Adhesion and Dentin bonding agents

Contents HistoryAdhesion - types

- factors affectingIdeal requirementsAdhesion to enamel Bonding to dentin and its problemsConditioning of dentinPrimersSmear layerHybrid layerClassification of dentin bonding agentsConclusion

Introduction

History –

1955 - Buonocore acid etching with phosphoric acid

1956 - Buonocore developed the first dentin bonding agents.

1957 - Bowen gave BISGMA resin system

1982 - Bowen, Cobb and Rapson developed multilayer adhesive system

Nakabayashi reported the presence of hybrid layer

Adhesion – latin - to stick

It is the force or the intermolecular attraction that exists between molecules of two unlike substances when placed in intimate contact with each other.

Adhesive Adherend

Types of adhesion –

1. Chemical - based on primary valence forces – covalent, ionic, metallic bonds2. Physical - based on secondary forces – Van der Waals forces,

hydrogen bonds, dispersion forces( specially when surfaces are smooth and polished )

3. Mechanical - based on penetration of one material into another at microscopic level. ( when surfaces are rough )

Factors affecting adhesion –

1. Surface energy –

2. Wetting -

3. Contact angle -

Adhesion to enamel – mainly by micromechanical method

Acid etchingThe procedure that leads to removal of smear layer and increase the microscopic roughness by removal of prismatic and the interprismatic mineral crystals.

Effects of acid etching –

1. Removes residual pellicle2. Creates a porous layer of depth 5 – 10 microns3. Increases wettability4. Increases surface area5. Increases surface energy

Bond strength to acid conditioned enamel 16 – 21 M Pa.

Various etchants used –

1. Phosphoric acid – as liquid or gel

)

2. Sulphuric acid 2 %

3. Phosphoric acid 10 %

4. Maleic acid 10 %

5. Nitric acid 2.5 %

6. Oxalic acid

7. Citric acid

Teeth requiring longer application - acid resistant teeth

( with high F content)

primary teeth. ( more aprismatic )

Patterns of etching –

Type 1 – Most common.Preferential removal of enamel prism cores & periphery intact.

Type 2 - Reverse of type 1. Periphery removed and cores intact.

Type 3 – Etching pattern less distinct. Both types 1 & 2 present.

Macrotags – resin tags formed at periphery of enamel rods ( between prisms )

Microtags - smaller tags formed at end of rod where individual hydroxyapatite crystal has been dissolved.

More important because of greater number and surface area.

Problems in bonding to dentin –

Presence of 30% organic material

Random arrangement of hydroxyapatite crystals in the organic matrix.

Tubular nature of dentin that permits fluid flow, adversely affects bonding.

Sclerosed dentin is difficult to penetrate as peritubular dentin becomes and wider.

Permeability of dentin differs at various sites.

Variation in arrangement of tubules & their shape.

Ideal requirements of dentin bonding agents:

1. Provide high bond strength to dentin that should be present immediately after placement and that should be permanent.

2. Provide bond strength to dentin similar to that of enamel.

3. Show biocompatibility to dental tissue including the pulp.

4. Minimize microleakage at the margins of the restorations.

5. Prevent recurrent caries and marginal staining.

6. Be easy to use and minimally technique sensitive.

7. Possess a good shelf life.

8. Be compatible with a wide range of resins.

9. In addition it should be non toxic and non sensitizing to the operators or patients.

10. Bonding agents should seal the tooth surfaces from oral fluids.

SMEAR LAYER

Suggested by Skinner (1961)

Coined by Boyde et al (1963)It is a thin layer of grinded debris left on tooth during tooth preparation.Thickness – 12 micron metersCompositionDenatured collagen Hydroxyapetite & cutting debris.

Of 2 types : - Superficial – loosely adherent smear layer - Deep – tightly packed smear plug

Smear plugs-Thickness 1-5 microns metersPrevents bacterial invasion but does not prevent diffusion of bacterial toxins into the pulp .Decreases dentinal permeability

Composition of dentin bonding agents

ConditionersPrimersAdhesivesInitiators and acceleratorsFillersOther ingredients.

Conditioning of the dentinDefined as alteration done after the creation of

dentin cutting debris, termed the smear layer, to create a surface, capable of micromechanical and chemical bonding to dentin bonding agent.

Changes after conditioning:Removal or modification of smear layerOpening of dentinal tubule orifices. Modifications in organic matterDecalcification of inorganic portion.

Can be done by -ChemicalsAcidsCalcium chelators

ThermalLasers

MechanicalAbrasion

Acid washing / rinsingRemoves smear layerDissolves HA

Drying shrinks remainingCollagen polymer molecules

Rehydration / primingSwells collagen

Mechanism of dentine bonding

Monomer penetration

Mechanism of dentine bonding

Monomer polymerisationAnd collagen entanglement

1. Phosphoric acid : First dentin conditioner37% H3PO4 should be used. Use of more than 50% results in the formation of monocalcium phosphate monohydrate that is not readily soluble and would not be completely washed away.

Calcium chelators:Chelators are used to remove the smear layer

without decalcification or significant physical changes to the underlying substrate as opposed to strong acid etchants.

EDTA:- Removes smear layer, not plugs. - It is scrubbed on the surface of the smear layer for a few seconds, then left passively for another 60 seconds followed by additional scrubbing. Disadvantage:- Removes small amount of Ca, thus causing fall in the bond strength.

Primers

Adhesion promoting agents

Contains monomers with HYDROPHILIC properties which have affinity for exposed collagen fibrils &HYDROPHOBIC properties for copolymerization with the adhesive resin.

Monomers used are : HEMA, NPG-GMA.

XM R

PRIMER FORMULA :

Hydrophilic part: (REACTIVE GROUP)Hydrophobic part: (Methacrylate group) R- Spacer Adhesive molecule is

bifunctional X enters into chemical union with the tooth structure M Co-polymerizes to the resin through the double bond of methacrylate R- spacer group responsible for making the molecule large enough to keep the methacrylate groups spatially located for optimal chemical reaction with composite

The solvents used in primers

1. Acetone based primers – Advantages :

Excellent water chaser,Dries quickly

2. Ethanol based primers – Advantages :

Evaporates less quicklyLess sensitive to wetness of dentin.Good surface energy and good

penetration.3. Water based primers – Advantages :

Slow evaporationNot sensitive to wetness of

dentin. Have capacity to raise collapsed

fibers.(9%-50%)

Hybrid layer :Nakabashi -1982

Resin reinforced zone, or resin infiltrated layer

Hybrid layer is the structure formed in dental hard tissues by demineralisation of the surface and subsurface, followed by infiltration of monomers and subsequent polymerisation. The resulting structure is neither resin or tooth but a hybrid of both.Shag carpet appearance

Appears when dentin surface after being acid etched is actively scrubbed with an acidic primer solution.

The combined mechanical and chemical action of rubbing the acid etched dentin with an acidic primer dissolves additional mineral salts while fluffing and separating the entangled collagen at the surface.

Fillers:Most of the bonding agents are unfilled

May contain inorganic fillers 0. 5% to 40% by wt.

microfillers, or nanofillers, and submicron glass.

Advantages of nanofillers:

Prevents nanoleakage.Causes uniform thickness of adhesive

layerBetter flexibility to adhesivesBetter dissipation of forcesScatter of light is prevented.Provides radio opacity

Classification of Dentin bonding agents

1. According to generation

2. According to adhesion strategy (number of clinical applications)

3. According to treatment of smear layer. 4. According to mode of curing

5. According to Ph

I. On the basis of generations:

First Generation: By Bowen in 1965

Agents used: NPG GMA

Mechanism of adhesion: Chelation of NPG GMA with calcium on the tooth

surface to generate water resistant chemical bonds

of resin to dentinal calcium.

Bond strength : 2 – 3 MPa

Second Generation: Early 1980s

Agents used: BIS-GMA / HEMAPolyurethane based compounds

Interaction b/w negatively charged phosphate grps in the resin with positively charged calcium ions in the smear layer.

Bond strength: 1- 5 Mpa

Third Generation: Mid 1980s

Negative effect of smear layer known by then. Thus additional step of conditioning and priming to -

remove smear layermodify smear layer

Mechanism of adhesion: Conditioner removes smear layer and

opens up the tubules.

Advantages:Increased bond strengthImproved clinical performanceReduced microleakage relied on mechanical means of bonding as opposed to the less reliable chemical adhesion.

Bond strength: 9-15 Mpa.

Disadvantages:More number of steps, thus complexTechnique sensitiveRetention decreased with time ( longevity questionable )

Fourth generation – Universal bonding systems

Characterized by Complete removal of smear layer Total etch technique

( simultaneous etching of enamel and dentin. )

Mechanism of action Etching removes smear layer, opens the tubules upto 7.5 microns

and increases the permeability.Primer wets & penetrates the collagen meshwork and increases its

surface energy and wettability into which the resin flows.

Based on diffusion and impregnation of resin into partially decalcified dentin followed by polymerisation creating a resin reinforced layer

Hybrid layer

AdvantagesLess technique sensitivitysimilar bond strengths to both enamel and dentin

Bond strength: 18 Mpa

Fifth generation

Developed with aim towards reducing the total number of clinical steps. Characterized by combinations of primer and adhesive in one step.

( One bottle adhesive systems )

Mechanism of action : complex combination of micromechanical retention

Bond strength 17 – 24 MPa.

Sixth generation: Introduced in the late 1990s.

Here the primer and etchant are combined in one step.( Self etching primers).

Available in two types –two step self etching primerssingle step self etching primers ( all in one

systems )

Seventh Generation: One up Bond F

Fluoride releasing Self etching type Colour changing capacity (visual confirmation of complete curing) Characterized by etching, priming and bonding in a simple application with no rinsing or drying. Available in two bottles, which have to be mixed and applied to the cavity.

(A & B mixed) (Completely cured)

Yellow Pink White

II. Classification based on the number of clinical application steps

and how they interact with tooth structure

Total etch adhesives:3 step2 step

Self etch adhesives:2 step1 step

Resin modified glass ionomer adhesives

Total etch adhesives - Fusayama

Total etching is the simultaneous etching of enamel and dentin.

The ph of these formulations is between 2.5 to 4.5

to demineralise smear layers and etch dentin.

Hence a separate etch and rinse phase involved

Mechanism of action – ( hybrid layer formation )

Smear layer removed by etch and rinse phase,

3-5micron deep demineralization of the dentin surface.

Collagen fibrils are completely removed from hydroxyapatite

a microretentive network for micromechanical interlocking of monomers.

3 step Total etch: ( Fourth generation of dentin bonding agent )

ConditioningPriming Application of adhesive resin

Etchant Primer Adhesive

2 Step total etch ( Fifth generation of dentin bonding agent. )

ConditioningPrimer and adhesive

Etchant

Adhesive Primer and adhesive

Primer

Self etch adhesives -

These adhesive systems are based on the use of non rinse acidic monomers that simultaneously condition and prime dentin and enamel.

Self etching primer

EtchantPrimer

Adhesive

Two step self etching primer - ( sixth generation )

Advantages : . The self etching primers are designed to be used on dry dentin. Avoids the removal of smear plugs in the dentinal tubules and

the dentin bond strengths is comparatively high.

Disadvantages : Less effective bonding of enamel. Initial bond might deteriorate with ageing, which could lead to premature failures. Bonding to sclerotic and caries affected dentin might be problematic

All in one adhesive

EtchantPrimer Adhesive

Single step self etching primer

Clinical uses of dentin bonding systems:

Bonding of directly placed resin based restorative materials. Bonding of indirectly placed restorative materials. Bonding of ceramic restorations. Bonding of amalgam restorations. Repair existing restorations. Sealing of pits and fissures of posterior teeth. Treatment of cervical sensitive dentine. Reattachment of fractured tooth fragment

Advantage of single step SEPs :

Fast and easy to use.

Disadvantages:

Marginal staining and debonding may occur in some of the Systems.May not bond durably to enamel.Lack of polymerization..

VII. According to their mode of curing:

Chemical cure:Amalgabond plus

Light cure:One bond Gluma comfort bond

Dual cure:Clearfil liner bond 2VPrime and Bond NT dual cure.

Failures in bonding :

Can occur at various levelsbetween mineralised and demineralised dentinbetween demineralised dentin and bonding agentwithin layer of bonding agentbetween bonding agent and composite resin.

Thank you !!