2.2 Glass Ionomer Cements, Resin-Modified Glass Ionomers & Bioactive Materials

Bioactive materials represent a dynamic class of dental biomaterials that elicit a specific biological response at the interface between the restoration and host tissue. Rather than serving as passive space-fillers, these materials release therapeutic ions, chemically bond to tooth structure, and promote tissue remineralization or hard-tissue bridge formation.

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Conventional Glass Ionomer Cements (GIC)

Conventional glass ionomer cements (GIC), introduced by Wilson and Kent in 1972, are generic acid-base cements formed by reacting an ion-leachable fluoroaluminosilicate glass powder with an aqueous polyalkenoic acid liquid.

1. Chemical Composition

• Powder: Calcium fluoroaluminosilicate glass ($SiO_2-Al_2O_3-CaF_2-AlF_3-Na_3AlF_6$). Silica ($SiO_2$) and alumina ($Al_2O_3$) form the cross-linked oxide glass framework. Fluoride ($CaF_2$) acts as a ceramic flux to lower melting temperature and provides the therapeutic source of leachable fluoride.
• Liquid: An aqueous solution of 40–50% polyacrylic acid or a copolymer of acrylic acid with itaconic, maleic, or tricarboxylic acids. Copolymerization prevents premature hydrogen bonding and gelation during shelf life, reducing liquid viscosity. Tartaric acid (5–10%) is added as an optical isomer accelerator to sharpen the setting transition without shortening manipulation time.

2. The Acid-Base Setting Cascade: Three Stages

The setting reaction of GIC is a continuous acid-base neutralization cascade occurring in three distinct, time-dependent phases:

1. Stage 1: Dissolution (Acid Attack): Upon mixing, ionized polyacrylic acid releases hydronium ions ($H_3O^+$). Protons attack the outer layer of the glass particles, extracting calcium ($Ca^{2+}$), aluminum ($Al^{3+}$), fluoride ($F^-$), and sodium ($Na^+$) ions into the aqueous matrix. A hydrated silica gel layer forms on the surface of the decomposing glass particles.
2. Stage 2: Gelation / Precipitation (Initial Set, 4 to 10 Minutes): Divalent calcium ions ($Ca^{2+}$) migrate more rapidly than trivalent aluminum ions. Within 4 to 10 minutes, $Ca^{2+}$ binds to the carboxylate groups ($-COO^-$) along adjacent polyacrylic acid chains, forming an initial calcium polyacrylate gel matrix. At this stage, the cement is rigid enough to carve but remains structurally fragile. It is exceptionally vulnerable to water contamination (which leaches out unbonded cations, causing chalky friability) and water desiccation (which dries out the matrix, causing microcracking and loss of adhesion).
3. Stage 3: Hardening / Maturation (Final Set, 24 to 48 Hours): Over the next 24 to 48 hours, trivalent aluminum ions ($Al^{3+}$) displace calcium ions. Because aluminum is trivalent, each $Al^{3+}$ ion coordinates three carboxylate groups across polyacrylic acid strands, creating a dense, heavily cross-linked aluminum polyacrylate matrix. The refractive index of the gel matrix shifts closer to that of the unreacted glass cores, increasing compressive strength, wear resistance, and optical translucency.

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3. Chemical Adhesion to Enamel and Dentin

Unlike resin composites that depend on micromechanical interlocking via hybrid layers, conventional GIC bonds chemically to tooth mineral without requiring an adhesive resin:

• Ionic Chelation: The ionized carboxylate groups ($-COO^-$) of polyacrylic acid displace phosphate ions ($PO_4^{3-}$) from the hydroxyapatite lattice ($Ca_{10}(PO_4)_6(OH)_2$) in enamel and dentin, forming permanent ionic bridges with structural calcium ions ($Ca^{2+}$).
• Ion-Exchange Layer: An interfacial, acid-resistant layer of calcium-polyacrylate complex forms, measuring 1–2 μm in thickness.
• Dentin Conditioning Protocol: Before placing GIC, the cavity preparation must be treated with 10% to 20% polyacrylic acid (Cavity Conditioner) for 10 to 20 seconds. This gently dissolves the loose smear layer and increases substrate surface energy while preserving smear plugs inside dentinal tubules and leaving hydroxyapatite calcium ions available for chemical chelation. Etching dentin with 37% phosphoric acid prior to conventional GIC is contraindicated, as it strips away the very calcium ions required for chemical bonding.

4. Fluoride Kinetics & Thermal Expansion

• Fluoride Burst & Sustained Release: GIC exhibits a high "burst effect" of fluoride release during the first 24 to 48 hours following placement, providing acute antimicrobial inhibition against Streptococcus mutans. This is followed by a prolonged, steady-state diffusion release over months and years.
• Recharge Potential: GIC functions as a "fluoride sponge" or rechargeable battery. Exposure to topical fluoride (fluoridated dentifrices, 5000 ppm gels, or professionally applied fluorides) recharges the cement matrix, which subsequently leaches fluoride back into adjacent enamel and dentin.
• Coefficient of Thermal Expansion (CTE): The CTE of conventional GIC is approximately $10-11 \times 10^{-6}/^\circ\text{C}$, which mirrors that of natural human dentin ($8-11 \times 10^{-6}/^\circ\text{C}$). This thermal compatibility prevents microleakage, marginal gap formation, and thermal shear stress during hot and cold oral ingestion.

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Resin-Modified Glass Ionomers (RMGI)

To overcome the poor early physical strength, brittle fracture, and extreme moisture sensitivity of conventional GIC, resin-modified glass ionomers (RMGI) incorporate a light-cured resin component.

• Composition: Formulated by adding hydrophilic methacrylate monomers—predominantly HEMA (2-hydroxyethyl methacrylate)—and photoinitiators (camphorquinone) to the polyacrylic acid liquid, paired with fluoroaluminosilicate glass powder.
• Dual-Cure Chemistry:
  • Reaction 1 (Photopolymerization): Visible light activation initiates immediate free-radical cross-linking of HEMA, producing a rigid polymer network within 20 to 40 seconds. This confers immediate early fracture toughness, low solubility, and resistance to water contamination.
  • Reaction 2 (Acid-Base Reaction): The classic acid-base ionic cross-linking continues in the dark at a sustained pace, producing the mature aluminum polyacrylate matrix over 24 hours.
• Toxicity Warning: Uncured free HEMA monomer is cytotoxic to pulp cells and readily diffuses through open dentinal tubules. RMGI is strictly contraindicated for direct pulp capping.

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Compomers and Giomers

• Compomers (Polyacid-Modified Resin Composites):
  • Anhydrous formulations containing dimethacrylate resins (Bis-GMA, UDMA) and filler particles embedded in a dehydrated polyacid matrix. No water is present in the syringe.
  • Setting is initiated 100% by visible light photopolymerization. Only after placement in the oral cavity does saliva absorption introduce water, triggering a minor, secondary acid-base reaction.
  • Compomers behave mechanically like resin composites; their fluoride release is significantly lower than GIC and rechargeability is negligible.
• Giomers (Pre-Reacted Glass Ionomer Technology):
  • Formulated with pre-reacted glass ionomer filler particles (PRG particles) embedded within a resin matrix. The glass particles are pre-treated with polyacrylic acid prior to compounding into the resin.
  • Giomers offer the high polishability, compressive strength, and wear resistance of composite resin while maintaining sustained, rechargeable fluoride release.

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Bioactive Calcium Silicates (Bioceramics)

Bioactive calcium silicate cements are hydraulic biomaterials that set in the presence of water, establishing an alkaline environment that induces hard-tissue deposition.

1. Mineral Trioxide Aggregate (MTA)

• Formulation: Developed by Torabinejad in 1993, MTA consists of 75% Portland cement (tricalcium silicate, dicalcium silicate, tricalcium aluminate), 20% bismuth oxide (radiopacifier), and 5% gypsum (calcium sulfate dihydrate).
• Hydration Setting: When mixed with sterile water, tricalcium silicate hydrates to form a rigid calcium silicate hydrate gel and releases copious calcium hydroxide ($Ca(OH)_2$): $\text{Tricalcium Silicate} + \text{H}_2\text{O} \rightarrow \text{Calcium Silicate Hydrate Gel} + \text{Ca(OH)}_2$
• Mechanism of Bioactivity: The release of $Ca(OH)_2$ elevates the local pH to 12.5, producing potent antibacterial activity. In contact with phosphate-rich interstitial tissue fluids, the material precipitates a superficial layer of carbonated hydroxyapatite, creating a biological seal and stimulating pulpal mesenchymal cells via bone morphogenetic proteins (BMP-2) and transforming growth factor-beta 1 (TGF-$\beta 1$) to deposit a reparative dentin bridge.
• Clinical Limitations: Original ProRoot MTA has a prolonged setting time (2.5 to 4 hours), sandy handling consistency, and risk of gray coronal tooth discoloration caused by bismuth oxide oxidation.

2. Biodentine (Active Biosilicate Technology)

• Formulation: Synthetic, high-purity tricalcium silicate powder blended with calcium carbonate (filler) and zirconium oxide (radiopacifier). The liquid consists of an aqueous solution of calcium chloride ($CaCl_2$) as a setting accelerator and a water-soluble polycarboxylate polymer.
• Setting & Handling: $CaCl_2$ accelerates the hydration reaction, shortening setting time to 10 to 12 minutes. Compressive strength reaches ~300 MPa within 28 days (matching natural dentin).
• Esthetic Advantage: By replacing bismuth oxide with zirconium oxide, Biodentine eliminates the coronal staining and discoloration associated with MTA, making it the material of choice in the anterior esthetic zone.

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Biomaterials Comparison Table