September 2010, Volume 6, Issue 8
Published by AEGIS Communications
Glass-Ionomer Cement Systems
A product review of some of the systems available for contemporary adhesive tooth repair.
Glass-ionomer cement systems have gained a prominent place in modern clinical dentistry.1 “Glass ionomer” has become the common term for glass polyalkenoate cements. These materials are made of calcium alumino-fluoro-silicate glass powder (base) combined with a water-soluble polymer (acid). When mixed together, the glass and acid components undergo a setting reaction involving neutralization of the acid groups by the powdered solid glass base. When describing this reaction to our young, curious, scientifically oriented patients, we tell them that “the acid solution chemically melts the glass powder and creates a solid glass-like repair material that actually bonds to the enamel and dentin and in many ways resembles tooth structure.”Significant amounts of fluoride ions are released from the glass powder during this reaction, without diminishing the physical properties of the hardened cement. The first glass-ionomer compound was invented in 1969 and Wilson and Kent reported about the new dental material concept in the early 1970s.2,3
Regardless of the favorable properties of glass ionomers, the initial materials of the 1970s were quite difficult to use and had distinct disadvantages. They had long hardening times and were very susceptible to breakdown or fracture if they became too wet or too dry during the extended setting reaction. Their important physical properties, once set, were inferior to the resin-based composites, even though the glass ionomers, per se, were more compatible with, and comparable to, dentin. As time passed, materials scientists did not give up on glass-ionomer systems; realizing their potential they searched for ways to the improve the products.Alterations in the glass-particle sizes and distributions, and types of acid formulations, improved the handling and physical properties of the cements.
In the 1980s and 1990s, metal-modified glass ionomers and those containing a light-polymerized liquid-resin component were introduced. The resin-modified glass-ionomer cements have significantly enhanced physical properties along with the great advantage of rapid, “on command” hardening by concentrated visible-light exposure. Products in this class of glass ionomers not only have greatly improved physical properties, but also have been manufactured and packaged in such ways to enhance handling, delivery of the material in the mouth, and overall user convenience.
There has been considerable confusion among dentists as to the nature of each of the various tooth-colored restorative materials that have been developed since the 1960s. The resin-based composites and glass-ionomer systems are the two major groups of such materials, but there are many variations and overlaps that make it difficult to sort out the product types. McLean et al addressed this issue in 1994,4 and Mount et al brought the concept up to date in 2009.5 A true glass-ionomer cement, whether it be used for tooth repair or luting purposes, has a hardening mechanism that involves a significant acid-base reaction (the acid is a water-soluble polymer and the base is a special glass).4-8
All glass-ionomer systems have certain properties in common. Besides their chemical bonding to dentin and enamel, good biocompatibility, and easy handling by syringe injection, the hardened cements have coefficients of thermal expansion quite similar to that of tooth structure. That means when cold or hot foodstuffs influence expansion and contraction of enamel and dentin, the cement expands and contracts in concert. That property, along with the chemical bond, helps to maintain marginal integrity and facilitate fluoride-ion dynamics.
Glass-ionomer cement systems can be considered “therapeutic” dental materials. Fluoride ions are released by glass ionomers and taken up by associated enamel and dentin. That tooth structure becomes less susceptible to acid challenge.9-27 In addition, water-based glass-ionomer materials act as fluoride-ion “reservoirs” by taking in salivary fluoride from dentifrices, mouthwashes, and topical fluoride solutions in the dental office. The fluoride content in true glass-ionomer systems and its ongoing activity is not only beneficial for young patients, but is particularly advantageous for all people with high susceptibility to dental caries.
Another major development in tooth restoration related to resin-modified glass-ionomer systems is that the light-curing resin component of the glass-ionomer cement chemically combines with resin-based composite material. Because certain glass-ionomer cements are the best direct-application dentin-replacement materials presently available, and resin-based composites simulate enamel closely, using the two in combination makes for biomimetic, or “tissue-specific” tooth repair.28,29 This concept has been called “layering,” “the sandwich technique,” or “stratification.”
In the past decade, many new glass-ionomer products have been introduced into the dental materials marketplace. These include tooth-repair materials, dentin-replacement liners and bases, and luting cements. While certain manufacturers have been the main source of glass-ionomer products and innovations, other dental companies have developed and introduced their own brands of glass-ionomer systems. The vast array of available glass-ionomer cement systems can pose difficulties for clinical dentists selecting materials for use in their patients.
To assist the readers of Inside Dentistry, the authors searched the Internet and current dental product catalogs to identify glass-ionomer products currently available (as of May 2010) in North America, and some of these are featured in this month’s Buyers Guide. Because of space constraints, it is likely that some products, or even manufacturers, were overlooked, and the authors apologize for any such unintentional omissions. The authors recommend that their list be used as a starting place for clinicians to investigate the wide scope of available glass-ionomer products. In addition to manufacturer descriptions, research reports about certain materials can be found in current dental journals and research newsletters. Such reports can be most helpful to dentists in identifying the best products to use for their patients.
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About the Authors
Theodore P. Croll, DDS
Private Practice in Pediatric Dentistry
Adjunct Professor, Pediatric Dentistry
University of Texas Health Science Center at San Antonio
San Antonio, Texas
Joel H. Berg, DDS, MS
Associate Dean for Hospital Affairs
Professor and Chair
Department of Pediatric Dentistry
University of Washington, School of Dentistry
Department of Dentistry
Seattle Children’s Hospital