Both Composites and Glass Ionomer Cement were developed in the mid 1970's. Traditional Glass Ionomer releases fluoride ions. The Release rates after 180 days are 1/10 that same rate in the first 48 hours. The fluoride ions help prevent and reverse very early decay, especially in the area right around the newly placed filling. There has also a decrease in the population of Streptococcus mutans. Glass Ionomer is a very brittle material, poor in high wear areas, and not very hard. It is the only material that chemically adheres to tooth structure without prior preparation. In terms of Aesthetics the material is acceptable but not perfect. They tend to look "dull". The opacity of a light cured material is 40-45% whereas the glass ionomers are 60%.
In Children, glass ionomer fillings are very good. They release fluoride, are easy to place and require minimal preparation. In adults, they make good fillings in areas of low stress or in situations where rampant decay is a problem. Also, they are an excellent material to use under another more durable material as a "base".