Сraig. Dental Materials

solvent levels can be as high as 90%. A few fourthand fifth-generation bonding agents are solvent-free. Therefore primers have different evaporation rates, drying patterns, and penetration characteristics,all of which can influence the resulting bond strength. Advantages and disad-

vantages of primers with various solvents are listed in Table 10-6.

Adhesives Adhesives are generally hydrophobic, dimethacrylate oligomers (see Table 10-5) that are compatible with monomers used

in the primer and composite. These oligomers are usually diluted with lower-molecular weight monomers.

Initiators and Accelerators Most bonding agents are light cured and contain an activator such as camphoroquinone and an organic amine. Dual-cured bonding agents include a catalyst to promote self-curing.

Fillers Although most bonding agents are unfilled, some products contain inorganic fillers ranging from 0.5% to 40% by weight. Filler particles include micro fillers, also called nanojillers, and sub-micron glass. Filled bonding agents tend to produce higher in vitro bond strengths.

Other Ingredients Bonding agents may contain fluoride or antimicrobial ingredients. One bonding agent contains glutaraldehyde as a desensitizer. The effectiveness of fluoride release from a bonding agent has not been demonstrated.

PROPERTIES

Laboratory Properties

Bond Strength Most bonding agents produce bond strengths to human enamel and superficial dentin of 15 to 35 MPa. Bond strengths

Evaporates quickly after being dispensed; can evaporate from container; sensitive to wetness of dentin; multiple coats may be required; offensive odor

Extra drying time

Long drying time; water can interfere with adhesive if not removed

determined for deep dentin tend to be lower than superficial dentin. A variety of clinical problems can reduce bond strength. Some clinical problems and suggested solutions are listed in Table 10-7.

Fatigue Strength Over long periods (>I0 years), the bonded interface will undergo extensive fatigue cycling. Combinations of mechanical and thermal cycling stresses may produce as many as 1 million cycles of loading on the interfaces per year. Weak or compromised interfaces will debond and allow microleakage and fluid flow. The latter is detectable as pain. Only a limited number of low-cycle fatigue tests have been performed on dentin bonding systems in the laboratory. Fatigued systems failed at the bonded interface and displayed 50% reduction in bond strength even after only 1000 cycles of loading. To date there are no laboratory data that would strongly support long-term success of bonding agents.

Biological Properties Solvents and monomers in bonding agents are typically skin irritants. Certain material, such as 2-hydroxyeth- ylmethacrylate (HEMA), is not considered biocompatible as a monomer. Bonding agents may produce local and systemic reactions in dentists and dental assistants sufficient to preclude their

I

I

Self-curedcomposite or resin cement debonds from adhesive

Use dual-cured bonding agent with self-cured composite or resin cement

further use in the dental office. It is critical that dental personnel protect themselves from recurring exposure. Protective techniques include wearing gloves, immediately replacing contaminated gloves, using high-volume evacuation where the materials are being used, keeping all bottles tightly closed, and disposing of materials in such a way that the monomers cannot evaporate into the office air. Even with double gloves, contact with these aggressive solvents and monomers will produce actual skin contact in a few minutes. Follow all reasonable precautions, and if unwanted contact occurs, immediately flush affected areas with copious amounts of water and soap. Once the materials are polymerized, there is very little risk of side effects; although patients should be protected during bonding operations, properly polymerized materials have

not been shown to create any subsequent patient hazard.

Clinical Properties Enamel bonding has a long history of success when enamel is etched appropriately, microtags are formed, and components are cured. Clinical evaluations of the performance of bonding agents were initiated in the late 1970s but were primarily focused on the ability of enamel bonding agents to suppress microleakage. There were no standardized clinical procedures for evaluating bonding systems until the ADA developed "Guidelines for Dental Adhesion" in 1994. These guidelines require the testing of adhesives for bonding restorations to non-retentive Class 5 lesions. The lesions, which may be saucer-shaped or notch-shaped, are found on enamel margins along the coronal mar-

gin and dentin along the apical margin (Fig. 10-14, A). The success of a bonding agent is evaluated indirectly by examining the performance of the restorations for: (1) postoperative sensitivity, (2) interfacial staining, (3) secondary caries, and (4) retention or fracture from insertion to 18 months. These clinical trials test short-term retention and initial sealing.

Most commercial products for enamel and dentin bonding are successful in clinical trials. However, these clinical trials generally combine enamel and dentin bonding. There is no acceptable clinical regimen for critically testing dentin bonding only in retentive preparations. Because clinical trials are usually highly controlled, they are often not predictive of average clinical usage in general practice. Longevity of bonding in general practice may only be 40% that achieved in clinical trials, because of technique or other failures. Long-term clinical performance of bonding systems (>I0 years) for a wide range of materials has not yet been reported.

Sites of failure for most bonded restorations occur along cervical margins where the bonding is primarily to dentin. Examination of bonded composites in Class 2 restorations demonstrates that 95% of all secondary caries associated with composite occurs interproximally. These margins are the most difficult to fabricate during place-

ment of the restoration, are typically bonded to dentin and cementum rather than enamel, and are hard to access with visible light for adequate polymerization.

MANIPULATION

Enamel Bonding Bonding to enamel occurs primarily by micro-mechanical retention after acid etching is used to remove smear layers and preferentially dissolve hydroxyapatite crystals in the outer surface of the interface. Fluid adhesive constituents penetrate into the newly produced surface irregularities and become locked into place after polymerization of the adhesive.

Gel etchants are dispensed from a syringe onto tooth surfaces to be etched (see Fig. 10-9, A). Etching times for enamel vary depending on the type and quality of enamel. Generally, a 15-second etch with 37% phosphoric acid is sufficient to produce microtags. However, until macro-spaces are evident, the characteristic clinical endpoint of a frosty enamel appearance will not develop. Deciduous enamel generally contains some prismless enamel that has not yet worn away. It takes longer to etch through this outer layer and to reveal characteristic etch patterns on the underlying prism enamel. It is there-

276 Chapter 10 BONDING TO DENTAL SUBSTRATES

fore commonplace to recommend 120 seconds of acid etching for primary enamel.

Some enamel may have been rendered more insoluble as a result of fluorosis. In those cases, extended etching times are required to ensure that sufficient micro-mechanical bonding can occur. It is not uncommon to extend etching times for several minutes to accomplish an adequate etch. The only caution is that dentin should be protected from exposure to acid for this long a time.

After the intended etching time, the materials are rinsed away and the tooth structure is maintained in a moist surface condition for the next stage of bonding. Then, primer can be flowed onto the surface to penetrate into the available surface irregularities. Primer and adhesive that flow into larger irregularities, such as the prism peripheries shown in Fig. 10-9, D, produce resin tags once the adhesive is cured. These tags are actually "macrotags." Inspection of the details of the surfaces for a single prism reveals that smaller tags, "microtags," form where adhesive flows into the spaces between partially dissolved individual hydroxyapatite crystals (see Fig. 10-9, E and F).Microtags are much more numerous and contribute to most of the micro-mechanical retention.

Dentin Bonding Dentin bonding involves three distinct processes-etching (conditioning), priming, and bonding. In contrast to enamel, dentin contains more water (see Table 10-2) and is strongly hydrophilic. To manage this problem, primers have hydrophilic components that wet dentin and penetrate the surface. The goal is to produce microtags for micro-mechanical adhesion.

Modern bonding agents are applied to moist dentin surfaces. Any drying must be done cautiously. Once the hydroxyapatite component of the outer layer of dentin is removed, dentin contains about 50% unfilled space and about 20% remaining water. Even a short air blast from an air-water spray can inadvertently dehydrate the outer surface and cause the remaining collagen sponge to collapse onto itself. Once this

happens, the collagen mesh readily excludes the penetration of primer and bonding will fail. It is then necessary to rehydrate the dentin surface before priming. Hold a cotton pellet moistened with water against the surface for about 10 to 15 seconds or apply a rewetting agent (surfactant solutions in water). Do not bond to dentin unless the surface looks glistening. This is the critical clinical clue that the surface is sufficiently hydrated. An overly moist condition is equally bad. Excess moisture tends to dilute the primer and interfere with resin interpenetration. Generally, etch enamel and dentin simultaneously. Apply etchant to enamel first and then to dentin. Rinse excess material from the surfaces. Use gentle air-drying to check the enamel for a frosty appearance, but guard against over-drying etched dentin as you d o so. If necessary, rehydrate the dentin to a glistening appearance before applying primer.

The process of resin impregnation of etched dentin by the primer is called hybrid layer formation, as described in Fig. 10-15. The result of this process has been called the resininterpenetration zone or resin-interdiffusion zone. This layer is critical to bonding and creates the necessary microtags. The hybrid layer is revealed to be quite variable in thickness (see Fig. 10-15, D). Concurrent with hybrid layer formation is the penetration of primer into the fluid-filled and/or open dentinal tubules. This generates quite large macrotags. However, these appear to be of little value to overall bonding at the present time. This material is generally undercured and behaves as soft flexible tags. If dentin is dehydrated before priming and bonding, these macrotags are more likely to be quite extensive.

The thickness of a hybrid layer is not a critical requirement for success. Dentin bond strength is probably proportional only to the number of microtags, not the length of the microtags. Effective etching and priming of dentin does not require long times to produce acceptable dentin bond strengths. Inadequate etching or priming, however, is a cancern; therefore the tendency is to err routinely on the side of longer-than- needed etching times. If the etched zone is too

deep, it is more challenging to fill the zone with primer. A potential problem presents if decalcified dentin is not fully impregnated. The etched but not impregnated space may reside as a mechanically weak zone and promote nanoleakage. While this zone has been detected in laboratory

experiments, the clinical results of this process have never been demonstrated to be a problem.

Primers contain solvents to enhance their wetting and to co-solubilize the range of monomers involved. During application of the primer, most of the solvent evaporates quickly. Very little of

278 Chapter 10 BONDING TO DENTAL SUBSTRATES

the original primer that is applied actually remains as polymerizable material to produce dentin impregnation. Thus several layers usually must be applied. The rule of thumb is that one should apply as many layers as are necessary to produce a persisting glistening appearance on dentin.

Once the primer is applied adequately, an adhesive is applied and light cured. Surfaces of the cured bonding agents are initially air inhibited and do not immediately react. However, as composite is placed against the surface, the air is displaced and copolymerization occurs.

BONDING SYSTEMS FOR OTHEf4 'i,,;

SUBSTRATES

AMALGAM

For bonding to dental amalgam, the objective is to cause the unset bonding agent and unset amalgam to intermingle before they set. Because dental amalgam is opaque, the bonding system must be chemically cured.

A bonding agent is applied to the cavity preparation. Dental amalgam is condensed against the unset bonding system. The thickness of the bonding layer must be increased by using multiple layers of bonding agent or by adding thickening agents to the bonding agent. One bonding agent uses an admixture of small, polymethyl methacrylate powder particles in the bonding agent to thicken it. Generally, the thickness of the bonding layer is increased to 20 to 50 ym. Fig. 10-16,A, portrays this process schematically. Fig. 10-16, B, provides a scanning electron photomicrograph of the resulting interface. In the absence of suitable commingling, the bond strength might be only 4 to 8 MPa. However, with micro-mechanical interlocking at the interface, the bond strength may be 20 MPa. In most restorations, amalgam bonding agents simply seal the cavity against fluid flow and microleakage. Amalgam bonding agents provide little or no retention for set amalgams, even if they are roughened, because the bonding systems do not wet or adapt to the set amalgam surfaces very well.

LABORATORY COMPOSITES

Restorations made with laboratory composites (see Chapter 9), include inlays, onlays, veneers, or milled restorations, and are fabricated indirectly in a dental laboratory and then bonded into existing cavity preparations. Bonding requires agents for both the tooth structure and the undersurfaces of the indirect restoration. Resin composite cements are usually used to fill the space between the two surfaces. Bonding to the tooth structure and composite cement is straightforward and reliable.

Bonding to the indirect composite surfaces is difficult. The goal is to swell the outer surfaces of the resin matrix and allow new monomers from the bonding agent to penetrate spaces among existing polymer chains. At the time of curing, the new polymer chains become micromechanically intertwined with the existing polymer chains, producing relatively strong bonding. Bonding can be enhanced by blasting (microetching) with particles of aluminum oxide, etching with hydrofluoric acid gel, or treating with primers. Sandblasting roughens the surface. Etching removes smear layers and partially dissolves glass filler particles. Primers provide good wetting and potential chemical bonding to exposed glass filler particle surfaces. Commercial primers for laboratory composites contain silane, unfilled resin monomers, or silane-monomer combinations.

Bonding composite cement to laboratory composites generally produces bond strengths in the range of 20 to 35 MPa.

CERAMIC

Ceramic restorations bonded to enamel and dentin include a number of bonding interfaces. Enamel and dentin are treated with whatever bonding agent is desirable. The under-surface of the all-ceramic inlay, onlay, crown, bridge, or veneer is treated with 5% to 9% hydrofluoric acid gel. The pH of this gel is low and capable of removing any smear layers while etching the phase boundaries and the more soluble silicate glass phases. Although this process does not

work on aluminaor zirconia-based ceramics, it is effective for other ceramics. Another popular option is to blast (micro-etch) the surface with 50-pm aluminum oxide particles.

Once the ceramic surface is prepared, an aqueous and acidified solution of silane is applied to enhance wetting and potentially function as a chemical coupling agent. Silane is a difunctional molecule capable of reacting to the hydroxyls on the silicate phases along the treated surface's restoration. The other end of the silane is capable of copolymerizing with resin composite cement, which is used to attach the two treated surfaces and fill in the typically 100 pm of intervening space after seating (Fig. 10-17).

Bonding systems used with resin cements for all-ceramic systems can be light cured but are

often dual cured or self cured. Bond strengths for a bonded ceramic restoration to tooth structure are generally in the range of 20 to 40 MPa.

COMPOSITE BONDED TO CAST ALLOYS

It is sometimes desirable to bond a resin composite to a cast alloy substrate rather than ceramic. Traditionally, resin bonding to metal substrates has relied on macro-mechanical retention (latticework, mesh, beads, or posts along the metal surface). Another approach to resin-metal adhesion was introduced in 1984 and trademarked "Silicoating." A silicon oxide coating is generated to create a surface that is capable of chemical bonding. This technique has been applied to gold, cobalt-chromium, silver-palladium,

and titanium alloys. For Silicoating, a metal surface is roughened by sandblasting with 250ym A1,0,, cleaned, silica coated, chemically silanated, primed, and the composite is bonded.

Other techniques for treating alloy surfaces to be bonded with composite include thermally applied silica and ceramic blasting (Table 10-8). Recently, liquid primers composed of thiophosphate monomers have become available. These techniques are equally effective in providing a relatively strong bond (18 to 30 MPa) of composites to alloy substrates.

REPAIR OF COMPOSITE, CERAMIC, AND PORCELAIN-FUSED-TO-METAL RESTORATIONS

It is quite common to encounter restorations that require repair because of long-term wear, unwanted color change, the appearance of small surface defects or chipping. These situations can be easily managed by veneering the surface with fresh composite. The damaged surface is removed to a depth of 1to 1.5mm below the final intended surface employing small burs, and the

excavation is extended to neighboring enamel margins. The margins are roughened and etched to remove any smear layer and provide micromechanical retention. Priming produces tags into the enamel and diffusion into the surface of the old restoration. It is critical that the old composite is not over-dried because water in the old restoration tends to hold open the old polymer network and allow some hybrid layer formation. The bonding agent is cured. Restorative material is added, cured, and polished. Excellent repair bond strengths (20 to 35 MPa) are possible.

I SELECTED PROBLEMS

Problem 1

The enamel portion of a tooth was etched for 15 seconds and rinsed. Upon observation, it did not appear frosty. What might explain the inadequate etch and what is a possible method to obtain a better etch?

Solution

The tooth may contain a higher than normal level of fluoride and thus may be resistant to normal etching. Apply the phosphoric acid for a longer period (up to 120 seconds) to obtain a frosty enamel appearance.

Problem 2

During removal of an impression of a first molar with a composite core build-up, the core material debonded. What might explain the failed bond and what is a possible solution to obtain an adequate bond of the composite core material to the tooth?

Solution a

Self-cured composite core materials are popular because they are esthetic and easy to use. However, some self-cured composite cores are incompatible with certain lightcured bonding agents. Be sure to pick compatible composite core materials and bonding agents for this application.

Solution b

Choose a dual-cured bonding agent or use a light-cured composite core material.

Problem 3

During fabrication of an all-ceramic crown, a dentist used a provisional composite cemented with eugenol-based temporary cement. Upon bonding the all-ceramic crown, there was a problem with setting of the bond-

ing agent and resin cement. Explain the problem and offer a solution.

Solution a

The polymerization of most bonding agents and resin cements is severely hindered in the presence of eugenol. Take care to remove all remnants of the eugenol cement, then rinse and re-etch the tooth.

Solution b

In the future, use a resin-based provisional cement.

Problem 4

A dentist learned at a study club that her colleagues were etching tooth structure with phosphoric acid before applying a sixthgeneration bonding agent. Is it desirable to etch with phosphoric acid before applying a self-etching primer-adhesive?

Solution

Current sixth-generation bonding agents bond effectively to enamel and dentin without prior etching with phosphoric acid. The additional etching achieved with phosphoric acid could result in over-etching of dentin and subsequent nanoleakage.

Problem 5

After etching, a dentist inadvertently overdried the tooth. Explain the problem and offer a solution.

Solution

Most modern bonding agents bond best to a moist tooth. If dentin is over-dried, it is best to rehydrate it by applying a moist cotton pellet for 15 seconds before applying the primer of the bonding agent.