Adhesive System

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Principles of adhesion

1950s: Michael Buoncore (father of adhesionin dentistry) found that applying acid to teeth

renders the tooth surface more receptive toadhesion.

1960s: First commercial restorative resin

composite. 1970s: Introduction of the acid-etch

technique in clinical practice.


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Factors Affecting Adhesion to Tooth Tissue

Compositional and structural aspects of

enamel and dentin.

Changes in dentinal structure.

The smear layer.

Internal and external dentinal wetness.


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Compositional and Structural Aspects of Enamel and Dentin

There are a lot of differences between adhesion to enamel andadhesion to dentin due to the difference in composition andStructural Aspects of Enamel and Dentin

1- Enamel has high inorganic content (88% by volume). While theorganic component and water form a little percent.

2-Enamel is homogenous in structure and composition irrespective ofits depth and location. Enamel on the surface is more mineralizedthan the enamel on the DEJ. But in general it is homogenousstructure formed of hydroxyapatite crystals and enamel prisms.


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1- Dentin has less inorganic content (50% by volume) and more water(25%).

2- Dentinal tissue is heterogeneous.

3- Dentin is a vital and dynamic tissue.

4- Dentinal tubules size and number increases from the DEJ to the pulpside. Dentin is wet. And as we go in depth there will be more flow.And as we drill more in dentin there will be more exposing ofdentinal tubules, means more irritation of the pulp. * And this willaffect the bonding agent because it is sensitive to moisture. So aswe expose more dentinal tubules > there will be more flow of water> the adhesion to dentin will be less reliable than adhesion toenamel.

5- Dentinal fluid is under a constant outward pressure from the Pulp.


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The smear layer can be removed by

Ethyl-enediaminetetraacetic acid (EDTA), mainly used inendodontic.

Acidic conditioners (citric, polyacyclic, lactic and phosphoric acids).An acid etchant to remove smear layer and open dentinal tubules,

so the adhesive can go through the dentinal tubules and forms itsfunction.

Although smear layer occlude the dentinal tubules, it is porous andallow small amount of dentinal fluids to pass through.

Dentinal tubules are wide, as we cut they will open which means

that dentin it has an outflow of water from the pulp through thedentinal tubules to the outer surface of the tooth. And this waterwill resist the flow of the adhesive into the dentinal tubules. Andthis is called inernal dentinal wetness. so internal wetness works inan opposite manner comparing to adhesion.


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Internal wetness try to go out throughdentinal tubules while the adhesive flow onthe surface and try to go in the dentinal

tubules. >> So the internal wetness will resist the

adhesive. We also have the external dentinalwetness which comes from the externalenvironment. And we can control it somehow,for example by moisture control.


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Internal and External Dentinal Wetness

Remove of the smear layer creates wet bondingsurface on which dentinal fluids exudes from thedentinal tubules.

This wet environment affects adhesion, because watercompetes by hydrolysis for all the adhesion sites on thehard tissues.

Bond strength of several adhesive systems decrease as

the depth of the preparation increased becausedentinal wetness is greater.

External humidity or environmental humidity can alsoaffect the bond strength.


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Wetting of the Adhesive

Wettabilty of the adhesive is its ability to spread overthe surface.

The higher the wettability is the better the adhesive. When the angle between the surface and adhesive is

big this means that the wettability of the adhesive willbe less. (and vice versa)

Adhesive systems must sufficiently wet the toothsurface: have a viscosity that is low enough topenetrate the micro porosities and be able to displacethe moisture and air during the bonding process.


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How to increase the wettability?

HEMA (2-hydroxyethyl methacrylate) is a surface activeagent that enhances the wettability of the adhesiveresins. It is present inside the adhesive to have a higher

flow over the dentinal surface and flows through thedentinal tubules.

We use it especially for dentine, we dont problemswith the bonding to enamel as far as we dont havewater and contaminants from the outer environment.

But we have problems in bonding in dentine because itis not homogenous and it has a surface flow of water(internal wetness).


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Enamel Acid -Etching Techniques

It is a straight forward step, we apply phosphoric acid to enamel tomake the surface porous and irregular so the adhesive can gothrough these irregularities and forms what we call resin tags.

Enamel etching transforms the smooth enamel surface to an

irregular high surface-free energy. Acid etching removes 10m of the enamel surface and creates a

microporous layer from 5 to 50 m deep.

The enamel bonding agent wet the etched enamel surface andpass through the micro porosities.

The bond between the bonding agent and the enamel ismicromechanical.

The bond between the bonding agent and resin composite ischemical.


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Note: we have 3 types of adhesion (chemical, micromechanical,mechanical).

Amalgam + tooth surface = mechanical

Composite or adhesive + tooth surface = micromechanical because we haveirregularities where the resin will go through.

Composite + adhesive = chemical

Enamel Etching Patterns

Two types of resin tags made by the acid etching:

Macrotags: Form circularly between enamel prism peripheries.Microtags: Form at the cores of enamel prisms.

Enamel Bonding Agents

Based on bis-GMA or urethane dimethacrylate (UDMA).


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Adhesion to Dentin

Development of Resin Dentin Adhesives.

First generation adhesives:

1- Very weak bond

2-Only 2 to 3 MPa (megapascal = Newton/mm2 ),3-How they measure this bond strength? They bring surface of dentinand they put the material into it and they use special instrument tobreak this bond, and they measure the amount of force needed tobreak this bond.

4-This bonding agent bonded to enamel and dentin by chelation to

calcium5-There was no acid etching, they thought if they acid etch the tooth

they will reach the pulp and cause irritation.


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Fifth-Generation Adhesives:

1- Simpler adhesive system than fourth-generation

2-Bond strength is comparable to fourth-generation

3-adhesives

4-One-bottle adhesive (primer + adhesive)

So here we have 2 steps (simpler)

1. Acid etching 2.primer &adhesive

Sixth-Generation Adhesives:

1- Slightly acidic primers or self-etching adhesives.

2- Possibility of single-dose package.

3- No post conditioning rinsing.

4- There is no acid etching and rinsing in a separate step.

Seven Generation

no-mix,

self-etching,

self-priming,

single-bottle adhesive

Insensitive to the amount of residual moisture on the surface of the preparation.


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Conditioning of Dentin

Any chemical alteration of the dentinal surface by acids with the objectiveof removing the smear layer and simultaneously deminerlizing thedentinal surface.

For conditioning of dentin we use the agent that we use to acid etchenamel but the effect is different.

In enamel it is going to form irregularities either between the prisms or atthe core of the prism, while Dentin acid etching will open the dentinaltubules by removing the smear layer and remove some of the mineralsaround the dentinal tubules.

The deminerlization process exposes the collagen fibrils and thusincreases the microporosities of the intertubular dentin.

Acid etching will work only on minerals.

Conditioners are applied to enamel and dentin (Total-etch technique).

The demineralization process exposing the collagen fibrils affect thesurface-free energy of dentin.


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Adhesive Resin

Consist of unfilled resin (GMA & UDMA) with surface active agentand they added to it the primer. (one bottle system)

Consist of hydrophobic monomers such as GMA and UDMA andhydrophilic monomers such as TEG-DMA and HEMA as wetting

agent. Adhesive resin stabilizes the hybrid layer and form resin extensions

into dentinal tubules (resin tags).

Self cured or light cured adhesive.

Light cure the adhesive before the application of the resin

composite. Oxygen inhibited layer on top of adhesive provide sufficient double

bonds for copolymerization with the resin composite


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Adhesion Strategies- A Scientific Classification of ModernAdhesives

Depends on the number of application steps and on theirinteraction with the dentinal substrate.

This classification depends on the smear layer Smear Layer-Modifying Adhesives.

Smear Layer-Removing Adhesives.

Smear Layer-Dissolving Adhesives.

The first two generations they didnt remove the smear layerthey modified it. While the 3rd & 4th &5th generationsthey removed the smear layer because they used acidetching. the sixth and 7th generation dissolved the smearlayer.


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Smear Layer-Modifying Adhesives

Based on weak acidity smear layer-modifying primers.

The smear layer is not removed that provides a natural barrier to thepulp.

The interaction of theses adhesives with dentin is very superficial, with

only limited penetration into dentin. Selective etching of enamel in a separate step.

Smear Layer-Removing Adhesives

Most common adhesives.

Removing of the smear layer using the total-etch concept.

Formation of hybrid layer and resin tags.

Three-step smear layer-removing adhesives (Acid etching + dentin primer+ adhesive)

Two-step smear layer removing adhesives or one-bottle adhesives (Acidetching + primer & adhesive)


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Smear Layer-Dissolving Adhesives

Smear layer dissolving adhesives or self-etching adhesives.

Slightly acidic primers or self-etching adhesives.

Partially demineralize the smear layer and the underlying dentin withsimultaneous resin infiltration.

Possibility of single-dose package.

No post conditioning rinsing. Critical Steps in Adhesion.

Isolation

Dentin and pulp protection

Enamel and dentin conditioning

Wet Vs Dry Bonding

Primer application

Adhesive resin application

Restorative procedure


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Enamel and Dentin Conditioning

Phosphoric acid etchant (35%) for both enamel and dentin in one application.

Etching time should be for 15 seconds staring with enamel.

The etchant should be thoroughly rinsed.

Wet Vs Dry Bonding

After acid etching enamel surface should be dried.

On dentin, a certain amount of moisture is needed wet bonding.

Dehydration of the acid-etched dentin surface through air drying can result in collapsing andshrinkage of the exposed collagen fibril network which can prevent resin infiltration.

On the other hand the acid-etched dentin surface should not be kept too wet.

Excess water should be removed with a short air blast or with a dry sponge.

Primer Application

The primer should be applied for at least 15 seconds. Primer should be actively rubbed into the dentin surface with disposable brush or sponge

applicator.

After short and gentle air drying the primed surface should appear glossy.


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Adhesive Resin Application

Spreading of the adhesive layer to the

bonded surface should be done by a brush.

The adhesive should be light cured for 20

seconds before the application of the

restorative resin composite


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Documents

Adhesive System