Chapter 9 The tooth-coloured restorative materials III
Glass ionomer cements
The next group of tooth-coloured restorative materials are the glass ionomer cements. This generic group of materials is distinguished by setting involving an acid–base reaction, requiring the presence of water. All commercially available glass ionomer materials involve a reaction of an acidic liquid with a basic glass. From a chemical and ISO terminological perspective, the term glass polyalkenoate cements is strictly speaking more correct when referring to this group of materials. The original term (glass ionomer cements) excludes some of the acids being used in the currently available products.
The original glass ionomer cement produced in the 1970s was derived from dental silicate cement. This was the restorative material of choice for use in the anterior segment of the mouth in the 1930s–1950s because it had better aesthetic properties than any of the other materials available at that time. It was based on a fluoro-alumino-silicate glass combined with phosphoric acid. It had limited applications as:
The term a light-cured glass ionomer is often used by dental manufacturers in promotional literature. It is important to stress that this refers to materials which are not true glass ionomer cements. Light curing is required because of the addition of a resin to the material (together with the chemicals needed to effect light polymerization of the material). These materials should be referred to as resin-modified glass ionomer cement. Chapter 10 deals with these materials.
However, silicate cement did have certain advantages in that it released fluoride ions, which was considered to provide some resistance to the development of recurrent caries. The initial objective underpinning the development of glass ionomer cements was to overcome the inherent disadvantages of silicate cement while retaining the perceived advantage.
Components of a Glass Ionomer
The material is the product of the chemical reaction between the glass and an acid when the two components are mixed together. There is an initial dissolution of the surface of the powder. The soluble components of the glass react with the polyacrylic acid to form a matrix. After setting, the residual unreacted material is encased in this salt matrix, which holds the cement together (Figure 9.1). The set material is therefore a cored structure in which only the surface of the glass has reacted to permit the binding of the glass particles together.
The glass is relatively similar to that used in silicate cement, being based on a combination of fluoro-alumino-silicate glasses. Different properties can be given to the final cement by the manufacturers by varying the composition of the glass, such as:
The glass has a formulation based on the firing of a combination of chemicals (Table 9.1). Figure 9.2 shows some typical glass formulations for commonly available cements and highlights the substantial variation in the formulations of glass which may be used to produce a glass ionomer cement.
|Alumina (aluminium oxide)||14.2–28.6|
|Silica (silicon dioxide)||30.1–41.9|
Manufacturing process for the glass component
The glass mixture is heated to a temperature between 1150°C and 1450°C. The molten mass is then poured onto a metal plate and then into water, a process termed shock cooling. The glass is then broken up to form a glass frit (Figure 9.3). All the glasses for the currently available cements are then either wet or dry milled to produce small particles of glass. Various ranges of particle size are used depending on the requirements and usage of the cement. The larger glass particle sizes are used in those cements intended as restorative materials (up to 20 μm) while the smaller particle sizes (<5 μm) are used for luting cements.
Frit: This is the product of pouring molten glass into water. It usually consists of small particles of glass which will be ground up to smaller size later in the glass preparation process. The size of the glass particles is determined by the use or application.
Wet milled: The glass particles are placed in a cylindrical ceramic vessel with a volume of water and a number of ceramic balls. The whole assembly is then rotated. The ceramic balls tumble around the vessel grinding the glass frit down in size. The longer the tumbling process the finer the particles of glass.
If the glass is mixed with a polyacid at this point, it would set very rapidly with too short a working time for clinical use. The manufacturers all now adopt a passivation treatment to reduce the reactivity of the glass. The glass is washed in acetic acid for up to 24 hours and then dried. This process leads to ion depletion of the glass surface, resulting in fewer ions being immediately available to form the salt matrix when mixing starts. Passivation of the glass is of greater significance when the particle size of the cement is reduced, as the smaller the particle the more rapid the set.
The acid used is part of a series of polyacids, including polyacrylic acid, polymaleic acid and a number of copolymers of polyacrylic acid. The combination of polyacids that are suitable for copolymerization can be varied widely so conveying different properties to the final product. The acid in the glass ionomer cement is a variant of either the homopolymer of acrylic acid, its co-polymer or maleic acid, depending on the manufacturer. The functional group which all these acids contain and which plays a part in the chemical reaction is a carboxylate group – which in water ionizes to carboxyl and hydrogen ions. A small number of cements also include a polyphosphonic acid. This is thought to provide a short sharp set but does not appear to enhance the physical properties to any great extent.
The performance of the cement is related to the molecular weight of the acid used and this explains why two cements with apparently similar compositions can behave differently. It is generally considered that the higher the molecular weight of the acid used, the better the mechanical properties. However, this benefit is offset by the increase in viscosity of the aqueous acid solution with increasing molecular weight. To overcome this, some manufacturers vacuum dry the acid solution and then mix the anhydrous powder so formed with the glass. Activation of the cement is achieved by adding water. This addition from a dropper bottle starts the chemical reaction as the vacuum-dried acid starts to go into solution, which then is followed by the initial dissolution of the glass surface.
Maleic acid, one of the alternatives to polyacrylic acid, is a less viscous aqueous solution and this has permitted one manufacturer (3M ESPE) to use the acid in aqueous form in the encapsulated presentation more easily.
Changes in molecular weight will also influence the working and setting time of the cement. The higher the molecular weight (and so the viscosity), the shorter the working and setting times. A small amount of tartaric acid is added to all materials to accelerate the setting phase of the reaction while maintaining the working time (Figure 9.4).
Fig. 9.4 Bar graph illustrating the effect of the addition of tartaric acid on working times (WT) and setting times (ST) of a glass ionomer cement. There is no change in working time but there is a marked reduction in setting time on addition of 2% tartaric acid.
(Data from Wilson AD, Nicolson J Acid-Base Cements: Their Biomedical and Industrial Applications. Cambridge University Press, Cambridge, 1993 (Chapter 5).)
The setting reaction of a glass ionomer cement involves many stages (Figure 9.5). On mixing the cement paste:
Fig. 9.5 (A) The stages of a setting reaction in a glass ionomer cement: (a) Initial stage with glass alone and insoluble (blue) metal ions in the glass; (b) Acid attack causes some metal ions to dissolve in the acid (red). The surface of the glass from where these ions have migrated is now an ion depleted zone primarily consisting of silica gel. (c) At the gelation stage sufficient metal ions are present in solution hence a number start to reprecipitate as the salt matrix. (d) During the hardening process this reprecipitation continues until the matrix is completely formed. (B) The ions released during the matrix formation in a glass ionomer or alumino-silicate polyacrylate (ASPA) cement.
The condensation process occurs more rapidly with the calcium ions. Within a minute of the commencement of the reaction, calcium polyacrylate gel is formed. Aluminium polyacrylate formation takes substantially longer. These salts do not appear until about an hour after the start of mixing. This is attributed to the fact that the aluminium ion is triple charged and less mobile. Complexes are also formed with the fluoride ions in the solution.
The cement then goes through a period of maturation. Further cross-linking of the matrix occurs and more cations become bound onto the polyanion chain. This maturation phase may be extended in time. This also has the effect of increasing the mechanical properties of the material substantially. The maturation phase can continue for some months.
Polyanion: a molecule carrying a large number of negative charges. In the case of a glass ionomer cement, there may be a number of carboxyl groups on one polymer chain and these will individually react with metal ions which are in solution.
One of the major limitations of the conventional glass ionomer cements is the time to full set after commencement of mixing. This is quite lengthy and it is in direct contrast with the rapid setting of resin-based composites when light activated. The ability to reduce in setting time is limited as any shortening of the set time usually results in a shortened working time.
The etching of glass ionomer cements has been recommended in some restorative procedures to enhance the union between the two materials such as in the sandwich or laminate technique, where a resin composite forms the surface of the restoration and the glass ionomer acts as a base. The etching may, however, damage the cement structure. The earlier the etching process is performed in the glass ionomer cement setting reaction, the more likely it is that the salt matrix will be damaged as it has not matured. If etching is carried out within 5 minutes of placement, the matrix is completely removed within 15 seconds of applying the 37% phosphoric acid etchant (Figure 9.6). Successful etching of the glass ionomer cement can only be achieved after the matrix has matured, that is, after 24 hours. After this time the rate of removal of the salt matrix is much slower and a partial surface removal of the salt matrix will occur.
Open sandwich: A restoration in a tooth in which two restorative materials are used. They are generally mechanically and/or chemically bound together. The open sandwich technique is usually used for Class II restorations where the underlying material forms part of the axial wall and is exposed to the oral environment (Figure 9.7).
• An ion exchange process where the polyacrylic acid displaces surface phosphate and calcium, enters the hydroxyapatite structure and forming a calcium polyacrylate salt. Thus an intermediate layer of calcium and aluminium phosphates and polyacrylates are formed at the tooth/restoration interface.
The bond strength is relatively low (5 MPa) but appears to be durable and fit for purpose. There is considerable evidence that the bond can re-form if broken, thus it can be termed dynamic. This is due to the polyacrylate and calcium ions being in close proximity to each other.
The reliability of the bond is improved by preconditioning the surface of the tooth by the use of an acid conditioner. This is primarily to remove smear layer and debris from the surface of the tooth. Both citric and polyacrylic acids have been used as conditioners. The best results are obtained using polyacrylic acid as this acid is not so highly ionized. This step may be included in a clinical protocol when using glass ionomer cements. However, evidence does exist that although restorations whose cavities were conditioned lasted longer, there was no statistically significant difference in longevity between those restorations placed without conditioning. It is likely that other factors will affect the performance to a greater degree.