Glass ionomer cement is a kind of dental cement that was developed in and began to be used in restorative dentistry in Made of a silicate glass. Whereas traditional glass ionomer cements were opaque, newer resin-modified glass ionomers have attained a much better esthetic match to dentin and. Glass Ionomer Cement. Glass ionomer cements (GIC) are the only direct restorative material to bond chemically to hard dental tissues owing to the formation of.
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This article is an updated review of the published literature on glass-ionomer cements and covers their structure, properties and clinical uses within dentistry, with an emphasis on findings from the last five years or so. Glass-ionomers iomomer shown to set by an acid-base reaction within 2—3 min and to form glasss, reasonably oonomer materials with acceptable appearance.
They release fluoride and are bioactive, so that they gradually develop a strong, durable interfacial ion-exchange layer at the interface with the tooth, which is responsible for their adhesion. Modified forms of glass-ionomers, namely resin-modified glass-ionomers and glass carbomer, are also described and ionmer properties and applications covered.
Physical properties of the resin-modified glass-ionomers are shown to be good, and comparable with those of conventional glass-ionomers, but biocompatibility is somewhat compromised by the presence of the resin component, 2 hydroxyethyl methacrylate. Properties of glass carbomer appear to be slightly inferior to those of the best modern conventional glass-ionomers, and there is not yet sufficient information to determine how their bioactivity compares, although they have been formulated to enhance this particular feature.
Glass-ionomer cements belong to the class of materials known as acid-base cements.
They are based on the product of reaction of weak polymeric acids with powdered glasses of basic character [ 1 ]. Setting occurs in concentrated solutions in water and the final structure contains a substantial amount of unreacted glass which acts as filler to reinforce the set cement. There are three essential ingredients to a glass-ionomer cement, namely polymeric water-soluble acid, basic ion-leachable glass, and water [ 4 ].
These are commonly presented as an aqueous solution of polymeric acid and a finely divided glass powder, which are mixed by an appropriate method to form a viscous paste that sets rapidly. However, alternative formulations exist which range from both the acid and the glass being present in the powder, and pure water being added to cause setting, to formulations in which some of the acid is blended with the glass powder and the rest is present in a dilute solution in water.
This solution is used as the liquid component in forming the paste for setting. The effect of these differences is not clear, because these formulations are proprietary, so that the exact amount of each component is not widely known. However, there appears to be no obvious effect on the final properties of presenting these materials with the components distributed differently between gllass powder and aqueous phases.
Glass-ionomer cements can be mixed using a spatula on a pad or glass block, so-called hand-mixing. The material can also be presented in a bespoke capsule, separated by a membrane.
The membrane is broken immediately before ionomed, and the capsule is vibrated rapidly in a specially designed auto-mixer. This mixes the cement after which the freshly-formed paste is extruded from the capsule and used for intra-oral application. Where a single brand is available as both a hand-mixed and capsulated version, the two types of cement have to be formulated differently.
A cement paste that sets in a satisfactory time when hand-mixed sets far too rapidly when subject to vibratory mixing. As a result, formulations for capsulation have to be less reactive than those for hand-mixing, and they rely on the accelerating effect of auto-mixing to give them satisfactory working and setting times.
The polymers used in glass-ionomer cements are polyalkenoic acids, either glqss poly acrylic acid or the 2: Poly vinyl phosphonic acid has been studied as a potential cement former [ 5 ], but its practical use is restricted to a single brand, where it is used in a mixture with poly acrylic acid and effectively acts as a setting rate modifier [ 6 ].
There is confusion in the literature about which polymers are used in glass-ionomer cements. This is because early research studied a range of mono- di- and tri-carboxylic acid monomers in polymers for gass formation, including itaconic and tricarballylic acid [ 7 ]. This has led some authors to assume that these substances must be used in practical cements. However, this is not the case, and commercial cements use either the homopolymer or copolymer of acrylic acid. The polymer influences the properties of the glass-ionomer cement formed from them.
High molecular weights increase the strength of the set cement, but solutions of high molecular weight polymers have high viscosities, making them difficult to mix. Molecular weights are therefore chosen to balance these competing effects.
Optimum properties are said to be achieved with inoomer molecular weights of 11, number average and 52, mass average [ 8 ]. These values give a polydispersity of 4.
Cements fement from homopolymers of acrylic acid show increases in compressive strength in the first 4—6 weeks. On the other hand, cements made from acrylic-maleic acid copolymers show an increase in compressive strength up to a point, but then there is a decline before an equilibrium value is reached. Compressive strength is ceement a fundamental property of materials, because compression causes a specimen to fracture in complex ways in directions approximately at right angles to the compressive force.
However, these alterations in measured compressive strength indicate that the material continues to undergo slow changes over time. In particular, this reduction has been attribute to the higher crosslink density cemsnt develops within copolymer cements compared with cements based on acrylic acid homopolymer [ 9 ]. It is vital that glasses for ionomer cements should be basic, i. In principle, several different glass compositions can be prepared that fulfil this requirement but in practice, only alumino-silicate glasses, with fluoride and phosphate additions, are fully satisfactory.
Commercial glasses for glass-ionomer cements are typically based on calcium compounds, with some extra sodium. There are also materials in which calcium has been substituted by strontium. Ionomer glasses owe their basic character to the fact that both alumina and silica are used in their preparation.
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Glasses based on silica alone lack reactivity and also basicity, because their structure contains mainly SiO 4 tetrahedra linked at the corners to form chains that carry no charge. When alumina is added, the aluminium is forced to adopt ionome similar 4-fold tetrahedral geometry to silicon, i.
These create basic character, and make the glass susceptible to attack by acids. Fluoride is also a vital component of the glasses used in glass-ionomer cements. An example composition is shown in Table 1for the glass known as G, which is similar to several tlass ionomer glasses. Practical ionomer glasses, including G, iomomer known to undergo at least partial phase separation as they cool [ 10 ].
This leads to regions of varying composition and typically adalay the occurrence of one phase that is more susceptible to acid attack than the others. In principle, this might be expected to alter the optical properties of the glass, and in turn the cement, but there have been no studies reported exploring this point. Studies of ionomer glasses have been carried out using MAS-NMR spectroscopy and these have provided useful structural information about these materials.
What is glass ionomer cement? | HowStuffWorks
Aluminium has been shown to occur in both 4- and 5-co-ordination in various glasses [ 1112 ], which confirms the effect of silica on the co-ordination state of aluminium [ 12 ]. Fluorine occurs in these glasses bound exclusively to aluminium [ 13 ]. The substitution of calcium with strontium in glasses of this type can be achieved by using the compounds SrO and SrF 2 in the place of CaO and CaF 2 in the glass-forming mixture [ 14 ].
Strontium has the effect of increasing radiopacity compared with calcium in these glasses without ce,ent adverse effect on the appearance of these cements.
Fluoride release is enhanced from these cements, though the reason for this is not known. The reasons for this are not clear. Certainly this is consistent with the cejent that the bands due to aluminium polyacrylate appear later when tartaric acid is present than when it is absent.
The bands arising from the various possible metal carboxylates occur in distinct regions of the infrared spectrum, as shown in Table 2. It then sets sharply to give the finished, hardened material that can be completed within the tooth. However, its effectiveness varies between glasses, depending on their composition.
Glass-ionomers set within 2—3 min from mixing by an acid-base reaction. The first step is a reaction with hydrated protons from the polyacid at basic sites on the surface of the glass particles. These ions then interact with the polyacid molecules to form ionic crosslinks, and the insolubilised polysalt that forms becomes the rigid framework for the set cement. When this setting reaction occurs, all of the water becomes incorporated into the cement, and no phase separation occurs.
The overall reaction appears to take place in two steps in a diffusion-controlled process [ 18 ]. The first step is the formation of ionic crosslinks, as we have seen, and this is responsible for the immediate hardening process.
This glsss step is slow, and continues for approximately a day [ 20 ]. After this initial hardening, there are further reactions, which take place slowly and are together known as maturation. They are associated with various changes in the physical properties of the resulting glass-ionomer cement [ 1 ]. Strength typically increases, as does translucency. In addition, the proportion of tightly-bound water within the structure increases.
The details of these processes are not known, and research continues on this question. Some years ago, it was shown that that hard, insoluble cements could be formed by reaction of ionomer glasses with acetic acid.
Glass ionomer cement
This is in spite of the fact that metal acetate salts are soluble in water [ 21 ]. It was also observed that these cements became progressively stronger in compression up to 3 months, though there were no discernible changes in the infrared spectra of the cements.
This led to the conclusion that there was an inorganic setting reaction that complemented the neutralization reaction in the setting of these cements.
In contrast, phosphate-free silicate glasses were shown not to undergo an equivalent glsss reaction [ 22 ]. This suggested that the proposed inorganic network cemwnt phosphate-based.
As mentioned, water is the third essential component of the glass-ionomer cement.
Several roles have been identified for water [ 9 ]. It is the solvent for the polymeric acid, it allows the polymer to act as an acid by promoting proton release, it is the medium in which the setting reaction takes place, and lastly, it is a component of the set cement [ 9 ].
Incorporation of water with glass-ionomers is associated with increases in the translucency of the glass-ionomer cement. Binding may occur partly by co-ordination to metal ions and partly by strong hydration of the polyanion molecules [ 9 ]. In addition, it may react with —Si—O—Si— units at the surface of the glass particles, leading to the formation of —Si—OH groups [ 23 ].
This has been confirmed by a few FTIR studies where the relevant region of the spectrum has been examined. The unbound water can be lost from the surface of a newly placed glass-ionomer cement. This causes an unsightly chalky appearance as microscopic cracks develop in the drying surface. To prevent this, it is important to protect the cement by covering it with an appropriate varnish or petroleum jelly [ 25 ].
Two types of varnish are available, namely simple solutions of polymer in solvent and light-curable low viscosity monomer.
There is evidence that the light-curable varnishes give superior protection in preventing drying out [ 25 ], because the lack of solvent means that the film formed has no porosities in it through which water can still escape. The physical properties of glass-ionomer cements are influenced by how the cement is prepared, including its powder: Care is needed therefore in making generalisations about the properties of these materials.
There is also the possibility that part of the success of glass-ionomers may arise because their performance is satisfactory even if they have not been properly mixed, or allowed to mature under ideal conditions.
The current ISO standard for glass-ionomers [ 3 ] gives minimum values for certain physical properties. These values, which are shown in Table 3are the least acceptable for a material to be allowed onto the market, rather than typical for materials known to perform well clinically. The only type of strength that the ISO standard deals with is compressive strength, but glass-ionomers also have reasonable flexural strength [ 1 ].
Their biaxial flexure [ 26 ] and their shear punch strengths [ 27 ] have also been determined.