Development of glass-ionomer cement systems

In the 1960s the idea of positive physico-chemical adhesion with tooth substance resulted in the invention of polyacrylic acid-based cements, first the zinc polycarboxylate and, subsequently, the glass-ionomer cements. These materials were shown to undergo specific adhesion with hydroxyapatite and proved to have properties satisfactory for a variety of clinical applications. The key properties of the glass-ionomer cements--fluoride release over a prolonged period and specific adhesion to enamel and dentine coupled with aesthetic qualities--are related to their characteristics as aqueous polyelectrolyte systems. In order to improve toughness, speed of setting and resistance to dehydration, hybrid materials in which some of the water content of the glass-ionomer system was replaced by water-soluble polymers or monomer systems capable of ambient polymerization were formulated in the late 1980s. These materials, which have been termed resin-modified glass-ionomer cements, involve, ideally, the formation of an interpenetrating polymer network combining the acid-base cross-linking reaction of the metal ion-polyacid with the cross-linking polymerization of the monomer system or additive action of the polymers. In the predominantly resin materials there is little polyelectrolyte character and it is controversial whether such materials should be categorized as glass-ionomer cement systems. The specific advantages of these materials over traditional glass-ionomer systems and over composite restorative systems remain to be fully documented. Studies of adsorption to hydroxyapatite of typical monomers using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF SIMS) indicate that resistance to water displacement decreases as hydrophobicity increases.