Glass ionomer cements

Introduction

GIC is the generic name for materials based on the reaction of glass powder and polyacrylic acid. The cements were developed in the 1970s to improve clinical performance compared with silicate cements and to reduce the risk of pulp damage.

The use of polyacrylic acid makes GIC capable of bonding to tooth structure. GIC is considered superior to many types of cements because it is adherent and translucent. Various formulas are available depending on the intended clinical application. Water-soluble polymers and polymerizable monomers have been replacing part of the liquid content. Particles of metal, metal-ceramic, and ceramic have been added to some products to enhance mechanical properties. Other new formulations are capable of being chemically cured, light-cured, or both.

Indications

• GICs have been used for the esthetic restoration of anterior teeth, e.g., Class III and V sites.
• As luting cements
• As an adhesives for orthodontic appliance and intermediate restorations
• As pit and fissure sealants
• As Liners and bases
• As core buildup materials

Classification

• Type I: Luting crowns, bridges, and orthodontic brackets
• Type IIa: Esthetic restorative cements
• Type IIb: Reinforced restorative cements
• Type III: Lining cements, base

Chemistry and Setting

The chemistry of GICs is essentially the same for all three types, with variations in powder composition and particle size to achieve the desired function. The consistency of the mixed GIC varies widely among manufacturers, from low to very high viscosity as influenced, by their use of various particle size distributions and the P/L ratio. Larger particles (about 50 μm) are used for the various restorative indications, and finer glass particles (about 15 μm) are used for cementing.

Glass Composition

The glass composition in GIC varies among manufacturers, but it always contains silica, calcia, alumina, and fluoride. The ratio of alumina to silica is the key to their reactivity with polyacrylic acid. Barium, strontium, or other higher atomic number metal oxides are added to the glass to increase the radiopacity. The silica glass is melted at temperatures between 1100 °C and 1500 °C, depending on the raw materials and the overall composition. The glass is ground into a powder with particles ranging from less than 15 μm to about 50 μm, depending on the indication.

Liquid Composition

Originally, aqueous solutions of polyacrylic acid (about 40% to 50%) were used, but such liquids were viscous and had a short shelf life because of gelation. Currently, the liquids are copolymers of itaconic, maleic, or tricarboxylic acids.

Tartaric acid is a rate-controlling additive in the GIC liquid that allows a wider range of glasses to be used, improves handling properties, decreases viscosity, lengthens shelf life before gelling of the liquid occurs, increases working time, and shortens the setting time.

A specialized GIC known as a water-settable glass ionomer is formulated with freeze-dried polyacrylic acid solid and glass powder, which is mixed with water or an aqueous solution containing tartaric acid. This type of GIC has an extended working time because additional time is needed to dissolve the dried polyacrylic acid in water and start the acid-base reaction.

Setting Reaction

• When the powder and liquid are mixed for a GIC, the acid starts to dissolve the glass, releasing calcium, aluminum, sodium, and fluorine ions. Water serves as a reaction medium.
• The polyacrylic acid chains are then cross-linked by the calcium ions; however, over the next 24 hours, the calcium ions are replaced by aluminum ions.
• Sodium and fluorine ions from the glass do not participate in the cross-linking of the cement.
• Some of the sodium ions may replace the hydrogen ions of carboxylic groups, and fluorine ions are dispersed within the cross-linked (matrix) phase of the set cement. The cross-linked phase becomes hydrated over time as it matures.
• The undissolved portion of glass particles is sheathed by a silica-rich gel that is formed on the surface of the glass particles.
• Thus, the set cement consists of undissolved glass particles with a silica gel coating embedded in an amorphous matrix of hydrated calcium and aluminum polysalts containing fluoride.

Mechanism of Adhesion

The adhesive mechanism of GIC primarily involves chelation of carboxyl groups of the polyacids with the calcium in the hydroxyapatite of the enamel and dentin. Because of the greater homogeneity and inorganic content of enamel, GICs bond better to enamel than to dentin.

Adhesion to the organic component of the dentin can also occur by either hydrogen bonding or metallic ion bridging between the carboxyl groups of the polyacid and the reactive groups within the collagen of dentin.

Biological and mechanical properties

GICs elicit a greater pulpal reaction than ZOE cement but less than zinc phosphate cement. Glass ionomer luting agents pose a greater pulpal hazard than glass ionomer restorations when the GIC is mixed with a low P/L ratio because the pH remains acidic longer. With any GIC, a protective liner such as Ca(OH)2 should be used if the preparation is closer than 0.5 mm to the pulp chamber.

The compressive strength is similar to that of zinc phosphate, and its diametral tensile strength is slightly higher. The modulus of elasticity is only about half that of zinc phosphate cement. Thus, the GIC is less stiff and more susceptible to elastic deformation. The higher elasticity (lower elastic modulus) makes GIC less desirable than zinc phosphate cement to support an all-ceramic crown, because greater tensile stress could develop in the crown that is supported by the GIC under occlusal loading.

GIC restoratives are more vulnerable to wear than are composites when subjected to in vitro toothbrush abrasion tests and simulated occlusal wear tests. Fracture toughness, which is a measure of the energy required to cause crack propagation leading to fracture, is another property pertinent to restorative materials. Restorative GICs are less tough than resin-based composites.

References

Glass ionomer cements Resources
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