Glass Ionomers
Inside Dentistry (ID): What exactly constitutes a glass-ionomer material?
Jed M. Best, DDS, MS (JB): Glass ionomers are basically a mixture of a fluoro-aluminosilicate glass powder and a polyacrylic acid. There are two categories: pure glass ionomers and resin-modified glass ionomers (RMGIs). RMGIs were created to harden on command, increase compressive strength, and be less susceptible to dehydration, among other improvements. However, in recent years, the development of high-viscosity pure glass ionomers has enabled a technique whereby heat is used to drive the reaction further so that the materials harden faster, and studies have indicated that, ultimately, they are also stronger. Glass ionomers are preferable to composite resins for situations in which there is interproximal decay in primary teeth because, among other factors, they have a similar coefficient of thermal expansion to the natural dentition, they chemically bond to tooth structure, and they release fluoride. The fluoride release helps to prevent secondary caries. On the other hand, composite resins micromechanically bond to tooth structure and do not release fluoride. There is one new resin that uses a quaternary ammonium silica nanoparticle to help prevent secondary caries, but that is just one materia—all glass ionomers help to precent secondary caries. It should be noted that some glass ionomers require a polyacrylic preconditioner.
ID: How do the adhesive properties of glass ionomers help in the treatment of caries in primary teeth?
JB: Years ago, the standard of care for pulpal therapy in primary teeth was a formocresol pulpotomy. If you were near the pulp, you performed a pulpotomy. Today, you would probably utilize mineral trioxide aggregate (MTA) as an alternative pulpotomy agent because formocresol has been listed by the World Health Organization as a possible carcinogen. However, the most successful pulpal therapy for teeth that do not demonstrate direct exposure is not an MTA pulpotomy, but rather indirect pulp therapy, which involves cleaning out the infected dentin, leaving the affected dentin, and sealing it, preferably with a glass ionomer.1 A micromechanical bond is not ideal for that; effective sealing requires a pure chemical bond, such as what can be achieved with a glass ionomer. As far back as 1982, Paul E. Starkey, DDS, wrote that indirect pulp therapy was probably the best option for primary teeth, but he emphasized that the available materials were a limiting factor.2 That was before glass ionomers became commercially abundant. The seal that glass ionomers provide is so important in this treatment because it prevents the ingress of bacteria near the deep part of the restoration.
ID: Why are glass ionomers also useful as luting agents for indirect restorations?
JB: Glass ionomer cement is a very viable option when, for example, placing a zirconia crown. When utilized as a cement, a glass ionomer does not require light because it reacts chemically. Curing lights are used in direct restorative applications to facilitate the polymerization of light-cure materials and harden them. When light-cure cements are used to place thicker restorations, such as full-coverage zirconia crowns, it can be difficult for the light to reach the deepest parts of the restoration and ensure complete polymerization. In addition to the adhesive properties of glass ionomers, their ability to release fluoride is a benefit in indirect restoration as well. Obviously, there is no standardized definition of "bioactivity" in dental materials, but the vast majority of the cements that manufacturers state are glass ionomers can be advertised as bioactive.
ID: Why aren't glass ionomers used more frequently?
JB: The biggest drawbacks to pure glass ionomers are that they are not as esthetic as resins and that, in many instances, they demonstrate greater wear. For primary teeth, however, these drawbacks are less of a concern because the restorations will rarely be seen by anyone, and the teeth will ultimately exfoliate. Although glass ionomers are popular in Europe, Australia, and New Zealand, their use in the United States has remained relatively stagnant for more than a decade. Most US dental schools teach resin technology. I do not know why glass ionomers have not caught on here, but clearly, they should have.
ID: Could that be attributed to the challenges involved in using them correctly?
JB: I do not believe so. When using glass ionomer as a restorative material, the major issue is that hand mixing can introduce variables, such as the strength of the user's wrist, whether the material is being mixed on a cold glass slab or a paper slab, etc. However, those variables can be eliminated by using triturated options. One manufacturer also offers a system that mixes the material as it is being expressed out of the capsule. Beyond the factors related to mixing, pure glass ionomer can simply be injected into the cavity that you have prepared. The biggest mistake that some people make during placement is trying to compress it with a metallic instrument and subsequently lifting it up off the base. The trick is to coat an instrument or a gloved finger with some resin bonding agent and use it to compress the glass ionomer and swipe across the occlusal surface. Once it hardens, you then trim it down and adjust for occlusion. To me, using a glass ionomer is easier than using a composite resin.
ID: Is the need for a primer a drawback to using glass ionomers?
JB: Some manufacturers recommend using a primer, but I do not believe it is always a must for glass ionomers because several manufacturers do not require it. If you are using a primer, however, it is usually a polyacrylic acid, and that means that you are etching the dentin and the enamel in addition to the pure chemical dentin bond that is obtained with glass ionomers. You get a stronger seal than what can be achieved with the exclusively micromechanical bond created by composite resin. In indirect pulp therapy, for example, do you really want resins, which are exothermic—potentially to a significant extent—close to the pulp? Again, I do not believe that etching is necessary with glass ionomers, and because etching and rinsing results in the generation of more aerosols, avoiding it may be particularly relevant in a world in which infectious disease experts agree that there will eventually be another pandemic.
ID: Can light curing glass ionomers prove to be harmful to the pulp?
JB: There is a consensus that raising the interpulpal temperature 5.5 degrees can result in pulpal necrosis in a certain percentage of teeth. However, Asha Patel, DDS, recently studied the four most popular glass ionomers on the market while she was a second-year pediatric dental resident at the University of Alabama at Birmingham, and she demonstrated that the time required to harden them—she used 40 seconds—did not increase the interpulpal temperature by that amount with any of the four materials. Eliminating that concern is very significant. I was taught that if a child has a proximal lesion, he or she is classified as high-risk and in need of help preventing secondary caries on the adjacent surfaces. At the moment, the only thing that dentistry has for that is fluoride, and what materials contain fluoride and readily release it? All glass ionomers do. Therefore, I do not understand why they are not more heavily utilized, at least in primary teeth.
About the Expert
Jed M. Best, DDS, MS, practiced pediatric dentistry for 42 years in New York, New York, and he currently holds academic appointments as a clinical professor at the Case Western Reserve University School of Dental Medicine, an associate professor at the Columbia University College of Dental Medicine, and an instructor at the University of Alabama at Birmingham School of Dentistry.
References
1. Seale NS. Indirect pulp therapy: an alternative to pulpotomy in primary teeth. Tex Dent J. 2010;127(11):1175-1183.
2. Starkey PE. Pulp Therapy in Dentistry for Children. Indiana University Pedodontic Alumni Association; 1982.
