BONDING AGENTS

Let’s know why was it introduced..

Bonding agents were introduced to improve or to create the bond between the tooth and the Restorative material.

Which was the first bonding agent ??

It’s sevriton cavity seal. It’s based on glycerophosphoric acid dimethacrylate. It had limitations like high polymerization shrinkage and high thermal expansion.

Composition of a dentin bonding agent:-

Primer + Etchants + adhesive , solvents, initiators, fillers particles and others ingredients like polyalkenoic copolymer

Use of primer it’s to wet the surface properly or to reduce the contact angle. They maintain an expanded collagen network to allow filtration of hydrophobic monomer as well. Ex:- HEMA( Hydroxy ethyl methacrylate ) and TEGDMA ( Tri ethanol glycol dimethacrylate ).

Conditioners are also called as Etchants. 10% Malic acid for 1 minute , and 37% phosphoric acid for 15 sec and if the concentration of phosphoric acid exceeds more than 50% then it forms monocalcium phosphate monohydrate which inhibits dissolution. Etching is done for both enamel and dentin.

Mode of action of etchant on enamel :- by selective dissolution of enamel rods at the centre or peripheries or both. They act by increasing the surface energy and increases the surface area which creates microporosities on the surface of enamel.

Mode of action of etchant on dentin :-It involves removal of smear layer and opening of dentinal tubules

Smear layer :- During tooth preparation the cut material along with water forms a thin film on the floor of the cavities known as smear or debris layer

Smear layer is it desirable? ?

In dentin the smear layer becomes burnished into the underlying dentinal tubules and it lowers the dentin permeability which has a protective effect.

Developing a bonding agent for enamel was easier than those of dentin, What would be the reason ?

Dentin contains water and due to inhomogeneous composition and presence smear layer will be a problem, whereas for enamel it contains more inorganic content that is calcium hydroxyapatite crystals.

The purpose of using solvents is in order to increase the diffusion of primers and adhesive into micro retentive tooth surface and most common solvents used are ethanol and acetone.

Adhesives in 1st generation are GPDM, 2nd generation BISGMA, TEGDMA, 3rd generation NPG- GMA,4th generation NPG-GMA, HEMA, 5th generation PENTA , Methacrylated phosphonates, 6th and 7th generation methacrylated phosphonates in water

The etchant generally was 37% phosphoric acid almost for all generations; and primer was HEMA.

Initial generations had all the components primer+ etchant + adhesive separately; as time progressed new generations evolved with all in one bonding agent.

• 2 step method :- 5th and 6th generation; 1 step method :- 7th generation ( all in one system )

• Indications for use of bonding agent:- When composite resin is used as Restorative material; And even when the porcelain veneers are bonded and when the exposed dentin is to be desensitized.

• Conditions that satisfy the true adhesion of Restorative material with tooth structure are sound tooth structure must be conserved; optimal retention must be achieved, microleakage should be prevented.

Hybrid layer :- Structure formed in dental hard tissues by deminerlization of surface followed by infiltration of monomers into collagen mesh and subsequent polymerization is called hybrid layer. It was reported by Nakabayshi in 1982.

Reference :- Philips and Manapalil text book of dental materials

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