Dental Bonding: Enamel vs Dentin

Enamel and Dentin Bonding

• Enamel is a highly mineralized tissue composed of more than 90% hydroxyapatite.
• Dentin contains a substantial proportion of water and organic material, primarily type I collagen.
• Dentin has a dense network of tubules that connect the pulp with the dentinoenamel junction (DEJ).
• Peritubular dentin lines the tubules and is a cuff of hypermineralized dentin.
• Intertubular dentin contains collagen fibrils and is less mineralized.

Smear Layer

• Residual organic and inorganic components form a ‘smear layer’ of debris on the surface after tooth preparation.
• The smear layer fills the orifices of dentin tubules, forming ‘smear plugs’ and decreases dentin permeability by nearly 90% in vitro.
• The composition of the smear layer includes hydroxyapatite and altered denatured collagen.

Enamel Adhesion

• Phosphoric acid etching facilitates adhesion of paints and resins to metallic surfaces.
• Phosphoric acid gels with concentrations of 30%–40% are used for etching, with 37% phosphoric acid gel being the most common.
• The recommended etch time is 15 seconds.
• Shear bond strengths of composite to phosphoric acid-etched enamel usually exceed 20 Mpa.

Resin Microtag Mechanism of Enamel Adhesion

• Acid etching transforms the smooth enamel into an irregular surface and increases its surface free energy.
• Penetration of a fluid resin-based material into the irregular etched surface facilitates adhesion.
• Monomers in the material are then polymerized, and the material becomes interlocked with the enamel surface.
• Formation of resin microtags within the enamel surface is the fundamental mechanism of resin–enamel adhesion.

Dentin Adhesion

• Bonding to dentin presents a much greater challenge.
• Adhesion is the state in which two surfaces are held together by interfacial forces.
• Four possible mechanisms of adhesion in dentistry include:
  • Mechanical—penetration of resin and formation of resin tags within the tooth surface
  • Adsorption—chemical bonding to the inorganic component (hydroxyapatite) or organic components (mainly type I collagen) of tooth structure
  • Diffusion—precipitation of substances on the tooth surfaces to which resin monomers can bond mechanically or chemically
  • A combination of the previous three mechanisms

Uses of Adhesives

• Restore Class I, II, III, IV, V, and VI carious lesions or traumatic defects.
• Change the shape and color of anterior teeth (e.g., with full or partial resin veneers).
• Improve retention for porcelain-fused-to-metal (ceramometal) or metallic crowns.
• Bond all-ceramic restorations.
• Seal pits and fissures.
• Bond orthodontic brackets.
• Bond splints for tooth luxations and periodontally involved anterior teeth and conservative tooth replacement prostheses.
• Repair existing restorations (composite, amalgam, ceramic, or ceramometal).
• Provide foundations for crowns.
• Desensitize non-carious cervical lesions (NCCLs) and exposed root surfaces.
• Bond fractured fragments of anterior teeth.