Dental Biomaterials I - Adhesion Concepts

Dental Biomaterials I - Dental Adhesion

• Course Title: Dental Biomaterials I
• Course Code: BDS011
• Topics to be covered include: basic concepts and definitions, importance of adhesion in dentistry and dental considerations, surface tension and surface energy, classification of adhesion, factors affecting the strength of an adhesive junction and method of testing, bonding to enamel and dentin.

Adhesion and Cohesion

• Adhesion: Bonding between dissimilar atoms and molecules.
• Cohesion: Bonding between similar atoms and molecules (e.g., gold foil dental restoration)

Adhesive and Adherend

• Adhesive: Liquid material used to produce adhesion.
• Adherend: Solid substance to which the adhesive is applied.
• Key to adhesion: materials must be in close contact, adhesive must be in liquid state with low viscosity, and produce a thin layer.

Types of Adhesion

• Chemical (true adhesion): Chemical bonding between dissimilar materials (e.g., glass ionomer and zinc polycarboxylate with the tooth). Glass ionomer and polycarboxylate cements contain COOH groups that react chemically with calcium in the tooth.
• Mechanical (macro-mechanical & micro-mechanical): Bonding between dissimilar materials through mechanical interlocking. No actual bond is formed. Examples include implants, amalgam, composite, and zinc phosphate cement.
• Physical: Weaker than chemical, using physical force.
  • Example: complete denture.

Factors Affecting Adhesive Junction Strength

• Wetting: Ability of adhesive to spread over the surface of the adherend.
  • Measured by contact angle:
    • Θ > 90°: Poor wetting
    • Θ < 90°: Good wetting
    • Θ = 0°: Excellent wetting
  • Factors affecting wetting include surface energy, surface tension, viscosity, and surface irregularities (roughness) of the adherend.
  • Surface energy: Force of attraction between surface atoms of the solid. Surface atoms have higher energy than atoms inside.
  • Surface tension: Force of attraction between surface atoms of a liquid. Atoms on the surface are strongly attracted to the interior. Increasing surface tension of adhesive decreases wettability.
• Stresses due to setting contraction of adhesive
• Thermal stresses: Large difference in coefficient of thermal expansion and contraction between adhesive and adherend can produce stresses in the bond. Needs close matching to minimize these stresses and increase the strength of adhesion.
• Cleanliness of the adherend: Debris or surface contaminations prevent intimate contact, essential for adhesion. Adhesion to clean and dry surfaces (enamel and dentin) is better than to wet, contaminated surfaces.
• Thickness of adhesive: Thinner adhesive films lead to stronger adhesive junctions with fewer air voids and less thermal stress due to setting contraction.
• Type of bond formed: Primary bonds between adhesive and adherend produce stronger adhesion compared to secondary bonds (e.g., soldered joint stronger than glued joint).

Failure of Adhesive Junction

• Adhesive failure: Adhesive-adherent separation
• Cohesive failure of the adhesive
• Cohesive failure of the adherend

Obstacles to Adhesion in the Oral Cavity

• Inhomogeneous composition of enamel and dentin: Enamel and dentin are comprised of organic and inorganic parts; adhesion is not uniform over the entire surface.
• Surface irregularities in the prepared cavity: Prepared cavity surfaces are full of pits and fissures, resulting in increased roughness, making it difficult for adhesives to flow and wet the entire surface.
• Debris in prepared cavity: Debris prevents intimate contact, essential for adhesion.
• Presence of water in the prepared cavity: Water from dentinal tubules forms a film preventing intimate contact between the adhesive and tooth.

Bonding to Tooth Structures

• Enamel bonding involves acid etching, bonding adhesive application, and composite filling.
• Acid etching (30-50% phosphoric acid for 30 seconds) removes 5 um of enamel and produces micropores, increasing surface energy and area for adhesion.
• Application of adhesive and composite filling completes the process.
• Dentin bonding presents greater obstacles due to higher water content, presence of a smear layer, and lower surface energy of dentin; it requires etching, primer application, bonding agent, and final composite application to complete the process.

Formation of Hybrid Layer

• Resin infiltrated dentin, a resin-reinforced layer part tooth and part resin.

Additional Notes

• Image descriptions and detailed materials are omitted per prompt instructions.