ACID ETCH &BONDING

DONE BY

DR. worood Hashim

Composition of human enamel.

Restorative materials can be bonded to the surface of enamel by the topical application of acid. The most commonly used acid is ortho-phosphoric acid, which, when applied to enamel for around 15 s, removes around 10 μm of the surface, This surface etching increases the surface area of enamel hugely and creates pores, up to 20 μm deep, that become filled on contact with restorative materials.

Etching also raises the surface energy of enamel and increases the ability of applied bonding resins to wet the surface, ensuring intimate adaptation. Photo-polymerisation (light-curing) of the bonding resin creates micro-mechanical retention between the enamel and the resin.

The optimum concentration of phosphoric acid is around 37%. Higher concentrations cause precipitation of salts that inhibit further de-mineralisation of the enamel.

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It is important to adequately wash away the de-mineralised enamel components for 5–10 s and to dry the enamel with compressed air from a three-in-one syringe.

The bond strength can be greatly reduced if the enamel is contaminated with blood or saliva during the bonding process. It is therefore important to maintain a clean, dry field by isolating the tooth, ideally with a rubber dam.

The material which promotes adhesion between two different substances, or between material and tooth surface is called bonding agent

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An adhesive bond between any two structures can be created by one or more of the following mechanisms:

1. Mechanical—penetration of resin and formation of resin tags within the tooth surface

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• Adsorption—chemical bonding to the inorganic component (hydroxyapatite) or organic components (mainly type I collagen) of tooth structure, the forces involved may be primary (ionic and covalent) or secondary (hydrogen bonds, dipole interaction, or van der Waals) valence forces

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3. Diffusion—precipitation of substances on the tooth surfaces to which resin monomers can bond mechanically or chemically.

4. A combination of the previous three mechanisms.

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The Structure of Enamel

Enamel consists (by volume) of 86% inorganic hydroxyapatite crystals, 12% water and 2% organic matrix

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Bonding to Enamel:

The ability of clinician to bond restorative materials to enamel has been an important achievement of modern dentistry. Bonding to enamel occurs by micromechanical retention after acid etching is used to remove smear layer and preferentially dissolve hydroxyapatite crystals. Then primer and adhesive are flown into the surface irregularities produce resin tags.

These tags are called “macro-tags” and several smaller tags “micro-tags”. Micro-tags contribute to most of micromechanical retention .

during cavity preparation produces a smear layer with a low surface free energy. Therefore, the tooth surface should be thoroughly cleaned and pretreated prior to bonding procedures to increase its surface free energy and produce bonding.

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Challenges in Dentin Bonding Substrate:

Lower bond strengths to dentin are the result of a number of factors:

1. dentin contains less mineralized tooth structure and more water than does enamel.

2. the presence of the smear layer makes wetting of the dentin by the adhesive more difficult; even when good wetting does occur, polymerization shrinkage can pull the smear layer away from the dentin and produce a microgap.

• 3. fluid in the dentin tubules reduces the stability of the composite resin to dentin bond.
• dentin contains a substantial proportion of water and organic material, Dentin also contains a dense network of tubules that connect the pulp with the dentino-enamel junction (DEJ).

• The relative area occupied by dentin tubules decreases with increasing distance from the pulp. The number of tubules decreases from about 45,000/mm² close to the pulp to about 20,000/mm² near the DEJ. Adhesion can be affected by the remaining dentin thickness after tooth preparation.

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• First generation dentin bonding agents The first generation bonding agents were used in 1960‟s. These ignored the smear layer and did not recommend etching the dentin. NPG-GMA (N-pherylglycine glcidyl methacrylate), cyanocrylates and polyurethanes were included.
• The bond strength of first generation dentin bonding agents was 2 to 3 MPa. glycerophosphoric acid dimethacrylate containing resin would bond to acid etched dentin. These bonding agents were designed for ionic bonding to hydroxyapatite or for covalent bonding (hydrogen bonding) to collagen.

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• Second generation bonding systems Second generation bonding agents performed better results than first generation bonding agents and were used in late 1970‟s & early 1980‟s. These systems leave the smear layer largely.
• The bond strength ranges from 4.5 to 6 MPa, The 2nd generation of dentin adhesives primarily used polymerizable phosphates added to BIS-GMA resins to promote bonding to the calcium in mineralized tooth structure.
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Third generation:

A newer generation adhesive system have been developed that use a conditioning step on dentin in conjunction with a bonding agent. It includes dentin conditioner, dentin primers, bonding agent. With third generation system, the acid etching of dentin partially removes or modifies smear layer.

This group removes the smear layer before bonding & gives bond strengths ranging from 16-26 MPa.3 components are used i.e. conditioner, primer and adhesive

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• 2. Primer- It is a bifunctional monomers (i.e. it has one hydrophilic end and one hydrophobic end) in a volatile solvent such as alcohol or acetone. It includes HEMA, NMSA, NPG, PMDM & 4-META. It promotes infiltration of demineralized peritubular and intertubular dentin by its own monomers, adhesive resin and links hydrophobic adhesive resin to hydrophilic dentin. It increases wettability of conditioned dentin surface to increase contact of dentin to resin.

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3. Adhesive- It promotes bond strength. It is an unfilled/partially filled resin which may contain some components of primer. It forms resin tags to seal dentinal tubules and provides resin composites to bond with methacrylate groups.

 To summarize, the application of third-generation dentin bonding agents involves three steps: etching with an acidic conditioner, priming with a bifunctional resin in a volatile solvent, and bonding with an unfilled or partially-filled resin

Fourth generation:

With the fourth generation bonding system ,complete removal of smear layer was achieved.

Concept of total etch and moist dentinal bonding are hallmarks of fourth generation materials . Bond strength – 3to 25 MPa,This generation is characterized by the process of hybridization at the interface of the dentin and the composite. Hybridization is the replacement of the hydroxyapatite and water in the surface dentin with resin This resin, in combination with the remaining collagen fibers, constitutes the hybrid layer. Hybridization involves both the dentinal tubules and the intratubular dentin, dramatically improving bond strength to dentin. Total etching and moist dentin bonding, concepts developed.

The “total-etch” term refers to the procedure whereby both enamel and dentin are etched before bonding. Total-etch adhesives involve an initial etching step with phosphoric acid (H3PO4) which removes the smear layer and conditions the preparation. The total-etch technique is also often referred to synonymously as the “etch-and-rinse” technique. The phosphoric acid is rinsed off with water together with the smear layer and the exposed dental tissue is carefully dried. Enamel is usually etched for longer than dentin.

Fifth generation:

The fifth generation consists of two different types of adhesive materials:

1. One bottle system: systems combined the primer and adhesives into one solution to be applied after etching enamel and dentin simultaneously with 35 to 37 percent phosphoric acid for 15-20 sec. In this mechanical interlocking with etched dentin occurs by means of resin tags, adhesive lateral branches and hybrid layer formation and show high bond strength both to enamel and dentin .

These materials adhere well to enamel, dentin, ceramics, and metal, but most importantly, are characterized by a single component, single bottle. There is no mixing, and thus less possibility for error.

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Sixth generation:

This system eliminates the need for etching with phosphoric acid by use of an acidic primer.

Type I (Self-etching Primer + Adhesive) acidic primer applied to tooth first, followed by adhesive.

Type II (Self-etching Adhesive) Two bottles or unit dose containing acidic primer and adhesive; a drop of each liquid is mixed and applied to the tooth. In the two-bottle sytems acidic primer is applied to the prepared area first, then air-thinned, and a son of bonding resin (which includes acidic additive) is then applied to the primed t surface.

This generation bonding agents were introduced in late 1990‟s & early 2000‟s. Self etching step was eliminated, instead acidic primer was included which was placed after the tooth preparation. Some of the variations involved either leaving the primer on the tooth and then placing adhesives over it or mixing the acidic primer and adhesive before placement on the enamel and dentin. The incidence of post treatment sensitivity was reduced in this system compared from the previous systems, but the bond strength is lower than fourth and fifth generation system to enamel and dentin.

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• The demineralized smear layer is incorporated into the hybrid layer as the etched surface is not rinsed. The hybrid layer thickness is 0.5μm to 5μm. Collagen fibers is infilterated by the acidic primers and adhesive monomers, as the primer decalcifies the inorganic component in dentin to the same depth, which should minimize potential leakage,
• postoperative sensitivity or voids. single-solution appears more appropriate because they consist of a single solution When applied to the tooth structure. These are self etching adhesives that require no mixing.

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• Seventh generation:
• This generation was introduced in the early 2000‟s, eliminating separate etching steps & containing acidic primers and adhesive monomers in a single bottle. Earlier this generation was available as light cured formulations, but now several dual cured products are offered. The smear layer was used as a bonding substrate. The smear layer and the top layer of underlying dentin surface is demineralized by acidic primer. The exposed collagen along with hydrophilic monomers is infiltrated by acidic primer which then is copolymerize.

The eighth generation bonding agents are dual cured , self etching, nano reinforced agents and produce bond strength of more than 30 MPa to dentin and enamel with no postoperative sensitivity These adhesives are one-bottle systems that may be used in either the total- or self-etch mode, on either dentin or enamel.

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C-factor

The C-factor is the ratio of bonded surfaces to the un-bonded, the greater is the potential for bond disruption from polymerization effects. A Class IV restoration (one bonded surface and four un-bonded surfaces) with a C-factor of 0.25 is at low risk for adverse polymerization shrinkage effects. A Class I restoration with a C-factor of 5 (five bonded surfaces, one un-bonded surface) is at much higher risk of bond disruption associated with polymerization shrinkage, particularly along the pulpal floor, Class I preparations with a high C-factor) by using.

(1) "Soft-start" polymerization instead of high-intensity light curing.

(2) Incremental additions to reduce the effects of polymerization shrinkage.

(3) A stress-breaking liner, such as a filled dentinal adhesive, flowable composite, or RMGI.

Glass-lonomer

1-Conventional Glass-lonomers: Conventional glass-ionomer Systems same of silicate cements-they release fluoride into the surrounding tooth structure, yielding a potential anti cariogenic effect, and possess a favourable coefficient of thermal expansion. Glass-ionomers use poly-acrylic acid, which renders the final restorative material less soluble. Because of their low resistance to wear and relativity low strength compared with composite or amalgam.

Although conventional glass ionomers are relatively technique-sensitive regarding mixing and insertion procedures, they may be good materials for restoration of root-surface caries because of their inherent potential anti-cariogenic quality and adhesion to dentin.

Similarly, glass ionomers may be indicated for other anterior restorations in patients exhibiting high caries activity. Glass-ionomers are not recommended for the restoration of occlusal areas of posterior teeth. Glass-ionomer cements also have been widely advocated for permanent cementation of crowns.

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2-Resin-Modified Glass-lonomers: In an effort to improve the physical properties and aesthetic qualities of conventional glass-ionomer cements, resin-modified glass-ionomer (RMGI) materials have been developed probably best described as glass-ionomer to which resin has been added. An acid-base setting reaction is present.

The resin component affords the potential for light curing, auto curing, or both. RMGIs are easier to use and possess better strength, wear resistance, and aesthetics than conventional glass-ionomers. They have the potential advantage of sustained fluoride release; they may be best indicated for Class V restorations in adults who are at high risk for caries and for Class I and II restorations in primary teeth.

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Compomers (Polyacid-Modified Composites): A composites to which some glass-ionomer components have been added, their physical properties are superior to traditional glass-ionomers and RMGIs, but inferior to those of composites.

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