cured hybrid ionomers (or resin-modified glass ionomers) are available for cementation. Hybrid ionomer restorative materials are described in Chapter 8.

COMPOSITION

One self-cured hybrid ionomer cement powder contains a radiopaque, fluoroaluminosilicate glass and a micro-encapsulated potassium persulfate and ascorbic acid catalyst system. The liquid is an aqueous solution of polycarboxylic acid modified with pendant methacrylate groups. It also contains 2-hydroxyethylmethacrylate (HEMA) and tartaric acid. Another self-cured cement contains a mixture of fluoroaluminosilicate and borosilicate glasses in the powder. Its liquid is a complex monomer containing carboxylic acid groups that can undergo an acid-base reaction with glass and vinyl groups that can polymerize when chemically activated. A light-cured hybrid ionomer cement contains fluoroaluminosilicate glass in the powder and a copolymer of

acrylic and maleic acids, HEMA, water, camphorquinone, and an activator in the liquid.

SETTING REACTION

Setting of hybrid ionomer cements generally results from an acid-base glass ionomer reaction and self-cured or light-cured polymerization of the pendant methacrylate groups. Some cements, however, are light-cured only.

MANIPULATION

The powder is fluffed before dispensing. The liquid is dispensed by keeping the vial vertical to the mixing pad. For one product, the powder/ liquid ratio is 1.6 g of powder to 1.0 g of liquid, and the powder is incorporated into the liquid within 30 seconds to give a mousse-like consistency. The working time is 2.5 minutes. The cement is applied to a clean, dry tooth that is not desiccated. Some products recommend the use of a conditioner for enhanced bonding to dentin. No coating agent is needed. HEMA is

a known contact allergen; therefore use of protective gloves and a no-touch technique are recommended.

PROPERTIES

Requirements for light-activated cements, which are water-based and set by multiple reactions, including an acid-base reaction and polymerization (Type I), and by cements that set only after light-activation (Type 111, are described by ANSI/ ADA Specification No. 96 (IS0 9917, Part 2). Properties for liners and bases and restoratives are given in Table 20-12.

The compressive and tensile strengths of hybrid ionomer cements are similar to those of glass ionomer cements (see Table 20-3). The fracture toughness is higher than that of other waterbased cements but lower than composite cements. The bond strength to moist dentin ranges from 10 to 14 MPa and is much higher than that of most water-based cements. Hybrid ionomer cements have very low solubility when tested by lactic acid erosion. Water sorption is higher than

for resin cements. Delayed fracture of ceramic restorations cemented with hybrid ionomer cements has been reported. Recently, some hybrid ionomer cements have been modified to have less water sorption. Fluoride release and rechargeability are similar to glass ionomer cements. The early pH is about 3.5 and gradually rises. Clinical experience indicates minimal postoperative sensitivity.

APPLICATIONS

Self-cured hybrid ionomer cements are indicated for permanent cementation of porcelain-fused- to-metal crowns; bridges; metal inlays, onlays, and crowns; post cementation; and luting of orthodontic appliances. Additional uses include adhesive liners for amalgam, bases, provisional restorations, and cementation of specific ceramic restorations. Light-cured hybrid ionomer cements are used primarily for liners and bases. Restorative applications of light-cured hybrid ionomer cements are discussed in Chapter 8. One light-cured product is recommended for direct bonding of orthodontic brackets and bands.

monomer, multifunctional acrylate/phosphate monomer, diacrylate monomer, and water.

SETTING REACTION

Setting is the result of selfand light-cured polymerization. Once the cement comes into contact with oral fluids, an acid-base reaction may occur. The carboxylic acid groups contribute to the adhesive capability of the cement.

MANIPULATION

Dry the tooth to be cemented but do not desiccate. The powder/liquid ratio is 2 scoops to 2 drops. Tumble the powder before dispensing. Mix the powder and liquid rapidly for 30 seconds. Place the mixed cement in the crown only and then seat the crown. A gel state is reached after 1 minute, at which time the excess cement is removed with floss and a scaler. Light-cure the exposed margins to stabilize the restoration. Setting occurs 3 minutes after start of mix. Once set, compomer cement is very hard.

PROPERTIES

COMPOMERS

Compomer cement is the newest resin-based cement indicated for cementation of cast alloy crowns and bridges, porcelain-fused-to-metal crowns and bridges, and gold cast inlays and onlays. Cementation of all-ceramic crowns, inlays, onlays, and veneers, with some exceptions, is contraindicated. The cement should not be used as a core or filling material. Compomers are also known as poly aci&modijied composites.

A compomer cement was recently introduced for orthodontic bonding.

COMPOSITION

The cement powder contains strontium aluminum fluorosilicate glass, sodium fluoride, and selfand light-cured initiators. The liquid contains polymerizable methacrylate/carboxylic acid

Compomer cement has high values of retention, bond strength, compressive strength, flexural strength, and fracture toughness (see Table 20-3). The cement has low solubility and sustained fluoride release.

COMPOSITES AND

Cements based on resin composites have been used for cementation of crowns, conventional bridges, and resin-bonded bridges; for bonding of esthetic ceramic and laboratory-processed composite restorations to teeth; and for direct bonding of orthodontic brackets to acid-etched enamel. Recently, composite cements have been developed for cementation of provisional restorations.

IS0 4049 for polymer-based filling, restorative, and luting materials (ANSI/ADA No.

27) describes the following three classes of composite cements:

Property requirements based on IS0 4049 can be summarized as follows:

Class 1, 2, 3: film thickness, max.-50 pm

CEMENTATION OF ALLOY CROWNS AND BRIDGES, RESIN-BONDEDBRIDGES, AND PROVISIONAL RESTORATIONS

Synthetic resin cements based on methyl methacrylate have been available since 1952 for use in cementation of inlays, crowns, and appliances. In the early 1970s, a resin composite was introduced as a crown and bridge cement.

Composition and Setting Reaction

Self-cured, composite cements are typically twopaste systems. One major component is a diacrylate oligomer diluted with lower-molecular weight dimethacrylate monomers. The other major component is silanated silica or glass. The initiator-accelerator system is peroxide-amine.

One adhesive resin cement is a self-cured, powder-liquid system formulated with methacryloxyethylphenyl phosphate or 4-methacry- loxyethyl-trimellitic anhydride (4-META). The 4-META cement is formulated with methyl methacrylate monomer and acrylic resin filler and is catalyzed by tri-butyl-borane. Another adhesive resin cement is a phosphonate cement supplied as a two-paste system, containing Bis-GMA resin and silanated quartz filler.The phosphonate molecule is very sensitive to oxygen, so a gel is

provided to coat the margins of a restoration until setting has occurred. The phosphate end of the phosphonate reacts with calcium of the tooth or with a metal oxide. The double-bonded ends of both 4-META and phosphonate cements react with other double bonds when available. Setting of resin cements results from selfor light-cured polymerization of carbon-carbon double bonds.

Properties Some properties of composite and adhesive resin cements are listed in Tables 20-3 and 20-4. A comparison of bond strengths of adhesive and conventional resin-bonded bridge cements is given in Table 20-13. Adhesive resin cements have superior bonding to sandblasted Ni-Cr-Be and Type IV gold alloys. Composite cements used for cementation of provisional restorations (25 to 70 MPa) have a substantially

Uchiyama Y: Adhesive prosthodontics-adhesive cements and techniques, Nijmegen, 1986, Academy of Dental Materials; and Watanabe F, Powers JM, Lorey RE:J Dent Res

67:479, 1988.

lower compressive strength than composite cements used for permanent cementation (180 to 265 MPa).

Applications Adhesive resin cements and composite cements in conjunction with bonding agents are being used as cements for posts and cores. Bond strengths of 14 MPa have been reported for silica-treated posts cemented with 4-META resin cement in extracted teeth. The use of resin-bonded bridges declined dramatically in the late 1980s.

BONDING OF ESTHETIC RESTORATIONS

The bonding of all-ceramic, tooth-colored crowns, veneers, inlays, and onlays became popular in the late 1980s.Dual-cured composite resin cements are ideal for bonding cast or CAD/CAMprepared ceramic restorations or laboratoryprocessed composite inlays. Light-cured composite cements are useful for bonding thin ceramic veneers where achieving adequate depth of cure is not a problem.

Composition Composite cements are microfilled or hybrid composites formulated primarily from Bis-GMA or urethane dimethacrylate resins and fumed silica or glass fillers (20% to 75% by weight) or both. Dual-cured cements come in a base-catalyst form and must be mixed before use. Light-cured composites are photoinitiated in the presence of a camphoroquinoneamine system. They provide a wide selection of shades, tints, and opaquers.

Manipulation A bonding agent is necessary to bond the resin cement to tooth structure, whereas various surface preparations (sandblasting) and treatments (silanation or chemical softening) are used to prepare the ceramic or laboratory-fabricated composite restorations for bonding. Based on in vitro studies, composite cements bond well to post-cured composite inlays.

Properties Compressive strengths of dualand light-cured resin composite cements have

been reported from 180 to 265 MPa (see Table 20-3). Viscosity has been measured subjectively to range from low to high. Film thicknesses on vented crowns range from 13 to 20 pm. The cements are radiopaque for use in the posterior portion of the mouth.

RESIN-METAL BONDING

Bonding composite to the metal framework of a bridge and denture acrylic to a partial denture framework can be improved by the use of silica coatings. Presently there are three processes for applying silica to either noble or base-metal alloys. One method applies pyrogenic silica using a propane flame. Other methods use heat in an oven or ceramic blasting to coat the restoration or appliance. Bond strengths of composites to silica-coated Au-Pd or Ni-Cr-Be alloys range from 16 to 22 MPa. Silica coating of noble alloys eliminates the need for tin plating these alloys to improve adhesion of composites. The bond strength of denture acrylics to Ni-Cr-Be alloys range from 7 to 23 MPa when the alloy is treated with a silica coating or primed with adhesive resin cement. Liquid primers based on thiosulfates have recently become available for treatment of alloys. Recently, metal primers based on thiophosphate chemistry have been introduced as a treatment for resin-metal bonding.

BONDING OF ORTHODONTIC BRACKETS

Resin cements were evaluated for direct bonding of orthodontic brackets (without bands) in the late 1960s. Advances in acid etching of enamel substantially increased the popularity of the technique in the mid-1970s. Now resin composite cements have completely replaced acrylic resin cements for orthodontic bonding. Composite cements are used with metal, plastic, or ceramic orthodontic brackets.

Composition and Setting Composite cements are formulated from various diacrylate oligomers diluted with lower-molecular weight dimethacrylate monomers and fillers of silica, glass, or colloidal silica. The highly filled cements

typically contain silanated inorganic particles (more than 60% by weight). The slightly filled cements contain 28% colloidal silica. The initiator-accelerator systems of these composite cements depend on the mode of initiating the polymerization. Amine-cured systems include conventional two-paste products and one-step products. Light-cured systems are polymerized by visible light.

Manipulation The success of composite cements used for direct and indirect bonding is highly dependent on proper isolation and acid etching of the enamel. The acid-etching technique involves etching the tooth for 15 to 60 seconds with a solution of phosphoric acid, followed by rinsing and drying. If the enamel is contaminated, re-etching of the tooth is necessary. Acid etching of enamel is discussed in greater detail in Chapter 10. A commercial selfetching primer is available as an alternative to phosphoric acid etching for orthodontic bonding to enamel.

Two-paste composite cements require mixing for 20 to 30 seconds before they are applied to the enamel and bracket base. A primer such as methyl methacrylate monomer in a solvent usually must be applied to a plastic bracket base. Sometimes a sealant formulated from an unfilled diacrylate is applied initially to the acid-etched enamel. The two-paste cements set several minutes after mixing.

The one-step (no-mix) cements require no mixing. A priming liquid is applied to the etched enamel, and the paste is applied to the bracket base. A plastic bracket may require a bracket primer. Polymerization is initiated when the bracket is placed on the primed tooth. The effect of film thickness on the polymerization of these cements has been investigated. Generally, there is a decrease in tensile bond strength as the thickness of one-step cements increases. Failures are characterized by incomplete polymerization of the resin. Bond strength of one-step cements decreases if the primer is exposed to a simulated oral environment for a minute or more, so bases should be placed promptly after the primer is applied to the teeth.

Light-cured composite cements are singlepaste systems that require no mixing. The resin is applied to the tooth and bracket base, and polymerization is activated by the light source. A sealant may be used for bonding to the teeth, and a primer may be required for bonding to a plastic bracket. Recently, high-intensity, visible lights and argon lasers have been used in orthodontics to save time.

Properties Two important properties of composite cements for orthodontic bonding are esthetics and bond strength to tooth structure and the bracket base.

Changes in color of composite cements can result from staining or from the formation of colored reaction products. After accelerated aging or exposure to a tea stain, the cements were darker and more chromatic. Exposure to the tea stain caused a greater change in color than the aging test.

The bond strength of composite cements to tooth structure appears clinically adequate if proper isolation and manipulative techniques are followed. Bonding to tooth structure results from the resin matrix penetrating into the etched areas of enamel.

Bonding to orthodontic bracket bases depends on the type of bracket base (metal, plastic, or ceramic) and the type of cement (hybrid ionomer, highly filled composite, or slightly filled composite), as shown in Table 20-14. Failures typically occur at the cement-base interface or, less often, within the cement or base. Bonding to plastic bases appears to be chemical, whereas bonding to metal and ceramic bases is mechanical. Failures at the cement-metal base interface are initiated at areas of stress concentration in the metal base, such as weld spots or damaged mesh (Fig. 20-6). Plastic brackets tend to fail at the wings rather than debonding in laboratory testing. Failure at the interface of the cement-ceramic bracket is influenced by the amount of penetration of resin into the retention areas of the base.

Improved metal bases, including photographically etched and grooved types, have been tested with surface treatments such as silanation, etching, and activation. Etching of the grooved