Bonding and bonding agents

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Direct Restorative Materials Dr. Md. Arifur Rahman BDS (DU), MPH Assistant Professor & Head Northeast Medical College Dental Unit [email protected]

Transcript of Bonding and bonding agents

Page 1: Bonding and bonding agents

Direct Restorative Materials

Dr. Md. Arifur RahmanBDS (DU), MPHAssistant Professor & HeadNortheast Medical College Dental [email protected]

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Bonding and Bonding Agents

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Bonding

Bonding and adhesion comprise a complex set of physical, chemical and mechanical mechanisms that allow the attachment and binding of one substance to another.

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Function of Banding agentA dental bonding system performs three essential

functions: (1) Provides resistance to separation of an

adherend substrate (i.e. enamel, dentin, metal, composite, ceramic) from a restorative or cementing material;

(2) distributes stress along bonded interfaces; and (3) seals the interface via adhesive bonding

between dentin and/or enamel and the bonded material, thus increasing resistance to microleakage and decreasing the risk for postoperative sensitivity, marginal staining, and secondary caries.

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Mechanism of adhesionthe fundamental mechanism of

adhesion to tooth structure can be regarded simply as an exchange by which inorganic tooth material (hydroxyapatite) is replaced by synthetic resins.

This process involves two parts: (1) removing hydroxapatite to create

micropores.(2) infiltration of resin monomers into

the micropores and subsequent polymerization. As a result, resin tags are formed that micromechanically interlock or interpenetrate with the hard tissue.

FIGURE : Scanning electron

microscopy image of tags formed by the penetration of

resin into etched areas of enamel.

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Mechanism of adhesion (cont.)In general, the following factors can play

major or minor roles in achieving adhesive bonds:

1. Surface energy and wetting2. Interpenetration (formation of a hybrid

zone)3. Micromechanical interlocking4. Chemical bonding

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ACID-ETCH TECHNIQUE• ENAMEL ETCHING• DENTIN ETCHINGBoth dentin and enamel are etched typically

using 37% phosphoric acid. Dentin etching is more technique sensitive than enamel etching because of the complexity of the dentin structure. Unlike enamel, dentin is a living tissue, consisting of 50 vol% of calcium phosphate mineral (hydroxyapatite), 30 vol% of organic material (mainly type I collagen), and 20 vol% fluid.

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ACID-ETCH TECHNIQUE (cont.)Acid etching removes hydroxyapatite almost

completely from several microns of sound dentin, exposing a microporous network of collagen suspended in water. Whereas etched enamel must be completely dry to form a strong bond with hydrophobic adhesive resins, etched dentin must be moist to form a hybrid layer.

Acid etching is a very effective way to improve bonding and durability as well as to ensure a sealed interface. It has markedly expanded the use of resin-based restorative materials because it provides a strong bond between resin and teeth, forming the basis for many innovative dental procedures.

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PROCESS AND PROCEDURAL FACTORSSeveral aspects of the total-etch, etch-and-

rinse, and related techniques are critical to the success or failure of bonding Systems

• Etching time• Rinsing and Drying stage • Cleanness of the Bonding Surfaces

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Etching timeCurrently, the etching time for most

etching gels is approximately 15 seconds. The advantage of such short etching times is that they yield acceptable bond strength in most instances, while conserving enamel and reducing treatment time.

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Rinsing and Drying stage Once the tooth is etched, the acid should be rinsed awaythoroughly with a stream of water for about 20 seconds,

andthe rinsed water must be removed. When enamel alone is etched and is to be bonded with a

hydrophobic resin (e.g., bisphenol A glycidyl methacrylate [bis-GMA]–based resin), it must be dried completely with warm air until it takes on a white, frosted appearance.

Dentin, in contrast, cannot withstand such aggressive drying, which would cause bond failure because of the formation of impermeable, collapsed collagen fibers.

In the total-etch technique, a dentin bonding agent and primer must be used that are compatible with both moist dentin and moist enamel

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Cleanness of the Bonding Surfaces

The etched surfaces must be kept clean (free of contaminants) and sufficiently dry until the resin is placed to form a sound mechanical bond. Although etching raises the surface energy, contamination can readily reduce the energy level of the etched surface. Reducing the surface energy, in turn, makes it more difficult to wet the surface with a bonding resin that may have too high a surface tension to wet the contaminated surface. Thus, even momentary contact with saliva or blood can prevent effective resin tag formation and severely reduce the bond strength. Another potential contaminant is oil that is released from the air compressor and transported along the air lines to the air–water syringe.

If contamination occurs, the contaminant should be removed, and the surface should be etched again for 10 seconds.

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DENTAL BONDING AGENTS

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Dental Bonding Dental bonding agents are designed to provide a

sufficientlystrong interface between restorative composites

and toothstructure to withstand mechanical forces and

shrinkage stress.

The success of adhesives is dependent on two types of bonding:

1. Micromechanical interlocking, chemical bonding with

enamel and dentin, or both. 2. Copolymerization with the resin matrix of

compositematerials

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Requirement for successful bonding As discussed in the earlier section on adhesion

mechanisms, a successful dentin bonding system must meet several requirements:

1. Adequate removal or dissolution of the smear layer from enamel and dentin

2. Maintenance or reconstitution of the dentin collagen matrix

3. Good wetting4. Efficient monomer diffusion and penetration5. Polymerization within tooth structure6. Copolymerization with the resin composite

matrix

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Composition of Bonding Irrespective of the number of bottles or

components, a typical dentin bonding system includes :

1. etchants, 2. Primers / resin monomers, 3. solvents,4. Adhesives ,5. initiators and inhibitors, 6. fillers, and 7. sometimes other functional ingredients such as

antimicrobial agents.

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Etchants

Etchants are relatively strong acids (pH = 1–2) used to remove smear layers and to dissolve the mineral phase to allow formation of micromechanical interlocking in enamel and in dentin. A number of acidic agents have been used to produce the required microporosity. However, phosphoric acid at a concentration between 30% and 50%, typically 37%, is the preferred etching agent to produce consistent etching patterns while not damaging the pulp.

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PrimersDentin etching is highly technique sensitive because the demineralized collagen network readily collapses when dried. Therefore, priming is necessary to maintain an expanded collagen network while removing residual water to allow for the infiltration of the hydrophobic adhesive monomer. Primers are solutions containing hydrophilic monomers dissolved in a solvent such as acetone, ethanol, or water. Such monomers exhibit hydrophilic properties through phosphate, carboxylic acid, alcohol, or ester functional groups. HEMA (2-hydroxyl ethyl meth acrylate) is a widely used primer monomer because of its high hydrophilicity and solvent-like nature.

If a primer has the ability to both etch and prime, it is categorized as a self-etching primer. For this purpose, acidic monomers are frequently used to formulate self-etching primers.

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SolventsSolvents also play important roles in priming

systems. The most commonly used solvents are water, ethanol, and acetone.

In addition to the enhancement of wetting of hydrophilic dentin, each solvent has a specific contribution to improve bond adhesion.

-Water can ionize acidic monomers as well as re-expand the collapsed collagen network.

-Ethanol and acetone have better miscibility with relatively hydrophobic monomers, and their “water chasing” ability facilitates water removal.

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AdhesivesFor dentin bonding, the primary purpose of

adhesives is to fill the interfibrillar space of the collagen network, creating a hybrid layer and resin tags to provide micromechanical retention upon polymerization.

In addition, adhesive layers also should prevent fluid leakage along the restorative material’s margin, since they make up the major part of the intermediate layer between dentin and/or enamel and restorative composites.

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InitiatorsSimilar initiator systems are used in both

adhesives and restorative composites. Polymerization can be initiated either through a

photo initiator system consisting of a photosensitizer (e.g., camphorquinone) and an initiator (e.g., tertiary amine), through a self-cure system that includes a chemical initiator (e.g., benzoyl peroxide), or through a dual cure initiator system.

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Filler ParticlesNanometer-sized silica particles have been added to

some adhesives to reinforce the adhesive and thereby produce higher bond strengths. However, the strengthening effect of the fillers in adhesives is uncertain because it is not clear whether these fillers can actually penetrate into the demineralized collagen networks, since the interfibrillar space of the collagen networks is within the range of 20 nanometers (nm) while the filler particles have a size of approximately 40 nm.

The other reason for the addition of fillers is to effectively modify adhesive viscosity to a thicker, pastier consistency. When such an adhesive is applied to an etched dental surface, it yields a thicker bonding layer that can promote increased bond strength by preventing oxygen inhibition. Moreover, a thick adhesive layer can reduce shrinkage stress because it is more compliant compared with restorative composites.

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Other IngredientsA number of additional ingredients are used with

dentin bonding agents for a variety of specific purposes. A few examples include the following:

1. glutar aldehyde (Probond, Dentsply, York, PA) is added as a desensitizer.

2. The monomer MDPB [12-meth acryloyl oxydo decylpyridinium bromide] (Clearfil Protect Bond, Kuraray America, New York, NY) and parabene (Adper Prompt-L-Pop, 3M ESPE, St. Paul, MN) are used as antimicrobials.

3. Fluoride (Prime & Bond NT, Dentsply, York, PA) is added to prevent secondary caries.

4. Benzal konium chloride (e.g., Etch 37, Bisco, Schamburg, IL) and chlorhexidine (e.g., Peak LC Bond Resin, Ultradent Products, South Jordon, UT) are used to prevent collagen degradation.

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Classification of Bonding agent

This classification was developed by Van Meerbeek et al. (2003). As shown in Figure, it is based on two general approaches to etching, priming, and applying the bonding resin to dentin and enamel surfaces, and further subdivided into the number of steps in the process. Thus, the major categories of bonding systems are known as “etch-and-rinse” and “self-etch” systems, with two subcategories, each according to the number of clinical steps involved.

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Etch-and-Rinse AdhesivesThree-step (fourth generation): At this time, the

most established, most reliable adhesion method in this category consists of three steps: (1) an acid etchant application, (2) application of the primer, and (3) application of the actual bonding agent or bonding resin. The primer contains hydrophilic functional monomers dissolved in an organic solvent such as acetone, ethanol, or water.

Two-step (fifth generation): A simplified method in this category combines the primer and adhesive resin into one application. This etch-and-rinse strategy is the most effective to achieve efficient and stable bonding to enamel. Etching, usually with a 30% to 40% phosphoric gel that is rinsed away, promotes the dissolution of enamel rods, creating porosities that are filled by bonding agents through capillary action and then followed by polymerization of resin.

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Self-Etch AdhesivesTwo-step (sixth generation): This approach does

not involve a separate etching step. In this case an acidic monomer which is not rinsed, is used to condition and prime the tooth at the same time.

One-step (seventh generation): The simplified method in this category combines conditioner, primer, and bonding resin into a single step. Most one-step or “all-in-one” systems are delivered by a bottle, vial, or single-unit dose applicator, which are formulated as a single component. However, Adper Promp L-Pop (3M ESPE, St. Paul, MN) is a variation on this theme, in which two liquid components are packaged in separate “blister” compartments in a single dispenser.

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Aging Effects and Degradation of the Hybrid LayerSeveral investigations have found that

the bond strength of three-step adhesive systems show little or no decrease in contrast to two-step adhesive systems that decrease significantly during a 4- to 5-year span.

Research has also shown that peripheral bonding to etched enamel, which seals the resin bond from exposure to water, can significantly increase bond durability.

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Which one is best? General recommendations for specific situations are given

below: • For bonding composite cores, three-step, etch-and-

rinse (fourth-generation) systems are usually recommended.

• For bonding anterior and posterior composites and cementation of veneers with resin cements, two-step etch-and-rinse (fifth-generation) systems provide the best performance.

• For bonding posterior composites, self etch, two-step (sixth-generation) systems are the better choice.

• Dual-cure one-step, self-etch (seventh-generation) systems are advised for esthetic posts and ceramic restorations bonded with resin cement, while light-cured one-step, self-etch (seventh-generation) systems are recommended for bonding posterior composite restorations.

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Which one is best? (Cont..)Recently, however, van Landuyt et al.

(2011) showed in a randomized clinical trial that a onestep, self-etch adhesive had similar clinical performance after 3 years compared with that of an etch-and-rinse adhesive in class V restorations. However, the one-step group exhibited more incisal marginal defects and discoloration compared with the etch-and-rinse group.

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LUTING AGENTS AND RESIN CEMENTSFor many years, retention of indirect restorations

could only be attained by the use of favorable tooth preparations and by micromechanical interlocking of the luting agent into irregularities present on the surface of the restoration and the tooth. Adhesive bonding completely changed the use of direct and indirect restorations.

A number of materials are available for cementation, including zinc phosphate cements, polycarboxylate cements, glass-ionomer cements (GIC), RMGI cements, and resin cements.

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ORTHODONTIC BRACKET BONDING RESINSOrthodontic treatment with fixed appliances involves

the use of attachments such as brackets and bands. Bands are metal rings that go around teeth and are usually used on molars. Brackets are bonded to enamel. The development of bonding has contributed to a decrease in banding even in posterior teeth.

A variety of luting agents have been used in orthodontics such as polycarboxylate cement, GIC, and resin-modified glass ionomer (RMGI) cements.

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ROOT CANAL SEALERS

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Endodontic sealers are used in conjunction with a solid or semisolid core material to fill voids and to seal root canals during obturation. These sealers, sometimes called cements, should fill and promote a close seal between the core and the dentinal walls of the canal, preventing or minimizing leakage, entombing residual microorganisms, and possibly filling inaccessible areas of the root canal system. Sealer selection may influence the outcome of endodontic treatment.

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Traditionally, gutta-percha has been used as the solid core material in combination with a zinc oxide and eugenol (ZOE) or calcium hydroxide cement, or an epoxy-based sealer. Clinical studies have shown predictable results with these conventional root canal sealers. GICs and resinmodified GICs are also used as root canal sealers. These can chemically bond to root canal dentin, but no bond is obtained to gutta-percha.

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Recently, new materials have been introduced on the market to improve the seal of the root canal system. These include different types of core materials, the development of low-viscosity methacrylate resin-based sealers, and the incorporation of dentin bonding agents.

One system uses conventional cones that are coated with a proprietary resin coating (EndoRez, Ultradent, South Jordan, UT) in combination with a methacrylate-based, dual-cured resin sealer and no dentin-bonding agent. In this case, the endodontic seal is dependent on the penetration of the hydrophilic sealer into the dentinal tubules and lateral canals following removal of the smear layer.

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Another system, called Resilon (SybronEndo Corporation, Orange, CA) is a thermoplastic composite polycaprolactone-based core material that contains dimethacrylate resin, radiopaque fillers, and glass-ionomer particles. Resilon cones are applied in combination with a methacrylatebased sealer to root dentin treated with self-etching primers.

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