First developed in the U.K. in the 1970s, glass ionomers have long been used in Europe, Asia, and developing countries for restorative and lining purposes, offering a readily available, cost-effective alternative for caries treatment, particularly in children. But their use in the U.S. has been limited.
"The United States has had a glass ionomer barrier for many years," said Martin Tyas, B.D.S., Ph.D. (Birm), D.D.Sc., professor and director of the biomaterials evaluation unit in the School of Dental Science at the University of Melbourne and moderator of the IADR session on glass-ionomer cementers. "Over the years, it has been used very little in the United States, and that use has been disappointing. I think this is primarily an issue of lack of training and understanding."
What are glass ionomers?
Glass ionomer cements are available in two forms, both of which contain fluoride: conventional and resin-modified. The first glass-ionomer cements comprised a liquid acid mixed with a glass powder, creating an acid-base reaction. Early release of calcium ions is responsible for the initial setting of these cements; the subsequent release of aluminium ions significantly improves the strength over the next several days. Initial commercial glass-ionomer cements included the Fuji I and Shofu I.
Resin-modified glass ionomer cements such as the Fuji Plus and Vitremer Luting were first developed in the early 1980s. These are glass-ionomer cements that contain a small quantity of a water-soluble, polymerizable resin component. These combine an acid-base reaction of the traditional glass ionomer with a self-cure amine-peroxide polymerization reaction. These light-cured systems have been developed by adding polymerizable functional methacrylate groups with a photoinitiator to the formulation.
According to a 1999 study ( Journal of the Canadian Dental Association, October 1999, Vol. 65:9, pp. 491-495), the main limitation of glass-ionomer cements is their relative lack of strength and low resistance to abrasion and wear. Resin-modified glass-ionomer cements have been shown to be stronger and more flexible, although their wear resistance needs further clinical study. In addition, while it has been hypothesized that release of fluoride from glass ionomers may contribute to caries prevention, this clinical effect has not been well-established in the literature ( Journal of the American Dental Association, March 2008, Vol. 139:3, pp. 257-268). |
For example, according to Sharan Sidhu, B.D.S., M.Sc., Ph.D., of the Institute of Dentistry at Barts and The London School of Medicine and Dentistry, there is much documented evidence that glass ionomers are a viable alternative to conventional materials for fillings, linings, and bases. Her review of several multiyear clinical evaluations of resin-based glass ionomers (RMGICs) compared retention, marginal characteristics, material deterioration, secondary caries, color stability, and pulpal and biological effects. She found that, with the exception of color stability, RMGICs consistently performed well.
Results from several studies -- including Abdalla and Alhadainy (
American Journal of Dentistry, February 1997, Vol. 10:1, pp. 18-20) and Loguercio et al (
Journal of Adhesive Dentistry, 2003, Vol. 5:4, pp. 323-332) -- show retention rates of 93% to 100%, according to Dr. Sidhu. Similarly, two studies -- the 1997 Abdalla and Alhadainy study and another by van Dijken et al (
Swedish Dental Journal, 1999, Vol. 23:1, pp. 1-9) -- found no secondary caries development following the use of RMGIC over two years and only one (in an open sandwich restoration) over three years. A third study (Loguercio et al, 2003) found no secondary caries over five years.
Promise for developing world
Ionomers are particularly well-adapted to atraumatic restorative treatment (ART), a technique being used in conditions where minimal equipment and supplies are available. In this technique, dentists use hand tools to remove decayed tissue without anesthesia, then restore the tooth using only the glass ionomer.
"Most patients [in developing countries] present themselves when the pain is unbearable, and extraction is the treatment of choice," said Joseph Frencken, D.D.S., Ph.D., associate professor in minimal intervention dentistry at Radboud University in the Netherlands. "But this can have serious consequences, especially in children."
Research has shown that lack of proper oral health care and the resulting illness and pain it can cause is one of the leading reasons for school absenteeism for children in developing countries, he added.
Research teams are working to introduce ART in Mexico, South Africa, and Tanzania, Dr. Frencken said. While they have so far had mixed results, Dr. Frencken attributes this to lack of sufficient training, an inadequate supply of instruments and materials, and high patient loads. "High-viscosity glass ionomer is an essential element in oral healthcare systems worldwide, together with hand instruments, and both need to be readily available when ART is introduced into a system," he concluded.
Despite the evidence presented at the IADR meeting, convincing dentists in the U.S. that glass ionomers are safe and practical remains as much a challenge as convincing their counterparts in developing countries.
In fact, too many dentists in the U.S. think glass ionomers are only fit for developing countries, according to one audience member at the IADR session who asked not to be identified. "Some of the rhetoric is that this is Third World dentistry -- even though this [approach] is saving teeth and keeping kids in the classroom," she said.
