GLASS IONOMER CEMENT

Glass Ionomer Cement

Glass Ionomer Cement

Introduction

In dentistry adhesion of restorative materials to tooth substance is an important objective. It is believed that a restorative should resemble the tooth in all respects. It should possess identical properties and would adhere tenaciously to the surrounding enamel and dentin. The glass ionomer cements are one of the products developed in this direction. The invention of the glass ionomer cement resulted from previous fundamental studies on dental silicate cements and studies where the phosphoric acid in dental silicate cements was replaced by organic chelating acids. It was assisted by work on the zinc polycarboxylate cement in which Smith showed that dental cements exhibiting the property of adhesion could be prepared from poly (acrylic acid). Glass ionomer cement has therefore been described as a hybrid of dental silicate cements and zinc polycarboxylates (Reisbick, 1978, 23).

Glass ionomer cements, are materials made of calcium, strontium aluminosilicate glass powder (base) combined with a water-soluble polymer (acid). When the components are mixed together, they undergo a setting reaction involving neutralization of the acid groups by the powdered solid glass base. Because both components are materials of wide chemical diversity, the range of glass - ionomer cements is very wide indeed and the material has considerable potential for further development.

Identification and classification of glass Ionomers

The term glass ionomer cement should be applied only to a material that involves a significant acid - base reaction as a part of its setting reaction and show a continuing fluoride release. Within the scope of the above definition there are a number of applications for these cements and accordingly they are classified.

Type I: Luting crowns, bridges and orthodontic brackets

Type II a: Aesthetic restorative cements

Type II b: Reinforced restorative cements

Type III: Lining cements, Base.

The chemistry is essentially the same for all three categories, but there are variations in powder / liquid ratio and powder particle size to accommodate the desired function. Glass ionomer cements have certain characteristics that are attractive to the dentist. They bond adhesively to enamel and dentin, release fluoride ions over a prolonged period of time, are biocompatible and have approximately the same coefficient of thermal expansion as that of tooth structure (McLean, 1994, 587). In spite of these advantages, conventional glass ionomers suffer from the disadvantages such as short working times and rather long setting times, brittleness, low fracture toughness, poor resistance to wear, susceptible to moisture contamination or dehydration during the early stages of the setting reaction.

It is well - known that the mechanical and physico- chemical properties and the working and setting characteristics of a glass ionomer cement critically depend on several factors, such as the composition of aluminosilicate glass and the poly acrylic acid liquid used in the formulation, the particle size of the glass powder, the relative proportions of the constituents in the cement mix (glass/ poly acrylic acid/ tartaric acid/water) and the mixing process (McLean, 1994, 587). The manufacturer controls most of these factors; the clinician can control the mixing ...