3 ways glass ionomers are changing restorative dentistry

December 10, 2018

Glass ionomer cement is a material with many uses in today’s dental practice. Here’s how this material is changing restorative dentistry.

Glass ionomer dental materials have been around for almost 50 years. They’re perennial favorites for pediatric restorative dentistry, but these versatile materials have many uses in the dental office.

However, in the U.S., some experts say glass ionomer restorative material is under-used and under-appreciated. The earliest versions of the material developed a reputation for “washing out,” which, for some dental professionals, has stuck with the material still today.

Over the past five decades, glass ionomer restorative material has been refined and remade. Problems with the early versions have been addressed, making it useful for so much more than in the past.

Let’s take a closer look at the history of glass ionomers, their advantages and disadvantages, and how glass ionomers are changing restorative dentistry today.

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A brief history of glass ionomers

In the 1950s, dentists had two major options for restoring permanent teeth: dental amalgams for posterior teeth and silicate cements for anterior teeth. Clinicians and researchers of that era realized that the future of dentistry would involve adhesive bonding to tooth structure. Some of the first research to achieve that goal was the introduction of acid-etching enamel in 1955 by Dr. Michael Buonocore.

In 1969, Alan Wilson and Brian Kent, chemists in the UK, introduced a dental material formed of silicate glass powder (calciumaluminofluorosilicate glass) and polyacrylic acid. They called it a glass ionomer, and it was a new type of restorative material that could chemically bond with tooth structure. The chemists published a paper in 1971 that described the new cement as “the product of the reaction between ion-leachable glass and an aqueous solution of polyacrylic acid.”

Glass ionomers’ efficacy resulted from the combination of its parts. It had fluoride-release properties of the silicate fillers and the adhesion qualities of the polyacrylic acid. Glass ionomers were used as permanent restoratives that could bond to the tooth and release fluoride. Also, their inherent translucency allowed them to be shaded to match tooth structure. 

Dr. Nathaniel Lawson is an assistant professor in UAB’s School of Dentistry’s division of biomaterials. He says in addition to the fluoride release, glass ionomers were also great because of their adhesive properties with the tooth and their moisture tolerance. Unlike resin composites, which must be linked to tooth structure through the hybrid layer formed with an adhesive, polyacrylic acid of glass ionomers adheres directly to the hydroxyapatite in teeth.

“Salivary contamination is an enemy of adhesive dentistry, as water will compete with our adhesives for access to tooth structure. Therefore, the hydrophilicity of glass ionomer materials make them more moisture-tolerant,” Dr. Lawson explains.

In 1991, Wilson published a paper in Clinical Materials about the future of glass ionomer cement. He believed its chemical diversification had untapped potential. He also predicted that the restorative material had the potential to improve its strength, handling and esthetics, characteristics Wilson attributed to resin-modified glass ionomer cements.

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What is a resin-modified glass ionomer?

By the 1980s and 1990s, glass ionomers incorporated resins, creating resin-modified glass ionomers (RMGI). By adding resins, which can range from 10 to 20 percent of the content, glass ionomers enjoyed easier mixing and handling.

“It was mainly added to allow the material to light polymerize and cure on demand,” Dr. Lawson says.

Although there’s  a perception that RMGI outperform conventional glass ionomers in strength and wear properties, Dr Lawson states that “many of the mechanical tests we perform show similar performance for both RMGI and glass ionomer materials.”

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Daniel H. Ward, DDS, published in an article for the American Academy of Cosmetic Dentists detailing how the two types of restorative glass ionomers have different strengths:

  • Glass ionomers are better for occlusion areas.

  • RMGIs are better when they’re used in conjunction with a composite resin and not in an occlusal area.

  • Glass ionomers need moisture control longer than RMGIs, meaning up to the five-minute setting time.

  • RMGIs only need moisture control during the placement and light-curing phase.

  • Glass ionomers shouldn’t be used with phosphoric acid, but RMGIs can be.

  • RMGIs have more translucency than glass ionomers, which can lead to better esthetic outcomes.

  • What are the advantages of glass ionomers and RMGIs?

When compared to composite resins, glass ionomers and RMGIs offer several advantages over composite resins:

  • They have hydrophilic properties.

  • They release fluoride, although some formulations of pure glass ionomers release much more fluoride than RMGI.

  • They bond with the tooth structure.

  • They don’t experience polymerization shrinkage.

What are the disadvantages of glass ionomers and RMGIs?

Like all materials, there are disadvantages of glass ionomers, too. Dr. Lawson shared a few of the limitations of glass ionomers:

  • They’re not as strong as composites, so they may fracture if placed in load-bearing restorations.

  • They’re less wear-resistant than composites, so they may “wash out” if placed in load-bearing areas or if they’re exposed to excessive abrasive toothbrushing.

  • They have lower gloss and stain resistance than composites, so they may discolor over time, and they also don’t achieve the level of esthetics that ‘s possible with a composite.

However, improvement in the material’s composition from dental materials manufacturers have addressed some of these disadvantages. Some new formulations of glass ionomer restoratives have even addressed the perceived wear and strength issues.

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3 ways glass ionomers are changing restorative dentistry

The early uses of glass ionomers were for anterior restorations and as cement for crowns and bridges. Over time, however, dental professionals expanded their use of the material based on its strength, solubility and translucency.

Today, glass ionomer and RMGI restorative materials are also used for occlusal restoration in primary teeth, treating proximal lesions, affixing orthodontic brackets, and lining or using as a base for caries restorations.

All the properties available with glass ionomers and RMGIs also allow dental professionals to work in new ways. Here are three ways that glass ionomers are changing dental restorations.

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1. It’s one restorative material with many treatment uses

One of the ways that glass ionomers and RMGIs are changing restorative dentistry is in how many ways you can use them. This gives clinicians many options.

Dr. Timothy Bizga practices in Cleveland, Ohio, as a general dentist. He’s also a speaker, trainer and coach for John Maxwell. Part of his practice philosophy is to choose restorative materials based on what’s best for the individual case. He appreciates how glass ionomer cements gives dentists more options.

“With advances in esthetics, handling and wear characteristics, the glass ionomer line of restorative materials available today offers dentists outstanding treatment options for the routine, geriatric and caries-risk patient,” Dr. Bizga says. 

Dr. Richmond Chung is a general and cosmetic dentist in private practice in Orange County, California. He uses RMGIs in his practice primarily for fluoride supplement and as a buffer in dental restorations. When there are subgingival lesions and the prep design finished well into the dentin and below the gum line, Dr. Chung uses RMGIs to bond to the dentin and creates a base layer for the composite restoration.

“If I have an area where there are deep caries that approach the pulp, or if there is affected dentin, not carious, but not as solid as you would like, I place the RMGI as a liner,” Dr. Chung says.

There are many types of glass ionomers. The different formulations allow for various treatment applications. For example, low-viscosity glass ionomer cement can be used as a sealant. Because of its unique properties, including its adhesion and hydrophilic features, it can be a preferable alternative to resin sealants for certain patients.

High-viscosity glass ionomer cements can be used instead of resin composite in many situations as a filling material.  They cure by an acid-base chemistry that doesn’t require light polymerization. Dr. Lawson describes glass ionomers as some of the original bulk fill materials. Dr. Chung says he also appreciates how these restorative materials simplify some restorations.

“The big plus for me is that you can bulk fill it in an area that is tough to isolate,” he says.

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2. They decrease postoperative sensitivity that can occur with composite resins

“Glass ionomers do not experience polymerization shrinkage that occurs with resin composites,” Dr. Lawson says. 

As far as isolation and having incremental curing as you would with composites, Dr. Chung likes that glass ionomers and RMGIs have no polymerization shrinkage.

“With composites, as you light cure them, you get the polymerization shrinkage with the potential to put stresses on the tooth or post-operative sensitivity,” Dr. Chung explains.

Polymerization shrinkage can result in exposing the internal structures of the tooth. Exposed dentin is a cause of post-operative sensitivity, including dentinal hypersensitivity. Dentinal hypersensitivity is caused by the stimulation of exposed dentin. Exposed dentin has many causes, but one is preparations for cavity treatment.

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The stimulus to dentin causes fluid to flow in the dentinal tubules, activating the nerve endings and triggering pain, typically lasting no longer than 10 seconds or so. While 10 seconds is a short amount of time, it can feel like an eternity to a patient in pain.

Suppressing sensitivity is imperative to clinicians when working with patients. Usually one achieves it by sealing the dentin, which effectively closes the open dentinal tubules. Glass ionomers seal the teeth and reduce the sensitivity that can occur.

3. You don’t have to be as diligent about isolation as with composite resins

Dr. Buonocore, the pioneer of adhesive dentistry, identified early in the development of adhesive dentistry the significant impediment to adhesion that moisture in the oral cavity presents. Dr. Buonocore said moisture created the worst situation for bonding and should be avoided.

Composites need complete isolation from moisture for placement. However, achieving complete isolation is widely considered a difficult aspect of adhesive dentistry. In dental school, students learn to isolate with a rubber dam, but once in practice, dentists often abandon the rubber dam to save time.

Glass ionomers are hydrophilic. Dr. Lawson says that because glass ionomers and RMGIs are more moisture tolerant, they can maintain their bond to tooth structure even if some saliva contamination occurs.

“This makes glass ionomers and RMGI a preferred material for situations in which isolation is difficult, such as root caries, uncooperative children and subgingival restorations,” Dr. Lawson says.

References

Wilson, A.D. and B. E. Kent. “The glass ionomer cement, a new translucent dental filling material.”   Journal of Applied Chemistry and Biotechnology.  Vol. 21. No.11, 1971, pp 313-313. From Web. 29 November 2018. < https://onlinelibrary.wiley.com/doi/pdf/10.1002/jctb.5020211101

Wilson, Alan D. “Glass-ionomer cement origins, development and future.” Clinical Materials. Vol. 7, No. 4, 1991, pp. 275-282. From Web 29 November 2018: https://www.sciencedirect.com/science/article/pii/026766059190070V

Ward, Daniel, DDS, H. “20 Tips for Using Glass Ionomers: How to Use Dentistry’s Other Direct Tooth-Colored Material.” Aacd.com. Web. 29 November 2018. http://aacd.com/proxy/files/Students%20and%20Faculty/20%20tips%20Glass%20Ionomers.pdf.