17.5 Glass Ionomer Cements, RMGICs and Compomers

Glass Ionomer Cements (GIC), RMGICs, Compomers & Sandwich Techniques

Invented by Wilson and Kent in 1972, conventional Glass Ionomer Cements (GICs) set via an acid-base neutralization reaction between a polymeric acid and a basic fluoroaluminosilicate glass powder.

                                Glass Ionomer Setting Dynamics
                                               │
      ┌────────────────────────────────────────┼────────────────────────────────────────┐
      ▼                                        ▼                                        ▼
Stage 1: Dissolution                  Stage 2: Gelation (Initial Set)          Stage 3: Maturation & Hardening
• Polyacrylic acid attacks glass      • Divalent Calcium ions (Ca²⁺)           • Trivalent Aluminium ions (Al³⁺)
• Releases Ca²⁺, Al³⁺, Na⁺, F⁻          cross-link polyacid chains               form stable 3D cross-links
• Silica hydrogel forms               • Occurs within 4–10 minutes             • Requires 24–48 hours
• HIGH MOISTURE SENSITIVITY           • Weak, vulnerable matrix                • Cements reach full strength

Chemical Composition & Acid-Base Setting Stages

1. Composition:
   • Powder: Calcium fluoroaluminosilicate glass (containing $SiO_2$, $Al_2O_3$, $CaF_2$, $Na_3AlF_6$).
   • Liquid: Aqueous solution of polyacrylic acid copolymerised with itaconic or maleic acid (prevents high viscosity), plus tartaric acid (accelerates setting time and sharpens the set).
2. Three-Stage Setting Reaction:
   • Stage 1 (Dissolution): The polyacid ionizes in water, releasing $H^+$ ions that attack the surfaces of the fluoroaluminosilicate glass particles. This leaches out $Ca^{2+}$, $Al^{3+}$, $Na^+$, and $F^-$ ions, forming a silica gel coating on the unreacted core particles.
   • Stage 2 (Gelation / Initial Set): Within the first 4 to 10 minutes, the rapidly available divalent calcium ions ($Ca^{2+}$) cross-link the carboxyl ($-COO^-$) groups of adjacent polyacrylic acid chains, creating an initial, brittle gel matrix.
   • Stage 3 (Maturation / Hardening): Over the subsequent 24 to 48 hours, the slower-migrating trivalent aluminium ions ($Al^{3+}$) displace calcium, forming rigid, highly insoluble 3D cross-linked polyacrylate salt complexes.

Adhesion Mechanism & Fluoride Dynamics

• Chemical Adhesion via Chelation: GIC achieves true, intrinsic chemical bonding to tooth structure without an intermediate adhesive. Free carboxylate ($-COO^-$) groups on the polyacrylic acid chains displace phosphate ions and form ionic coordination bonds with calcium ($Ca^{2+}$) within tooth hydroxyapatite, creating an intermediate ion-exchange layer.
• Thermal Compatibility: GIC exhibits a coefficient of thermal expansion ($10 ext{-}11 \times 10^{-6}/\text{K}$) that closely matches natural tooth hard tissues ($11-12 \times 10^{-6}/\text{K}$), minimizing thermal contraction stresses at restoration margins.
• Fluoride Release & Recharge: High initial fluoride burst release over the first 48 hours, transitioning to a sustained, low-level release. Uniquely, the GIC matrix acts as a fluoride sponge / reservoir, absorbing fluoride ions from daily toothpastes or varnishes and re-releasing them into adjacent tooth margins during acid challenges.
• Moisture Sensitivity Management: During early setting (Stages 1 and 2), GIC is extraordinarily vulnerable to:
  • Moisture Contamination: Water influx dissolves free metal cations, weakening the matrix and resulting in opacity and low compressive strength;
  • Desiccation: Evaporative water loss halts maturation, inducing surface crazing, cracking, and loss of chemical adhesion.
  • Protection: Must be coated immediately following placement with a light-cured unfilled resin glaze or petroleum jelly.

Restorative Spectrum: GIC vs RMGIC vs Compomer vs Composite

  Fluoride Release, Mechanical Strength & Setting Continuum:
  
  Conventional GIC  ──────────►  RMGIC  ──────────►  Compomer  ──────────►  Resin Composite
  [Pure Acid-Base]           [Acid-Base + Light]    [Polymerisation only]   [Polymerisation only]
  • Max fluoride release     • Good fluoride        • Low fluoride release  • Zero fluoride release
  • Low tensile strength     • Dual-cure setting    • Requires bonding agent• High tensile strength
  • Intrinsic adhesion       • Intrinsic adhesion   • Composite matrix      • High aesthetics/wear

• Resin-Modified Glass Ionomer (RMGIC - e.g. Fuji II LC): Incorporates 2-hydroxyethyl methacrylate (HEMA) and camphorquinone into the GIC liquid. Undergoes a dual setting reaction: immediate light-cured free-radical polymerization of HEMA, followed by the continuing internal acid-base neutralization. Possesses higher early physical strength, immediate moisture resistance, and chemical adhesion.
• Compomer (Polyacid-Modified Resin Composite): Resin composite base containing fluoroaluminosilicate glass fillers and dehydrated acidic monomers. Sets exclusively via free-radical photopolymerization. Lacks intrinsic self-adhesion (mandates a dental bonding agent). It absorbs oral water over months, triggering a minor secondary acid-base reaction with limited fluoride release.

The Sandwich Technique

Used primarily in Class II restorations to combine the chemical seal and biological advantages of glass ionomer with the superior aesthetics and mechanical wear resistance of resin composite:

  Closed Sandwich Technique                       Open Sandwich Technique
  
  ┌─────────────────────────┐                     ┌─────────────────────────┐
  │ Composite Resin Cap     │                     │ Composite Resin Cap     │
  ├─────────────────────────┤                     ├─────────────────────────┤
  │   GIC / RMGIC Base      │                     │   GIC / RMGIC Base      │ ◄── Extends to
  └─────────────────────────┘                     └─────────────────────────┘     Cervical Margin
  All external cavosurface margins                GIC directly exposed at cervical
  are enamel (bonded with composite)              margin below the amelocemental junction (ACJ)

1. Closed Sandwich: GIC or RMGIC is placed strictly as a dentine-replacement base on the pulpal floor, completely enclosed within the cavity. Composite resin covers all external occlusal, proximal, and gingival margins. Indicated when the entire cavosurface perimeter is supported by sound enamel.
2. Open Sandwich: GIC or RMGIC is placed at the gingival floor of a Class II box and extended directly to the external cavosurface margin, leaving the GIC exposed to the oral cavity at the cervical margin beneath composite. Indicated when the gingival margin sits below the cemento-enamel junction (CEJ) on root dentine/cementum, exploiting GIC's superior chemical seal, moisture tolerance, and continuous fluoride release where composite-dentine bonds frequently fail.

Clinical Indications Examiners Expect

Glass ionomer is the material of choice in a defined set of situations, and the exam tests recognition of them: as a luting cement for conventional crowns, as a base or liner under other restorations, as a fissure sealant where moisture control is impossible, in atraumatic restorative treatment, in root surface and cervical lesions in high-caries-risk or xerostomic patients, as a temporary or interim restoration, and as the cement for orthodontic bands. Its weaknesses — low fracture toughness, susceptibility to moisture during the initial set and to desiccation afterwards, and inferior polish and aesthetics — make it unsuitable for load-bearing occlusal restorations in permanent teeth.