Dental Materials: Glass Ionomer Restoration

Glass Ionomer Restoration (1)

Glass ionomer cement (GIC) is a water-based restorative material formed by an acid-base reaction between calcium-fluoro-alumino-silicate glass powder and an aqueous solution of polyacid.
GIC references include: Phillips Science of Dental Materials 11th edition, Craig's Restorative Dental Materials 13th edition, and Art and Science of Dental Materials 6th edition (chapter 10).
Learning Objectives (ILOs) include determining the characteristics, indications, contraindications, and manipulation techniques for glass ionomer restorative materials.

Outline

Definition: A water-based material created by an acid-base reaction between calcium-fluoro-aluminosilicate glass powder and a polyacid solution.
Composition: Two main components:
- Acid-soluble glass powder (calcium-fluoro-aluminosilicate glass) - Essential ingredients are alumina and silica. Sodium (Na+), Potassium (K+), Calcium (Ca++), and Strontium (Sr++) are added for improved reactivity with polyacids. Fluorides are excluded from the glass structure to allow for free diffusion.
- Polyacid liquid (aqueous solution of polyacrylic acid co-polymerized with organic acids like maleic, itaconic, and tartaric acid).
Setting Reaction: A three-phase acid-base reaction:
- Acid Attack: Upon mixing, the acid dissociates into carboxylate anions (RCOO-) and hydrogen protons (H+). The H+ ions attack the glass surface. This releases the cement-forming metal ions (Ca++ and Al+++).
- Gelation (Initial setting): The surface layer of the glass powder reacts with the acid, transforming into a silica hydrogel. The glass core remains intact. Calcium ions react with carboxylate anions to form a water-soluble calcium polysalt matrix.
- Maturation & Hardening (Final setting): In the presence of an aqueous medium, additional attack by H+ ions on the silicate glass releases Al+++. The incorporation of Al+++ into the preformed matrix forms a water-insoluble calcium-aluminum-carboxylate gel. This gel is no longer susceptible to hydration or dehydration. The final set cement consists of unreacted glass particles, silica hydrogel, and a hydrated Ca/Al polysalt matrix.
Properties, Advantages, and Disadvantages:
- Hydration & Dehydration (Moisture sensitivity):
  - Hydration (Disadvantages): Dissolution of poly-salts; loss of adhesive potential; disintegration; surface erosion. These issues lead to decreased strength, weakened restoration, increased opacity, and increased susceptibility to staining or leakage.
  - Dehydration (Disadvantages): Surface micro-cracks leading to weakened/weakened restoration and increased susceptibility to staining/leakage. It is thus vital to avoid moisture contamination.
- Biocompatibility: GIC displays resistance to bacterial plaque due to fluoride release, it is composed of weak organic acids, and its high molecular weight limits its diffusion through the dentinal tubules into the pulp. Minimal temperature rise during setting.
- Bonding to Tooth Structure: Chemical bonding with calcium phosphate crystals and hydrogen bonding with collagen matrix in dentin.
- Setting Shrinkage: Minimal shrinkage stresses (2 MPa) in comparison to composite resins (18 MPa), making it an advantageous property. The rubbery stage during setting permits flow at the free surface, leading to the dissipation of shrinkage stresses.
- Wear Resistance: Low initial wear resistance; high wear resistance upon aging. This suggests the GIC should not be utilized as a definitive restorative material in classes I and II.
- Fluoride release & Uptake: High fluoride release during the post-setting maturation (first few days) contributing to caries prevention; long-term fluoride release through equilibrium diffusion; released fluoride is primarily as NaF, which does not affect the matrix formation or lose desired properties.
- Aesthetics: Adequate initial color matching of GIC; translucency takes several days; opacity is a drawback.
- Strength: Brittle, comparatively weak with a lack of rigidity but strength increases with increasing P/L ratio (powder/liquid).
- Radiopacity: Incorporating radiopaque components (like Lanthanum).
- Advantages: Cariostatic (due to fluoride release), adhesive potential, low setting contraction, biocompatibility, thermal insulation, low thermal expansion, good aesthetics, multiple clinical applications, ease of manipulation, and reasonable cost.
- Disadvantages: poor mechanical properties/wear resistance preventing use in stress-bearing areas; very sensitive to hydration and dehydration; short working time; and relatively long setting time.