dentin bonding agents

BY:

Kritika Sarkar

American society for testing and materials specification D 907.

Buonocore reported in 1955 that acid, could be used to alter the surface of enamel to ’ render it more receptive to adhesion."1 He had discovered that acrylic resin could be bonded to human enamel after conditioning with 85% phosphoric acid.

Buonocore accurately predicted several potential uses for this new technique, including Class III and Class V restorations and pit and fissure sealants.

Bonding sytem for restorative materials by Edward J Swift DMD, MS

Dentin bonding agent

A thin layer of resin between conditioned dentin

& the resin matrix of composite.

Dentin bonding:

The process of bonding a resin to conditioned

dentin.

Phillip’s science of dental materials (11th edition)

The word adhesion comes from the latin

“adhaerere” (to stick to).

It is defined as the state in which 2 surfaces

are held together by interfacial forces which

may consist of valence/ interlocking forces

or both.

Sturdevant’s art and science of operative dentistry (5th edition)

Chemical bonding between the adhesive

and the adherent

Adhesive

Adherend

Van der waal’s interactions-attraction between

opposite charges on ions and dipoles

Dispersion forces- interaction of induced

dipoles

Hydrogen bonds-it is particularly strong bond

and can be included among physical forces

COVALENT BOND

It involves sharing

electron between 2 atoms

or molecules.

IONIC BOND

It involves an actual

transfer of electron form

one atom to another.

Interlocking of the

adhesive with

irregularities in the

surface of the substrate

or adherend.

This would involve the

penetration of resin &

formation of resin tags

with the tooth surface.

Interlocking between mobile molecules such as the adhesion of two polymers through diffusion of polymer chain ends across an interface.

This would involve the precipitation of substances on the tooth surfaces to which resin monomers can bond mechanically or chemically.

An electrical double layer at the interface of a

metal with a polymer that is a part of total

bonding mechanism.

Energy of a solid on the surface is higher than

its interior because:

Core atoms are surrounded by atoms on all sides

Equal interatomic distances

Minimal energy

•But the surface atoms are unequally distributed and hence have more energy.•So the surface atoms get attracted towards the core resulting in surface tension.•Due to this substances on the surfaces get attracted to the substrate.•Harder the surface, higher the surface energy & higher the adhesive properties.

When a film of water in

introduced between two

glass slides the

adhesion properties are

better illustrated . This

is called wetting.

Surface energy and

cleanliness of the

adherend influences the

wetting.

It is the measure of wettability & is the angle

formed by the adhesive with the adherend at

the interface.

Smaller the contact angle greater the

wettability.

Low viscosity of the adhesive is required to

allow its easy flow on the surface of the

adherent. This increases the strength of

adhesion

This prevents the liquid adhesive from

completely wetting the adherent due to the

presence of air pockets. This decreases the

strength of adhesion.

Any debris on the surface will prevent direct

contact between the adhesive and the

adherend. This decreases the strength of

adhesive.

The thinner the adhesive film, the lesser the

air voids and the stronger the strength of

adhesion.

Liquid adhesive undergo contraction during

setting (polymerization shrinkage). This

contraction results in the reaction of stresses at

the interface that severely decreases the

strength of adhesion.

No doubt that primary bonds between

adhesive and adherent produce stronger

adhesion than if secondary bonds are formed.

The mineral content increases in different situations, which include:Aged dentin

Dentin beneath carious lesion

Exposed dentin

In all the above mentioned situation the dentinal tubules become obliterated with tricalciumphosphate crystals.

There are compositional changes in sclerotic dentin, which is much more resistant to acid etching than normal dentin.

Consequently, the penetration of a dentin adhesive is limited.

Sclerotic dentin

Composite resins do not show an intimate microscopic contact with dentin when placed directly into the cavity.

In order to overcome this, an intervening layer of fluid is used, which fills in the microscopic space, polymerizes & combines with the composite resin & components of dentin.

The adhesive molecule is bifunctional & a part of which (X) enters into chemical union with the tooth structure, & the other part (M) copolymerizes to the resin through the double the double bond of methacrylate.

CH3

CH

2

C

C

O

O R X

Methacrylate group

Spacer

Reactive group capable

of bonding to dentin

•The spacer group (R) is responsible for making the

molecules large enough to keep the methacrylate groups

spatially located for optimal chemical reaction with

composite

Ideally dentin adhesives should be both hydrophilic & hydrophobic.

It has be hydrophilic so as to be able to displace dentinal fluids & thereby wet the surface, permitting penetration into porosities with the dentin & eventually react with organic or inorganic components.

Hydrophobic properties are needed to allow bonding to the composite resin matrix of which is hydrophobic.

It occurs through ionic interaction between Ca2+ ions on the surface of dentin & negative charges on the group X of the adhesive.

Group X can be:▪ Phosphates

▪ Amino acids

▪ Amino alcohols

▪ Dicarboxylates

This involves interaction with the following

components present in the collagen of dentin: Amino (-NH)

Amido (-CONH)

Hydroxyl (-OH)

Carboxylate (-COOH)

Removal of hydrogen from any of these

groups allows combination with chemicals

present in the dentin bonding agent.

Compounds that are capable of reacting with one or more groups of collagen are:

isocynates

carboxylic acid chlorides

carboxylic acid anhydrides

aldehydes.

Egs: Dentin Adhesit(isocynate based), Gluma (aldehydebased)

Bonding to enamel is relatively simple process, without

major technical requirements/difficulties.

Bonding to dentin presents a much greater challenge.

Several factors account for this difference between

enamel and dentin.

Although enamel is highly mineralized tissue composed

of more than 90% (by volume) hydroxyappetite, dentin

contains substantial proportion of water and organic

material, primarily type 1 collagen.

Dentin also contains a dense network of tubules that connect the pulp with the dentinoenamel junction (DEJ).

A cuff of hypermineralizeddentin called peritubulardentin lines the tubules.

The less mineralized intertubuler dentin contains collagen fibrils with the characteristic collagen banding.

Dentin is intrinsically a hydrated tissue.

Adhesion can be affected by the “remaining dentin thickness” after tooth preparation.

Bond strengths are generally less in deep dentin than in superficial dentin.

Whenever tooth structure is prepared with a bur or an instrument, residual organic & inorganic components form a “smear-layer” of debris on the surface.

The smear layer fills the orifices of the dentinal tubules, forming “smear-plugs”

Based on :

Generation

Mode of application

Number of steps

Etching pattern

These products ignored the smear layer. They included NPG-GMA (N-phenylglycine glycidyl

methacrylate), the polyurethanes, and cyanoacrylates. An example of an NPG-GMA bonding agent was S.S.

White's Cervident which became available in 1965. The bond strength of this first-generation dentin

bonding agent was on the order of 2 to 3 MPa. 6 Clinical trials of these products were largely

disappointing; 7th one 6-month study reported a failure rate of 50%.

Additional problems with them included loss in bond strength over time and a lack of stability of individual components during storage.

These systems leave the smear layer largely, if not wholly, intact when used.

Although second generation bonding agents produced variable results, they generally performed better than first-generation bonding agents.

They routinely produced bond strengths that ranged from

approximately 4.5 to 6 MPa10,11 and exhibited clinical failure

rates of 30% at one year. Many of these products were developed and

marketed in the late 1970s and early 1980s.

There were three types of second-generation

products:

1. Etched tubule dentin bonding agents

2. Phosphate ester dentin bonding agents

3. Polyurethane dentin bonding agents

These attempted to achieve retention to dentin

by etching the tubules with 25% citric acid

and employing ethylmethacrylate to

mechanically interlock with the etched tubules

representative brand: Dentin Bonding System

(Den-Mat)

▪ These used analogs of BIS-GMA with attached phosphate esters the

phosphate group of the dentin bonding agent apparently bonded with

calcium in the tooth structure and the methacrylate end of the molecule

bonded to the composite resin.

Most systems of this type employed a mild cleanser to modify the smear

layer bond strengths were approximately 10% to 30% as strong as etched

enamel to resin bonds representative brands: Bondlite (SDS/Kerr),

Creation Bond (Den-Mat), Prisma Universal Bond (Caulk), and

Scotchbond(3M)

These were based on the isocyanate group of the polyurethane polymer that bonds to various groups in dentin including carboxyl, amino, and hydroxy groups.

Most used diisocyanates which simultaneously bonded to both the dentin and composite resin.

The polyurethane's setting reaction was unaffected by the presence of fluid in the dentin tubules or smear layer.

Most of these systems left the smear layer intact, however some employed hydrogen peroxide for cleansing.

Representative brand: Dentin-Adhesit (Ivoclar Vivadent)

These systems alter or remove the smear layer prior to bonding and produce bond strengths ranging from 16 to 26 Mpa.

Some of the products produce bond strengths approaching those formed to enamel. Clinical retention rates of 100% at 2 years have been reported.

Most products use a three-component system consisting of a:

1. Conditioner

2. Primer

3. Adhesive.

It is usually a weak organic acid (e.g., maleicacid), a low concentration of a stronger inorganic acid (e.g., phosphoric or nitric acid), or a chelating agent (e.g., EDTA).

Main Actions:▪ Heavily alters or removes the smear layer

▪ Demineralizes peritubular and intertubular surface dentin and, thereby, exposes collagen fibrils

▪ Demineralizes up to a depth of 7.5 microns (depth of demineralization depends on type of acid, its concentration, and etching time). More mineralized peritubular dentin is etched more deeply than the intertubular dentin

▪ Increases dentin permeability by 4 to 9 times.

It is usually a bifunctional monomer in a volatile solvent such as acetone or alcohol.

A bifunctional monomer is one that has a hydrophilic end (i.e., one with an affinity for water) and a hydrophobic end (one lacking an affinity for water).

Examples of bifunctional monomers include HEMA (hydroxyethyl methacrylate), NMSA (N-methacryloyl-5-aminosalicylic acid), NPG (N-phenylglycine), PMDM (pyromelliticdiethylmethacrylate), and 4-META (4-methacryloxyethyl trimellitate anhydride).

Main Actions:▪ Links the hydrophilic dentin to the hydrophobic adhesive

resin; is able to do this because of its bifunctional nature

(i.e., primer's hydrophilic end bonds to the wet dentin and

its hydrophobic end bonds to the adhesive resin).

▪ Promotes infiltration of demineralized peritubular and

intertubular dentin by its own monomers and those of the

adhesive resin.

▪ Increases wettability of the conditioned dentin surface

and increases contact between the dentin and resin.

It is an unfilled or partially-filled resin; may

contain some component of the primer (e.g.,

HEMA) in an attempt to promote increased bond

strength.

Main Actions:▪ Combines with the primer s monomers to form a resin-reinforced

hybrid layer ( resin-dentin interdiffusion zone ) from 1 to 5 microns

thick.

▪ Forms resin tags to seal the dentin tubules.

▪ Provides methacrylate groups to bond with the subsequently placed

resin composite.

The next generation of dentin bonding systems appeared in the early 1990s and is still widely used.

Most of these systems are based on the "total-etch" technique, or simultaneous etching of enamel and dentin, typically with phosphoric acid. Improvement in dentin bond strengths by etching was first demonstrated by Fusayamain 1979 but the concept of total-etching only recently gained acceptance in the United States.

Etching of dentin traditionally was discouraged because data from early studies seemed to indicate that phos-