l 3 Adhesive

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L-3 Dental material Dr.Niaz

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Principles of adhesion

History;

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

renders the tooth surface more receptive to adhesion.1960’s: First commercial restorative resin composite.

1970’s: Introduction of the acid-etch technique in clinical practice.

Adhesion

Adhesion or bonding is the attachment of one substance to another.

In dentistry it is the process by which a restorative material form a proper union with the tooth

surface.

Adhesive or bonding agent: the material that is applied to the surface of substances to join

them together. ( i.e. tooth surface and resin composite).

Factors Affecting Adhesion to Tooth Tissue

1- Compositional and structural aspects of enamel and dentin.

2- Changes in dentinal structure.

3- The smear layer.

4- Internal and external dentinal wetness.

Compositional and Structural Aspects of Enamel and Dentin

There are a lot of differences between adhesion to enamel and adhesion to dentin due to the

difference in composition and Structural Aspects of Enamel and Dentin

1- Enamel has high inorganic content (88% by volume). While the organic component and water

form a little percent.

2-Enamel is homogenous in structure and composition irrespective of its depth and location.

Enamel on the surface is more mineralized than the enamel on the DEJ. But in general it is

homogenous structure formed of hydroxyapatite crystals and enamel prisms.

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 pulp side. 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 of dentinal tubules, means more irritation of the pulp.




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* And this will affect the bonding agent because it is sensitive to moisture. So as we expose more

dentinal tubules there will be more flow of water and the adhesion to dentin will be less reliable

than adhesion to enamel.

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

The Smear Layer

Definition: Any debris, calcific in nature, produced by reduction or instrumentation of

dentin, enamel or cementum. (Mineralized tooth structure if we cut it with a hand piece it

will produce smear layer that stick on the tooth surface).

The thickness of the smear layer varies from 0.5 to 5 μm. But it affects the adhesion

especially with dentin, because it blocks the dentinal tubules.This layer of debris has a

great influence on any adhesive bond formed between the cut tooth and the restorative

material.

The smear layer can be removed by

1- Ethyl-enediaminetetraacetic acid (EDTA), mainly used in endodontic.

2- 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 its function.

Although smear layer occlude the dentinal tubules, it is porous and allow 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 the dentinal

tubules to the outer surface of the tooth. And this water will resist the flow of the




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adhesive into the dentinal tubules. And this is called inernal dentinal wetness. so internal

wetness works in an opposite manner comparing to adhesion.

Internal wetness: try to go out through dentinal tubules while the adhesive flow on the surface

and try to go in the dentinal tubules.So the internal wetness will resist the adhesive. We also have the external dentinal wetness

which comes from the external environment. And we can control it somehow, for example by

moisture control.

Internal and External Dentinal Wetness

1-Remove of the smear layer creates wet bonding surface on which dentinal fluids exudes from

the dentinal tubules.

2- This wet environment affects adhesion, because water competes by hydrolysis for all the

adhesion sites on the hard tissues.

3-Bond strength of several adhesive systems decrease as the depth of the preparation increased

because dentinal wetness is greater.

4- External humidity or environmental humidity can also affect the bond strength.

Wetting of the Adhesive

Definition: Wettabilty of the adhesive is its ability to spread over the 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 will be less. (and vice versa)

Adhesive systems must sufficiently wet the tooth surface: have a viscosity that is low

enough to penetrate the micro porosities and be able to displace the moisture and air

during the bonding process.




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How to increase the wettability?

HEMA (2-hydroxyethyl methacrylate) is a surface active agent that enhances the wettability of

the adhesive resins. It is present inside the adhesive to have a higher flow over the dentinal

surface and flows through the dentinal tubules.We use it especially for dentine, we don’t problems with the bonding to enamel as far as we

don’t have water and contaminants from the outer environment.

But we have problems in bonding in dentine because it is not homogenous and it has a surface

flow of water (internal wetness).

Enamel Acid -Etching Techniques

1- It is a straight forward step, we apply phosphoric acid to enamel to make the surface

porous and irregular so the adhesive can go through these irregularities and forms what

we call resin tags.

2- Enamel etching transforms the smooth enamel surface to an irregular high surface-free

energy.

3- Acid etching removes 10μm of the enamel surface and creates a microporous layer from

5 to 50 μm deep.

4- The enamel bonding agent wet the etched enamel surface and pass through the micro

porosities.

5- The bond between the bonding agent and the enamel is micromechanical.

6- The bond between the bonding agent and resin composite is chemical.

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

Amalgam + tooth surface = mechanical

Composite or adhesive + tooth surface = micromechanical because we have irregularities where

the resin will go through.

Composite + adhesive =chemical

Enamel Etching Patterns

Two types of resin tags made by the acid etching:

1- Macrotags: Form circularly between enamel prism peripheries.

2- Microtags: Form at the cores of enamel prisms.




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Enamel Bonding Agents

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




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Difference between the composite and the adhesive

Adhesive is mainly made of resin with no filler loading & Diluted with a lower viscosity and

higher hydrophilicity monomers (that will make the adhesive more wet able) such as (TEG-

DMA) and (HEMA).Etching with Phosphoric Acid

1- Phosphoric acid 30% to 40%.

2- Etching time is not less than 15 seconds.

3- Washing time 5 to 10 seconds.

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 dentin and they put the material

into it and they use special instrument to break this bond, and they measure the amount of force

needed to break this bond.

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

5-There was no acid etching, they thought if they acid etch the tooth they will reach the pulp and

cause irritation.

Second generation adhesives:

1- Moderate bond strength

2- 5 to 6 MPa

3- Primary bonding was to the smear layer

4- Also here there was no acid etching.

Third-Generation Adhesives:

1- Early 1980’s.

2- Removal of the smear layer.

3- Total etch concept. Etching the enamel and dentin.

Fourth-Generation Adhesives:

1- Early to mid 1990’s.

2- Multiple dentin adhesives.




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3- Treatment of dentin by primer (make dentin more receptive to adhesion).

HEMA is a surface active agent & primer added on the adhesive to increase the wetability.

4- Application of low viscosity adhesive resin.

Here there are 3 steps1. Acid etching 2. Dentin Primer 3. Adhesive

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

1- no-mix,2- self-etching,

3- self-priming,

4- single-bottle adhesive5- Insensitive to the amount of residual moisture on the surface of the preparation.

Conditioning of Dentin: Any chemical alteration of the dentinal surface by acids with

the objective of removing the smear layer and simultaneously deminerlizing the dentinalsurface.

For conditioning of dentin we use the agent that we use to acid etch enamel but the effect

is different.

In enamel it is going to form irregularities either between the prisms or at the core of the

prism, while Dentin acid etching will open the dentinal tubules by removing the smear

layer and remove some of the minerals around the dentinal tubules.

The deminerlization process exposes the collagen fibrils and thus increases the

microporosities of the intertubular dentin.




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* 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 the surface-free energy of

dentin.Dentin Primers

* Primers are adhesion-promoting agents. It has hydrophilic and hydrophobic sides in its

chemical structure. The hydrophilic part will bind to collagen fibrils of the dentin which is

hydrophobic as well, while the hydrophobic side will bind to the adhesive which is also

hydrophobic.

* Contain hydrophilic monomers dissolved in organic solvents such as acetone or ethanol.

*Those solvents are volatile and so can displace water from the dentinal surface and collagen

network.

*This promotes infiltration of the monomer of the adhesive in the nanospaces of the exposed

collagen network.

* Primers have hydrophilic properties that have affinity to the collagen fibrils and hydrophobic

properties to copolymerize with the adhesive resin.

* Many Modern adhesives have HEMA as a primer and contain other monomers.

*One-bottle adhesives: combine priming and bonding function.

Adhesive Resin

_ Consist of unfilled resin (GMA & UDMA) with surface active agent and they added to it the

primer. (one bottle system)

_ Consist of hydrophobic monomers such as GMA and UDMA and hydrophilic 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.

Hybridization : Hybrid layer is the area or zone of the adhesive system which

micromechanically interlocks with the dentinal collagen.




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Resin Tag Formation

_ Penetration of the resin to the dentinal tubules to form resin tags.

_ Resin infiltrates even lateral tubule branches forming submicron resin tags.

_ The contribution of formation resin tags to bond strength has been a matter of speculation.

Adhesion Strategies- A Scientific Classification of Modern Adhesives

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

This classification depends on the smear layer

1- Smear Layer-Modifying Adhesives.

2- Smear Layer-Removing Adhesives.

3- Smear Layer-Dissolving Adhesives.

The first two generations they didn’t remove the smear layer they modified it. While the 3rd &

4th &5th generations they removed the smear layer because they used acid etching. the sixth and

7th

generation dissolved the smear layer.

Smear Layer-Modifying Adhesives

* Based on weak acidity smear layer-modifying primers.

*The smear layer is not removed that provides a natural barrier to the pulp.

* The interaction of theses adhesives with dentin is very superficial, with only limited

penetration into dentin.

* Selective etching of enamel in a separate step.




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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 (Acid etching + primer &

adhesive)

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 with simultaneous resin

infiltration.

* Possibility of single-dose package.

* No post conditioning rinsing.

Critical Steps in Adhesion.

1_ Isolation

2_ Dentin and pulp protection

3_ Enamel and dentin conditioning

4_ Wet Vs Dry Bonding

5_ Primer application

6_ Adhesive resin application

7_ Restorative procedure




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Isolation

_ Isolation and moisture control is very critical for the bonding procedure.

_ Bonding to acid etched enamel requires a dry surface to allow the bonding resin to flow into

the microporosities created by acid etching. _ Accidental contamination of the substrate with external fluids prevents effective contact

between the adhesive and the bonding substrate.

_ Salivary contamination is detrimental because salivary proteins block the microtentive

porosities on the acid etched enamel and dentin and may block infiltration of the resin.

_ Ideal isolation is by using the rubber-dam. But at least we have to do isolation by cotton rolls

and suction all the time while doing a composite restoration.

_ If an accidental contamination occur after acid etching and bonding it should repeat the steps

from the beginning (acid etching,rinsing, drying, bonding, light cure the bonding agent, and the

composite)

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 and

shrinkage 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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l 3 Adhesive