Glass ionomer (GI) is a unique, tooth-colored material used extensively in modern dentistry as an alternative to traditional fillings and bonding agents. Introduced in the 1970s, this material gained popularity because it adheres directly to the tooth structure without needing complex intermediate bonding steps. It is often chosen for procedures where a biological benefit is desired, such as helping to protect the tooth from further decay. The versatility of glass ionomer has made it an important material for dentists treating both children and adults.
Fundamental Composition and Mechanism
The material is supplied as two main components: a fine glass powder and an acidic liquid, which are mixed just before application. The powder consists primarily of a fluoro-aluminosilicate glass, which is ion-leachable. The liquid component is an aqueous solution of polyacrylic acid, which acts as the chemical initiator.
The setting process is an acid-base reaction that begins immediately upon mixing. The acid attacks the glass particles, causing metal ions like calcium, aluminum, and fluoride to be released. These released ions then cross-link with the carboxyl groups on the polyacrylic acid chains, forming a rigid, three-dimensional matrix known as a glass polyalkenoate.
This reaction results in a chemical bond directly to the tooth’s hard tissues, enamel and dentin. The polyacid chemically links with the calcium ions found in the hydroxyapatite crystals of the tooth structure. This direct chemical adhesion is a significant advantage over materials like amalgam, which rely on mechanical retention, and composite resins, which require multiple bonding agents. The chemical bond creates a tight seal against the tooth, helping to prevent microleakage.
Distinct Advantages for Dental Health
A primary benefit of glass ionomer is its ability to act as a reservoir for fluoride, which it releases over a prolonged period into the surrounding tooth structure. This initial release is characterized by a rapid “burst effect” in the first 24 to 48 hours, followed by a gradual, sustained release. The released fluoride ions are incorporated into the tooth mineral, making the enamel and dentin more resistant to acid attack and helping to prevent recurrent decay.
The material can also be “recharged” with fluoride from external sources, such as fluoridated toothpaste or rinses. The set cement absorbs fluoride ions from the oral environment and releases them later, extending its decay-preventing effect. This recharge capability makes the material a dynamic and biologically active component in maintaining oral health.
Glass ionomer offers good biocompatibility because the polyacrylic acid liquid is a weak acid with a higher pH, reducing the risk of pulp irritation. Furthermore, the material’s coefficient of thermal expansion is similar to that of natural tooth structure. This property means the material expands and contracts at a rate closer to the tooth when exposed to hot and cold temperatures, reducing stress on the bond and minimizing marginal leakage.
Primary Uses and Different Forms
Glass ionomer cement is highly versatile, with applications ranging from simple preventive measures to complex restorative procedures. It is commonly used as a luting cement to secure crowns, bridges, and orthodontic brackets to teeth. Due to its moisture tolerance and ease of use, it is an excellent choice for interim therapeutic restorations, especially in pediatric dentistry or for uncooperative patients.
It also serves an important function as a liner or base material beneath other fillings, where it acts as a protective layer between the restorative material and the inner tooth structure. A specific, practical application is in the Atraumatic Restorative Treatment (ART) technique, which uses hand instruments to remove decay and then restores the cavity with a high-strength glass ionomer, often without the need for an electric drill. The material is also applied as a pit and fissure sealant to protect the chewing surfaces of molars.
The material is available in two main forms: Conventional Glass Ionomer (CGI) and Resin-Modified Glass Ionomer (RMGI).
Conventional Glass Ionomer (CGI)
CGI is the original, slow-setting version that relies solely on the acid-base chemical reaction for setting. CGI is favored in situations where moisture control is compromised, as it is less sensitive to water during the setting phase.
Resin-Modified Glass Ionomer (RMGI)
RMGI incorporates a small amount of resin and a light-curing component. This modification allows for a dual-cure mechanism: the initial set is rapidly achieved by light-curing the resin component, followed by the slower, ongoing acid-base reaction. RMGI offers improved mechanical properties like higher flexural strength, making it more robust for certain restorations. Both forms retain the ability to release and recharge fluoride.