bonding agents in dentistry

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HISTORICAL PERSPECTIVE AND CURRENT STATUS OF BONDING AGENTS

NIRAJ KUMARJR-PROSTHODONTICS

AIIMS-NEW DELHI

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contents

• HISTORY• DEFINITIONS• IDEAL REQUIREMENTS• COMPONENTS• BONDING TO ENAMEL & DENTIN• GENERATIONS OF DBA• CURRENT STATUS• ROLE OF SMEAR LAYER • EXPANDED CLINICAL INDICATIONS• SUMMARY• REFERENCES

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HISTORY

Major turning points and events in dental bonding technologies

Historical evolution of bonding agents is an ongoing process which dates back to late 50s in a continuous effort to improve the bonding of the restoration to the tooth snd simplify the clinical steps.

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DEFINITIONS• Dentin conditioner - An acidic agent that dissolves the

inorganic structure in dentin, resulting in a collagen mesh that allows infiltration of an adhesive resin.

• Hybrid layer - An intermediate layer of resin, collagen, and

dentin that is produced by acid etching of dentin and infiltration of resin into the conditioned dentin.

• Primer - A hydrophilic, low-viscosity resin that promotes

bonding to an adherend substrate, such as dentin.

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• Resin tag - Extension of resin that has penetrated into etched

enamel or dentin.

• Smear layer - Poorly adherent layer of ground dentin

produced by cutting a dentin surface; also, a tenacious deposit of microscopic debris that covers enamel and dentin surfaces that have been prepared for a restoration.

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- Requirements for a successful dentin bonding system :

a) Adequate removal or dissolution of the smear layer from enamel and dentin.

b) Maintenance or reconstitution of the dentin collagen matrix.

c) Good wetting.d) Efficient monomer diffusion and penetration.e) Polymerization within tooth structure.f) Copolymerization with the resin composite matrix.

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COMPONENTSIrrespective of the number of bottles or components, a typical dentin bonding system includes : - etchant - relatively strong acids (pH:1-2) -removes smear layer - gel (colloidal silica) – precise placement -adhesive bisGMA, UDMA - primer - maintains expanded collagen network -hydrophillic monomers (phosphate, carboxylic acid, esters) in solvents. [HEMA , 4-META] - solvents used for primers (water, ethanol & acetone) - initiators (photo ,chemical or dual cure) - fillers silica(40nm)- reinforce - other ingredients - glutaraldehyde - desensitizer - MDPB & parabene – AMA - F and CHX (prevents collagen degradation)

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BONDING TO ENAMEL & DENTIN

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Bonding to enamel

- Acid etching transforms the smooth enamel into an irregular surface and increases its free surface energy.

Bonding agents (resins) penetrate into the surface (capillary action).

Monomers polymerize and become interlocked

-Formation of resin microtags – is the fundamental of resin-enamel adhesion.

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- Enamel etching results in 3 different micro-morphologic patterns:

a) Type I - dissolution of prism cores without involving the

prism peripheries.

b) Type II - dissolution of peripheral enamel with the rods intact.

c) Type III - less distinct than the other two patterns (a combination of the two types)

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(Type I) (Type II) (Type III)

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- In either of the cases the resin tags are approximately - 6µm (diameter) - 10-20 µm (length)

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concentration & time ( two important parameters )

• Buonocore – 85%• Gwinnet – suggested the use of lower concentrations to prevent the formation of precipitates.

- a number of acidic agents have been used to produce the required microporosity.

However , phosphoric acid at a concentration between 30% and 50% , (typically 37%) is the preffered etchant.

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>50%

Deposition of an adherent layer

monocalcium phosphate monohydate

Prevents further dissolution

<27%

Creates a dicalcium phosphate monohydrate precipitate

Cannot be removed easily

Interferes with adhesion.

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- An etching time of 60 seconds was originally recommended for concentrations between 30% to 40%.

- studies using scanning electron microscopy showed

that a 15 sec etch resulted in a similar surface roughness as that provided by a 60 sec etch.

- other in vitro studies have shown similar bond strengths and microleakage for both 15 and 60 sec.

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Bonding to dentin - Adhesion to dentin is relatively difficult.

- Dentin adhesion relies primarily on - the penetration of adhesive monomers into the filigree of collagen fibers left exposed by acid etching.

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Challenges in dentin bonding

- consists of a substantial proportion of water and organic material (type I collagen)

- dense network of tubules connecting the pulp with the DEJ.

- cuff of hypermineralized dentin (peritubular) lining the dentinal tubules.

- Presence of smear layer .

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Moist versus dry dentin surfacesIf the dentin suface is:

a) air dried -the collagen undergoes immediate collapse (spatial alteration) prevents resin monomers from penetrating the nanochannels formed by dissolution of hydroxyapatite crystals between collagen fibers.

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b) Pooled moisture on dentin surface:

- excess water dilutes the primer and renders it less effective.

c) Glistening hydrated surface:

- ideal condition for bonding.

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EVOLUTION OF DENTIN BONDING AGENTS

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FIRST GENERATION - surface active comonomer NPG-GMA (N- phenylglycine glycidyl methacrylate)

-chelates with calcium of tooth structure and generates water resistant chemical bonds.

-aimed only to bond with calcium ion and not the organic matrix (collagen).

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-- Since they could bond with calcium ions in tooth structure, they form stronger bonds with enamel than dentin.

- role of smear layer was ignored.• example - Cervident

Amino-carboxylate based bonding agent

Calcium ion in the tooth structure

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LIMITATIONS : - Low bond strength: 2-3Mpa. - Loss of bond strength over time. - magnetic resonance analysis showed no ionic bond formation

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SECOND GENERATION

• Clearfil Bond F - first product - introduced in Japan (1978)

- phosphate ester material. - phenyl-P and HEMA in ethanol solution.

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- polar interaction between the : - negatively charged phosphate group in resin and - positively charged calcium in smear layer..

Example: scotchbond, bondlite and prisma universal.

LIMITATIONS:

-loosely attached smear layer - weakest link -low Bond Strength : 1-5MPa

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THIRD GENERATION

- Introduced in 1979. -- designed not to remove the entire

smear layer, but rather to modify it and to allow penetration of acidic monomers.

- treatment of smear layer with acidic primer using an aq.solution of - 2.5%maleic acid - 55% HEMA - trace- methacrylic acid

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Scotchbond-2 - first DBA to receive “provisional” and “full acceptance” from ADA.

- overall aim of 3rd gen DBA was - preservation of a modified smear layer, with - slight demineralization of the underlying intertubular dentin.

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• LIMITATIONS:

- because of the hydrophobic nature of bonding agents , acid etching did not produce the significant improvement in dentin bond strength inspite of flow of resin into open dentinal tubules.

- Pulpal inflammatory responses were thought to be

triggered due to application of acid.

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- Early DBA were based on the successful model of silane coupling agents. (used in composites to bond the inorganic -- filler to the matrix resin)

M R XUNSATURATED

METHACRYLATE GROUP

-capable of copolymerizing with the composite resin.

GROUP

-that is capable of chemically reacting with the siliceous

substrate.

R

-is a spacer group that ensures mobility of the M group after

the X group has been immobilized by reaction.

Composite Tooth

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( γ-methacryloxypropyl trimethoxysilane) organosilane

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Changing concepts

- organization into generations is somewhat artificial to mark key advances in materials and techniques along a more-or-less continuous developmental pathway.

- a more logical and straightforward classification for contemporary adhesive systems based rather on their

- mechanism of adhesion and the - number of clinical steps involved.

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- developed by Van Meerbeek et al. (2003).

-is based on approaches to etching , priming, and application of bonding resin to dentin and enamel

& further subdivided into the number of steps in the process.

Thus, the major categories of bonding systems are known as: - “etch-and-rinse” and - “self-etch” systems, - with two subcategories, each according to the number of

clinical steps involved.

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Etch & rinse adhesives

Self etch adhesives

Three step (4th gen)

Two step (5th gen)

Two step (6th gen)

One step (7th gen)

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Three-step (fourth generation)- the most established, most reliable adhesion method - consists of three steps: (1) an acid etchant application,(2) application of the primer, and (3) application of the actual bonding agent .

-The primer contains hydrophilic functional monomers dissolved in an organic solvent such as acetone, ethanol, or water.

E P A

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Two-step (fifth generation)

- simplified method - this category combines the primer and adhesive resin into

one application. This etch-and-rinse strategy is the most effective to achieve

efficient and stable bonding to enamel.

E P A

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Two-step (sixth generation):- does not involve a separate etching step.

- an acidic monomer which is not rinsed, is used to condition and prime the tooth at the same time.

EP A

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- There are two types of self-etch adhesives : - mild and - strong - Strong self-etch adhesives -- have been documented with a

bonding mechanism that resembles the etch-and-rinse adhesives.

- Mild self-etch adhesives -- only partially dissolve the dentin surface, so a substantial amount of hydroxyapatite remains available within the hybrid layer.

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One-step (seventh generation)- this category combines conditioner, primer, and bonding resin

into a single step.

-Most one-step or “all-in-one” systems are delivered by a bottle, vial, or single-unit dose applicator, which are formulated as a single component.

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One-step, self-etch adhesives are an attractive approach for clinicians because of the : - reduced and less complex number of clinical steps required . - there is no need for rinsing or drying of the tooth structure.

E P A

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Current status - At this time, products in the one-step self-etch category

have undergone limited clinical experience and

consequently not enough is known concerning their performance and bond durability under long-term clinical conditions compared with the two-step self-etch and etch and- rinse products.

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- Recently, however, - van Landuyt et al.(2011)

showed in a randomized clinical trial that a one step, self-etch adhesive had similar clinical performance after 3 years compared with that of an etch-and-rinse adhesive in class V restorations.

However, the one-step group exhibited more incisal marginal defects and discoloration compared with the etch-and-rinse group.

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Adhesive system Enamel (Mpa) Dentin (Mpa)Older systems

Second generation 10-20 2-4

Third generation 10-30 3-13

Current options

Total-etch three steps 20-50 13-80

Total etch one bottle 20-45 3-75

Self-etch primer systems 5-35 10-75

All-in-one self etch adhesives

0-35 0-60

Bond strengths of several generations of adhesives

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SMEAR LAYER --- when enamel and dentin tissues are mechanically

cut, especially with a rotary instrument, a layer of adherent debris and organic film known as a smear layer is left on their surfaces and prevents strong bonding.

-Different quantities and qualities of smear layer are produced by the various cutting and instrumentation techniques,

for example, - during cavity or - root canal preparation.

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In dentin, -- the smear layer becomes burnished into the underlying dentinal tubules and -- lowers dentin permeability, which is a protective effect.

However, it is also a very weak cohesive material and interferes with strong bonding. - various cleaning or treatment agents and procedures

are employed to either remove the smear layer or enhance its cohesive strength and other properties.

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- application of acid is used to remove the smear layer from both enamel and dentin.

Alternatively, in dentin the smear layer can be left partially in place and modified such that adhesive resins penetrate through it and bond to the intact dentin structures below.

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Expanded clinical indications

• Desensitization - GLUMA desensitizer (aq. Sol of 5% glutaraldehyde and - 35% HEMA) • Adhesive amalgam restorations (amalgabond) - marginal discoloration -secondary caries & -pstoperative sensitivity• Indirect adhesive restorations

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SUMMARY- Reliable bonding of resins to enamel and dentin has

revolutionized the practice of dentistry.

- Improvements in dentin bonding materials and techniques are likely to continue.

- Even as the materials themselves become better and easier to use , however, proper attention to technique and a good understanding of the bonding process remain essential for clinical success.

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REFERENCES

• PHILLIPS’ Science of dental material; 12th ed.

• Strudevant’s Art & science of operative dentistry; 4th ed.

• Craig’s – restorative dental materials ;13th edition.

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Thank you