**2. Glass-ionomer cements**

The original glass-ionomer cements (GICs), which are governed ISO 9917.1-2007 are waterbased materials which set by an acid-base reaction between a polyalkenoic acid and a fluroaluminosilicate glass [86] and have been one of the most widely researched dental materials since their introduction in the 1970s. Since these were brittle materials, attempts were made to enhance the physical properties by the addition of either metal particles (silver or gold), by a fusion process resulting in a 'cermet' (ceramic-metal), or amalgam alloy particles by a simple addition ('admix'). An important characteristic of glass-ionomer is its ability to bond to tooth structure, one mechanism being that of a hydrogen bond between the carboxyl group of the polyacid and the camcium in the tooth structure. It has also been shown that there is a micromechanical penetration of the GI into the tooth. They have a coefficient of thermal expansion similar to the tooth, which may help reduce microleakage and therefore postoperative sensitivity and can be bulk-filled and finished faster than a composite. The newer generations of glass ionomer materials are faster setting and no longer sensitive to hydration or desiccation during setting. One main advantage of glass ionomer materials is their chemical bonding ability to tooth structure, making them more resistant to leaks. Compared with resin system bonding, glass ionomer bonding is more degeneration-resistant and does not breakup, unlike the hydrolytic degradation of the hybrid layer of the resin system. Further modification of water-based ('conventional') GICs took place in the early 1990s by the addition of water-soluble resin, to produce the 'resinmodified' GICs. The purpose of adding resin was to enhance the physical properties and to reduce the sensivity to water balance of the conventional GICs. The first of the 'resinmodified' GICs (RM-GICs) was Virtabond (3M Dental Products, St Paul, Minnesota, USA), now called Vitrebond (3M/Espe Dental). Other names for RM-GIC which have been used include 'resin-ionomers', 'resinomers', 'hybrid ionomers' and 'light-cured glass ionomers' [17, 79, 80, 84].
