Glass Ionomer: Preparation, Properties, Types, Uses

The glass ionomer is a material made of silicate glass and water soluble acidic polymer. It is widely used in dental repairs and especially in pediatric dentistry.

It belongs to a type of materials known as acid-base cements, as it is the product of the reaction between weak polymeric acids and basic glasses in powder form.

This material releases fluoride ions (F ) with ease, which helps prevent cavities, one of its advantages. Another of its capabilities is that it chemically adheres to dentin and enamel.

Additionally it is biocompatible and of low toxicity. The bond with the tooth is acid-resistant and durable. However, it has low resistance to fracture and wear, so it cannot be applied in highly stressed dental areas.

The acidic polymer that is generally used to obtain it is polyacrylic acid, which is a polyalkenoic acid. For this reason, according to the International Organization for Standardization or ISO ( International Organization for Standardization ), its correct name is “glass polyalkenoate cement”.

Nomenclature

  • Glass ionomer
  • Glass Polyalkenoate Cement
  • Ionomer glass

Preparation

Glass ionomer cements consist of calcium or strontium aluminofluorosilicate glass powder (basic) that has been mixed with a water soluble acidic polymer.

The polymers used are polyalkenic acids, in particular polyacrylic acid:

–CH 2 —CH (COOH) —CH 2 —CH (COOH) —CH 2 —CH (COOH) —CH 2 —CH (COOH) –

A 2: 1 copolymer of acrylic acid and maleic acid can also be used. Glasses must be basic, capable of reacting with acid to form salts.

What happens when they join

When these components are mixed, they undergo an acid-base neutralization reaction generating a hardened material. Its setting or solidification occurs in concentrated aqueous solutions.

The final structure contains a significant amount of unreacted glass, which functions as a reinforcing filler for the cement.

Chelating agents such as tartaric or citric acid are also added, whose action is not yet clear. It is estimated that they possibly prevent the precipitation of aluminum salts, since they trap the Al 3+ ion .

In this way, setting is delayed and the cement can be better mixed.

Chemical representation and composition

An example of how a glass ionomer can be represented chemically is the following formula: SiO 2 -Al 2 O 3 -P 2 O 5 -CaO-CaF 2 .

Although there are various glass ionomer compositions commercially, they are somewhat similar. An example is shown below:

Silica (SiO 2 ) = 24.9%; alumina (Al 2 O 3 ) = 14.2%; aluminum fluoride (AlF 3 ) = 4.6%; calcium fluoride (CaF 2 ) = 12.8%; sodium aluminum fluoride (NaAlF 4 ) = 19.2%; aluminum phosphate (Al (PO 4 ) 3 ) = 24.2%.

Properties

The behavior of glass ionomers depends on their composition, polyacid concentration, size of the glass powder particles and the powder / liquid ratio. Most show opacity to X-rays.

As an example, the minimum requirements that these materials must meet, specifically a restorative cement, according to ISO are shown:

Setting time

2-6 minutes

Compressive strength

100 MPa (minimum)

Acid erosion

0.05mm / h (maximum)

Opacity

0.35-0.90

Acid soluble arsenic

2 mg / Kg (maximum)

Acid soluble lead

100 mg / Kg (maximum)

Types of glass ionomers

Depending on their application they are divided into three classes:

Type I: Fixing and bonding cements

They have a low powder / liquid ratio, therefore they have a moderate resistance. Set quickly with good water resistance. They are used for the cementation of bridges, crowns, orthodontic appliances and inlays.

Type II: Cements for restoration

They are subdivided in turn into two classes.

Type II-a:

They have a high powder / liquid ratio, good harmony with the color of the teeth, they need protection from humidity for at least 24 hours with varnish or hydrocarbon gel.

They are used for repairs of the front teeth, where appearance is important.

Type II-b:

They have a high powder / liquid ratio, fast setting and quick water resistance. They serve in places where appearance is not important, such as back teeth repairs.

Type III: Cements for coatings or bases

Those used as coatings have a low powder / liquid ratio to allow the material to adapt well to the walls of the dental cavity.

If they are used as a base, their powder / liquid ratio is high and they act as a substitute for dentin to later associate with the resin that is placed on top.

Applications

Glass ionomers can be used to repair caries or cervical defects (i.e., in the neck of the tooth, between the crown and the root) caused by abrasion and erosion, for the repair of temporary teeth, incisors and canines and tunnel restoration.

They are used as a base underneath amalgam or gold, to temporarily fix large carious lesions, endodontic openings and cusp fractures.

As fissure sealants

They are placed in both primary and permanent molar fissures to prevent cavities, as it is retained in depth in the gaps and prevents them from being colonized by plaque or film of bacteria. The anticaries effect is also favored by the release of fluoride.

In the restorative treatment technique without trauma

This technique is applied in countries where the lack of electricity prevents the use of electric drills and mills. It is also used in children who do not cooperate with the dentist. Its acronym is ART, from the English Atraumatic Restorative Treatment .

Hand instruments are used to remove decayed dentin, and then glass ionomer cement is applied to repair the tooth. Due to its adhesiveness, this material can be used on teeth that have had minimal preparation, making the repair quickly and effectively.

The fluoride ions released by the glass ionomer penetrate the remaining cavities, killing any bacteria that may be present.

In modified resins or hybrid ionomer cements

They are prepared from mixtures that contain the same components as glass ionomers, but also include a monomer and a polymerization initiator.

The resulting material contains a structure based on both the acid-base reaction and the polymerization of the monomer, which is usually 2-hydroxyethyl methacrylate.

In order to develop its properties optimally, it must be irradiated with a curing lamp for a specific time. The application of light allows the activation of the polymerization reaction of the monomer by photons.

The combination of the resin with the glass ionomer increases its resistance, has less solubility and less sensitivity to humidity. However, it releases less fluoride and shows less biocompatibility than conventional glass ionomers.

Advantages of glass ionomers

Accession

The glass ionomer adheres very well to dentin and tooth enamel. This property is important because it helps it stay attached to the tooth and prevents harmful microorganisms from entering the repaired space.

The strong adhesion is initially due to the formation of hydrogen bonds between the carboxylic (—COOH) groups of the polyacrylic acid and the water molecules attached to the tooth surface. These hydrogen bonds are of the H — O — H type.

These bonds are then slowly replaced by stronger ionic bonds between calcium Ca 2+ cations in the tooth and the cement anions: (COO ) – (Ca 2+ ) – (COO ).

This material can also bind very well to metals used in tooth restoration.

How adherence is favored

To achieve better adhesion, the freshly carved tooth surface is pre-rinsed with an aqueous solution of polyacrylic acid, which slightly demineralizes the tooth surface by opening the dentin tubules.

In this way the available surface area for cation / anion bond formation is increased and an ion-rich layer is formed which is highly resistant to acid attack.

Other professionals in this area recommend pre-rinsing with phosphoric acid (H 3 PO 4 ) to clean the cavity and remove particles, including traces of oil, from the instrument that drilled the tooth.

Bioactivity

It is capable of releasing biologically active ions such as fluoride, sodium, calcium, phosphate and silicate to the surrounding environment.

Calcium is an essential mineral for teeth and favors their remineralization. Silicate can be incorporated naturally into the hydroxyapatite of the tooth, as well as phosphate. Fluoride forms fluoroapatite.

The ionomer can also take up calcium and phosphate ions from the surroundings, such as saliva, developing a harder surface.

Anticaries effect

According to recent reviews (year 2019) of publications on glass ionomers, it is confirmed that they have a measurable anticaries effect. The layer rich in ions they generate makes secondary cavities very rare around restorations made with these.

Regarding the proportion of cavities, they have proven to be as or more effective than composite resins.

Some studies suggest that the cariostatic property is probably due to the physical barrier that the glass ionomer provides in the cracks and not to a chemical effect on the inhibition of demineralization.

Fluoride release

It can release fluoride ion, a property that is maintained for very long periods of time and is considered clinically beneficial for the tooth, as it prevents decalcification of the enamel. Release increases under acidic conditions.

Certain sources indicate that the fluoride released by the glass ionomer reduces decalcification around orthodontic supports or bracketts and some professionals indicate that it acts as an antibacterial.

However, according to other authors, there is no clear evidence about whether fluoride release is beneficial or not for the tooth.

Easy removal

When new repairs are required, it can be removed with much less difficulty than other materials, since the cement that remains on the tooth surface can be dried by applying air, making it more fragile and easy to remove.

Disadvantages

Conventional glass ionomers have relatively low strength, so they can be brittle or brittle and have a tendency to wear.

This is associated with its microporosity, or the presence of small holes within its structure. Therefore, they show a propensity to fail with a higher speed than other restorative materials and cannot be used in areas that support high stress.

References

  1. Sidhu, SK and Nicholson, JW (2016). A Review of Glass-Ionomer Cements for Clinical Dentistry. J. Funct. Biomater. 2016, 7, 16. Recovered from mdpi.com.
  2. Attaie, AB and Ouatik, N. (2015). Esthetics and pediatric dentistry. Posterior glass ionomer and resin-modified glass ionomer restorations. In Esthetic Dentistry (Third Edition). Recovered from sciencedirect.com.
  3. Zheng, LW et al. (2019). Glass ionomer cements. In Encyclopedia of Biomedical Engineering. Volume 1. Recovered from sciencedirect.com.
  4. Uses of glass ionomer materials. (2007). Restoration of teeth (simple restorations) and preventative dentistry. In Restorative Dentistry (Second Edition). Recovered from sciencedirect.com.
  5. Nesbit, SP et al. (2017). Definitive phase of treatment. Glass ionomer restoration. In Diagnosis and Treatment Planning in Dentistry (Third Edition). Recovered from sciencedirect.com.
  6. Üsümez, S. and Erverdi, N. (2010). Adhesives and Bonding in Orthodontics. Glass ionomer cements. In Current Therapy in Orthodontics. Recovered from sciencedirect.com.
  7. Wells, MH and Dahlke Jr. WO (2019). Pit and Fissure Sealants. Glass Ionomer. In Pediatric Dentistry (Sixth Edition). Recovered from sciencedirect.com.
  8. Knight, GM (2018). Glass Ionomers: Why, Where and How. Recovered from oralhealthgroup.com.
  9. Gjorgievska, E. et al. (2020). Assessment of the Impact of the Addition of Nanoparticles on the Properties of Glass-Ionomer Cements. Materials 2020, 13, 276. Recovered from mdpi.com.

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