Dental Adhesives and Etching Processes

Operative Dentistry - Chapter 4: Adhesion

• G.V. Black described restoration retention based on cavity design and undercut dentin. Additional tooth structure removal was needed for retention. Materials available at the time (e.g., amalgam) had limitations.
• Adhesive systems have significantly changed cavity preparation design, transitioning from "extension for prevention" to a caries lesion-centered approach. This approach aims to conserve tooth structure.
• Enamel and dentin bonding techniques have advanced significantly since 1955. Buonocore's 1955 clinical technique used diluted phosphoric acid to etch enamel, creating a microscopically roughened surface for better resin retention. The resin forms "tags" within the microporosities and then polymerizes.
• Adhesion/Bonding: Two surfaces held together by interfacial forces (valence or interlocking forces, or both).
• Adhesive/Adherent: A material (often a viscous fluid) that joins substrates, solidifying to transfer load between them.
• Adherend: The surface or substrate to which something is adhered.
• Adhesive Strength: The load-bearing capacity of an adhesive joint.
• Adhesive Failure: The bond fails at the interface between two substrates.
• Cohesive Failure: The bond fails within one substrate, not at the interface.
• Abbreviations commonly used in resin chemicals include: Bis-GMA, HEMA, TEGDMA, 4-META, UDMA, and Phenyl-P.

Requirements for Adhesion

• Good substrate wetting (low contact angle) and a clean substrate are essential for adhesion. Wetting is the expression of attractive forces between adhesive and substrate molecules. Cleanliness increases adhesion, and higher surface energy of the substrate results in better adhesion. Surface tension of the bonding agent must be less than the surface energy of the enamel or dentin.
• Contamination (e.g., saliva, blood) reduces substrate energy and impairs wetting.

Mechanisms of Bonding

• Micromechanical: Resin penetration and resin tag formation within the tooth surface.
• Adsorption: Chemical bonding to inorganic (hydroxyapatite) or organic (type-I collagen) components of tooth structure.
• Diffusion: Substances precipitate on tooth surfaces for mechanical/chemical bonding with resin monomers.
• Combination: A combination of the above mechanisms is frequently employed.
• A good wetting ensures good adhesion.

Advantages of Adhesion

• Conservation of tooth structure.
• Reinforcement of remaining tooth structure.
• Restoration retention.
• Reduction/elimination of marginal microleakage.
• Expansion of esthetic possibilities.

Clinical Indications for Adhesives

• Treatment of dentin hypersensitivity.
• Bonded amalgam restorations.
• Indirect adhesive restorations.
• Repair of porcelain/composite.
• Restoring caries/fractures
• Altering shape/color of anterior teeth
• Bonding orthodontic brackets
• Repairing fractured restorative materials

Classification of Adhesive Systems

• Based on Bonding Strategy:
  • Etch-and-rinse (Total-etch)
  • Self-etch.
  • Glass ionomer adhesive systems
• Based on Clinical Application Steps:
  • Three-step systems.
  • Two-step systems.
  • One-step systems.
• Based on Generations:
  • First generation
  • Second generation
  • Third generation
  • Fourth generation
  • Fifth generation
  • Sixth generation
  • Seventh generation

First Generation

• Hydrophobic monomers.
• Acid etching is only for enamel, not dentin.
• Self-cure bonding agents.
• Chemically composed of surface-active monomers (NPG-GMA).

Second Generation

• Phosphate ester monomers.
• Enhanced surface wetting.
• Chemical bond to calcium smear layer.
• Low bond strengths

Third Generation

• Modified/altered smear layer.
• Acidic, hydrophilic monomers.
• Removal of smear layer by chelating agent.

Fourth Generation

• Developed in the early 1990s.
• Based on total etch/moist bonding.
• Three-step application.
• High bond strength.
• Characterized by hybridization at the dentin/composite resin interface.

Components of Fourth Generation Adhesives

• Includes conditioners, primers, and adhesives

Sixth Generation Bonding Agents

• i. Self-etching primer and adhesive: Available in two bottles. Primer is applied to the adhesive. Water is the solvent.
• ii. Self-etching adhesive: Available in two bottles. A drop of each bottle is mixed and applied.

Advantages of Self-etching

• Reduces postoperative sensitivity.
• Faster and simpler technique.

Disadvantages of Self-etching

• Lower bond strength to enamel relative to dentin, often requiring selective etching for better enamel bond
• pH is inadequate to etch enamel effectively, sometimes leading to decreased bond strength on enamel
• High hydrophilicity

Seventh Generation

• Simplifies multiple sixth generation materials into a single bottle one-step system.
• Avoids mixing errors.
• Shows minimal postoperative sensitivity.

Etching

• Process of selective demineralization of substrate (enamel/dentin) to create spaces for micro-mechanical bonding.

Primer/Adhesive Promoting Agents

• Hydrophilic monomers that increase surface wettability.

Bonding Agents

• Hydrophobic monomers that penetrate the demineralized surface and polymerize for micromechanical retention.

Classification based on application steps

• Three-step system
• Two-step system

Etch time for various teeth

• Depending on fluoride and if it is primary tooth or permanent.

Dentin

• Heterogeneous structure of tubules connecting the pulp to DEJ.
• Peritubular and intertubular dentin.
• High protein content contributes to dentin's hydrophilic nature and low surface energy.

Smear layer

• Thin porous layer of hydroxyapatite and altered collagen.
• Forms when tooth surface is prepared, trapping debris.
• Acts as a barrier, decreasing dentin permeability.

Stresses at Resin/Dentin Interface

• Resin composites shrink during polymerization.
• Residual stresses can lead to bond disruption, marginal gaps, and microleakage.

Microleakage

• Passage of ions, molecules, bacteria, and/or toxins between restoration margins and tooth walls.
• Causes sensitivity and recurrent caries

Etching time on dentin (15 seconds)

• Removes smear plugs and layer
• Opens dentin tubules, increases wetness and sensitivity
• Demineralizes dentin

Drying

• Air-drying enamel leads to frosty appearance
• Air drying dentin reduces collagen volume by 65%
• Minimal blotting sufficient for dentin drying

Priming

• Applied after etching and before bonding.
• Increases surface wettability.
• Hydrophilic monomers (HEMA) dissolved in organic solvents.

Solvent-based Primers

• Acetone-based: Excellent on wet dentin
• Water-based: Good on dry dentin
• Ethanol/Alcohol-based: Mid-point between acetone and water

Bonding Resin Infiltration

• Hydrophobic monomers (Bis-GMA, UDMA)
• Some hydrophilic monomers to regulate viscosity.
• HEMA as a wetting agent

Enamel and Resin Infiltration:

• Infiltration and polymerization into etched pores, forming resin tags and hybrid layer.

Dentin Resin Infiltration

• Infiltration and polymerization of hydrophobic monomers into interfibrilar spaces. Also infiltration and polymerization of monomers into dentinal tubules.

Hybrid Layer / Resin Reinforcement Layer / Interdiffusion Zone

• An interlocking of resin within the demineralized dentin surface.

Technique Sensitivity of Etch-and-Rinse Adhesives

• Postoperative hypersensitivity due to smear layer removal & dentin fluid movement.
• Sensitivity to overwetting/overdrying.
• Risk of overetching. (Incomplete resin infiltration )causing sensitivity, nanoleakage, & hydrolytic degradation

Self-etch Approach (Etch-and-Dry)

• Eliminates separate etching step.
• Acidic monomers simultaneously etch and prime dentin.
• Eliminates the risk of over-etching.
• Easier technique and avoids rinsing step, reducing moisture contamination

Advantages of SE over TE

• Faster and simpler
• Technique-sensitive
• Spontaneous demineralization and resin infiltration
• Reduced postoperative sensitivity

Classification according to acidity

• Strong (pH > 1): Deep demineralization in enamel and dentin.
• Intermediate (pH ≥ 1.5): Hybrid layer formation with a partially mineralized base.
• Mild (pH = 2): Superficial demineralization leaving some residual hydroxyapatite.
• Ultra-mild (pH=2.7): Minimal demineralization.

2-Step Self-Etch

• Two-bottle system: self-etch primer, and bonding agent.
• Self-etch primer (SEP): Hydrophilic, acidic primer that etches and primes simultaneously.
• Bonding agent: Hydrophobic monomers.

1-Step Self-Etch

• Single-bottle "all-in-one" adhesive system.
• Combines etching, priming, and bonding in one step.

Advantages of 1-Step Self-Etch

• Easy to use.
• Fewer steps.

Disadvantages of 1-Step Self-Etch

• Possible low enamel bond
• Not stable in nature, prone to water degradation

Universal Adhesives (UAs)

• Single-bottle, no-mix systems.
• Useable in total-etch, self-etch, and selective-etch procedures.
• Multifunctional monomers.
• Require water and organic solvents.

Glass Ionomer Adhesives (GI)

• Self-adhering.
• Removes smear layer.
• Activating calcium ions for increased reactivity.
• Mechanical bond via hybridization.

Factors Affecting Adhesion

• Physiological effects of both dentin and enamel.

  • Surface Energy
  • Capillary attraction
  • Osmotic pressure (water movement).

• C-factor (ratio of bonded/unbonded surfaces)

• Post-restorative care includes maintaining proper oral hygiene.

• Technique of restoration and skill of operator (use of rubber dam)

• Constituents of temporary restorations

This quiz explores the intricacies of etch-and-rinse adhesives in dentistry, specifically focusing on enamel etching techniques and the bonding process with dentin. Test your knowledge on the effects of various factors on etching, advantages and disadvantages of adhesives, and the clinical significance of each step involved.