DENTIN BONDINGMANASI MENONFOURTH YEAR- PART ONE

WHAT ARE BONDING AGENTS?

Bonding agents can be defined as materialS of low viscosity, when applied on the tooth surface forms a thin film after setting.

This thin film strongly bonds to the tooth surface, on which the viscous composite restorative resin is applied. This after setting forms an integrated resin restoration.

CHALLENGES IN DENTIN BONDING

Adhesive material interaction- mechanically, chemically or both.

Mainly relies on the penetration of adhesive monomer into the filigree of collagen fibers left exposed by acid etch technique.

Greater challenge- water, organic material, type one collagen , dense network of tubules that connect the pulp with DEJ.

Tubules are lined by peritubular dentin. Intertubular dentin penetrated by

submicron channels- allow passage of tubular fluid.

Intrinsically hydrated and movement of fluid from pulp to DEJ due slight constant pressure.

ENAMEL

MINERAL 88%WATER 10%ORGANIC 2%

Tubules enclose cellular extensions from the odontoblasts- in direct communication with pulp.

45,000/mm² tubules - closer to pulp ( 22%), 22,000/mm²- near DEJ ( 1%)

Bond strength less in deep dentin- 4- META MONOMER( not affected by dentin depth).

Formation of smear layer leads to smear plugs in dentinal tubules that decrease dentin permeability by 86%.

Smear layer- hydroyapatite and altered denatured collagen, its removal by acidic solutions results in increased fluid flow onto the exposed dentin surface( interference with adhesion)

Additional factors- vasoconstrictors in LA that dec. fluid flow and pulpal pressure, radius and length of tubules, viscosity of dentinal fluid.

STRESSES AT RESIN DENTIN INTERFACE

Polymerization shrinkage of composites creates stress within composite mass-7MPa

Configuration factor influences stress relief. number of bonded surfaces

number of un-bonded surfaces Stress relief depends on the number of free

surfaces Unrelieved stresses- internal bond disruption,

marginal gaps and microleakage

Immediate bond strengths of 17MPa- to resist contraction stresses, water sorption by resin compensates for the polymerization shrinkage, directly proportional to resin content.

Wide temp. variations- restoration undergoes volumetric changes

Linear coefficient of thermal expansion- 4 times greater for resin

DEVELOPMENT

BEGINNING

1950- Resin containing glycerophosphoric acid dimethacrylate (GPDM) could bond to hydrochloric acid etched dentin surface.

Primitive technique, bond strength low.

FIRST GENERATION ‘CERVIDENT’ NPG- GMA, surface active

comonomer. Comonomer could chelate with Ca on

the tooth surface to generate water resistant chemical bonds of resin to dental Ca.

Bond strenths of 2-3 MPa , poor clinical results

SECOND GENERATION Clearfil bond system- Japan First product, contained phosphate-

ester material( phenyl-P and HEMA in ethanol)

MOA: polar interaction between negatively charged phosphate groups in the resin and positively charged Ca ions in the smear layer.

In vitro bond strengths of 1-5 MPa Did not wet dentin well and did not

penetrate the entire depth of smear layer and could not reach the superficial dentin to establish ionic bonding or resin extensions into dentinal tubules.

Bonding material tended to peel from dentin surface after water storage

Clinically unacceptable after two yrs of placement.

THIRD GENERATION Concept of phosphoric acid etching of

dentin before applying a phosphate ester- type bonding agent, hydrophobic nature of bonding resin resulted in no significant increase of bond strength.

Pulpal inflammatory responses. Clearfil New Bond- long hydrophobic

and short hydrophillic component.

Removal of entire smear layer not seen butmodified and allowed penetration of acidic monomers, such as phenyl-P or PENTA.

Treatment of smear layer with acidic primers- using 2.5% maleic acid, 55% HEMA nad a trace of methacrylic acid

Scotchbond 2- first dentin bonding agent to recieve full acceptance from ADA.

CURRENT STRATEGIES FOR ADHESIONS OF RESINS TO DENTIN….

THREE-STEPTOTAL ETCH (E+P+B)

ETCHANT (E)Removes the smear layer.Exposes intertubuler and peritubular collagen.Opens tubules ina funnel configuration.

PRIMER (P)Bifunctional molecules.External surface of collagen fibrils enveloped.Reestablishes surface –free energy to levels compatible with a more hydrophobic material

BONDING AGENT (B)Mostly hydrophobic monomers, Bis-GMA; however can contain small % of hydrophillic monomers.Copolymerises with primer molecules.Penetrates and polymerizes into the interfibrillar spaces to serve as a structural back bone to hybrid layer.

ONE-BOTTLE TOTAL-ETCH(E+[PB])

ETCHANT (E)Removes smear layer.Exposes intertubuler and peritubular collagen.Opens tubbules in funnel configuration and decreases surface energy.

PRIMER + BONDING AGENT (PB)Penetrates into dentin tubules forming resin tags. First coat applied on etched dentin works as a primer- increases surface free energy of dentin.Second coat acts as bonding agent used in three step system-Fills spaces between the dense network of collagen fibers.

TWO-BOTTLE SELF ETCH ( [EP]+ B)

ETCHANT +PRIMER (EP)Self etching primer does not remove smear layer , fixes it and exposes about 0.5-1µm of intertubular collagen because of its acidity( pH- 1.2-2.0).Smear plug impregnated with acidic monomer, prepares pathway for penetration subsequently placed fluid resin into microchannels .

BONDING AGENT (B)Same type of bonding agent included in three -step, total-etch systems.The resin tags form on resin penetration onto the microchannels of the primer-impregnated smear plug.

ALL -IN -ONE SELF ETCH (EPB)

Etches enamel.Incorporates the smear layer into the interface.Being an aqueous solution of a phosphated monomer, it de-mineralizes and penetrates dentin simultaneously, leaving a precipitate on the hybrid layer.Forms a thin layer of adhesive, leading to low bond strengths; a multi-coat approach is recommended.Incompatible with self-cure composite resins.

HYBRID LAYER

Zone where the adhesive resin of the dentin bonding agent micromechanically interlocks within the intertubular dentin and surrounding collagen fibres. Formed in the following manner:

A) Etching removes the smear layer, exposure of collagen fibrils seen, hydroxyapatite within the intertubular dentin removed.

B) Primers penetrate collagen network.C) Resin microtags formed by adhesive resins and

primers within the intertubular dentin and surround the collagen fibres upon cooling.

MOIST VERSUS DRY DENTIN SURFACES

Vital dentin is inherently wet, water is considered as an obstacle for attaining an effective adhesion of resins to dentin.

“Moist bonding technique” ( combination of hydrophillic and hydrophobic monomers in same bottle dissolved in an organic solvent like ethanol or acetone) prevents collagen collapse.

The solvents help in displacing water from dentin surface and the moist collagen network and promotes infiltration of resin monomers.

Pooled moisture- excess water on tooth renders dilution of primer making it less effective.

Glistening hydrated surface preferred Usually tooth preparation dried to

check for classic etched enamel appearance but, dentin collagen collapses easily on air drying, resulting in closing of the micropores in the exposed intertubular collagen

Acetone- based, water free dentin bonding systems require rewetting of etched dentin surface with aqueous rewetting agents that restore bond strength values and raise the collapsed collagen network.

Collapse of collagen network due to change in molecular arrangement. Extra-fibrillar spaces being fewer in dried collagen, results in less penetration of monomers.

Rewetting dentin after air drying to check for frosty appearance is an acceptable procedure.

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