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        ENAMEL BOND

Bonding to enamel has been done for over 40 years.  Enamel consists of organic and inorganic components.  The inorganic component, hydroxyapatite, varies from 86% to 98% depending on the age of the enamel.  Application of 37% phosphoric acid removes about 10 microns of enamel to expose prisms of enamel rods and create the classic honeycomb effect.  Acid also increases surface energy for better wetting of the enamel.  Resins flow into micromechanical retentive areas. Resin tags fill microscopic holes to provide retention.  Retention is about 30 MPa.. 

Acid etching is done for a minimum of 20 to 30 seconds.  Etching is most effective when the acid is activated by movement such as mixing.  Thorough rinsing for 10 seconds removes acid and dissolved calcium phosphates.  37% Phosphoric acid is used to enamel etch, however, other acids including nitric, polyacrylic, oxalic, pyruvic and maleic acids are effective.  Use of acids for self etching materials on dentin often is much less effective on enamel.