Glass polyalkenoate cement

Another development has been the advent of the dual-cure resin cements. These are hybrids of glass polyalkenoate cements and methacrylates that set both by an add-base cementation reaction and by vinyl polymerization (which may be initiated by light-curing). In these materials, the solvent is not water but a mixture of water and hydroxyethylmethacrylate which is capable of taking dimethacrylates and poly(acrylic add)-containing vinyl groups into solution. In the absence of light these materials set slowly and... 

The glass-ionomer or glass polyalkenoate cement (Section 5.9)... 

The glass polyalkenoate cement, formerly known as the glass-ionomer cement, was invented by Wilson and Kent in 1969 (Wilson Kent, 1973) and is now well established as a material that has an important role in clinical dentistry. It has proved to have considerable development potential and has been subjected to continuous development, improvement and... 

Glass polyalkenoate cement has a unique combination of properties. It adheres to tooth material and base metals. It releases fluoride over a long period and is a cariostat. In addition it is translucent and so can be colour-matched to enamel. New clinical techniques have been devised to exploit the unique characteristics of the material. 

The powders used in glass polyalkenoate cement formulations are prepared from glasses and not opaque sintered masses. In this they resemble the traditional dental silicate cement from which they are descended. The glass plays several roles in the chemistry and physics of the glass polyalkenoate... 

Table 5.10. Examples of practical glasses used in glass polyalkenoate cement Crisp, Abel Wilson, 1979 Wilson McLean, 1988 Brook, Craig Lamb, 1991)...

Table 5.11. Effect of +)-tartaric acid on glass polyalkenoate cement properties...

The poly(alkenoic acid)s used in glass polyalkenoate cement are generally similar to those used in zinc polycarboxylate cements. They are homopolymers of acrylic acid and its copolymers with itaconic add, maleic add and other monomers e.g. 3-butene 1,2,3-tricarboxylic add. They have already been described in Section 5.3. The poly(acrylic add) is not always contained in the liquid. Sometimes the dry add is blended with glass powder and the cement is activated by mixing with water or an aqueous solution of tartaric add (McLean, Wilson Prosser, 1984 Prosser et al., 1984). 

Increase in concentration of the polyacid increases solution viscosity, quite sharply above 45% by mass (Crisp, Lewis Wilson, 1977). The strength of glass polyalkenoate cements also increases, almost linearly, with polyacid concentration. This is achieved at the cost of produdng overthick cement pastes and loss of working time. 

The molecular weight of the polyacid affects the properties of glass polyalkenoate cements. Strength, fracture toughness, resistance to erosion and wear are all improved as the molecular weight of the polyadd is... 

Table 5.12. Ejfect of the various tartaric acids on glass polyalkenoate cement properties Crisp, Lewis Wilson, 1979)...

The glass polyalkenoate cement system was not viable until Wilson and Crisp discovered the action of (+)-tartaric acid as a reaction-controlling additive (Wilson Crisp, 1975,1976,1980 Wilson, Crisp Ferner, 1976 Crisp Wilson, 1976 Crisp, Lewis Wilson, 1979). It may be regarded as an essential constituent and is invariably included in glass polyalkenoate cements as a reaction-controlling additive. 

Table 5.13. Effect of fluorides on glass polyalkenoate cement compressive strength, MPa (Crisp, Merson < Wilson, 1980)...

The glass polyalkenoate cement uniquely combines translucency with the ability to bond to untreated tooth material and bone. Indeed, the only other cement to possess translucency is the dental silicate cement, while the zinc polycarboxylate cement is the only other adhesive cement. It is also an agent for the sustained release of fluoride. For these reasons the glass polyalkenoate cement has many applications in dentistry as well as being a candidate bone cement. Its translucency makes it a favoured material both for the restoration of front teeth and to cement translucent porcelain teeth and veneers. Its adhesive quality reduces and sometimes eliminates the need for the use of the dental drill. The release of fluoride from this cement protects neighbouring tooth material from the ravages of dental decay. New clinical techniques have been devised to exploit the unique characteristics of the material (McLean Wilson, 1977a,b,c Wilson McLean, 1988 Mount, 1990). 

The glass polyalkenoate cement sets rapidly within a few minutes to form a translucent body, which when young behaves like a thermoplastic material. Setting time (37 °C) recorded for cements mixed very thickly for restorative work varied from 2-75 to 4-7 minutes, and for the more thinly mixed luting agents from 4-5 to 6-25 minutes. Properties are summarized in Table 5.15. 

Table 5.15. Mechanical properties of glass polyalkenoate cement Prosser et al. 1984 0ilo, 1988 Paddon Wilson, 1976 Wilson, Paddon Crisp, 1979 Pearson Atkinson, 1991 Williams Billington, 1989)...

Figure 5.18 This figure shows how the properties of a glass polyalkenoate cement change as it ages. S is the compressive strength, E the modulus, a a stress-relaxation function, and c a strain-conversion function from elastic to plastic strain (Paddon Wilson, 1976).

Fracture toughness values for glass polyalkenoate cement vary from 0-25 to 0-55 MN (Lloyd Mitchell, 1984 Goldman, 1985 Lloyd Adamson, 1987). The values are generally higher than those found for the traditional dental silicate cement but lower than those found for anterior composite resins (Lloyd Mitchell, 1984 Goldman, 1985) and much lower than those for posterior composite resins and dental amalgams (Lloyd Adamson, 1987). 

Figure 5.19 The translucent appearance of glass polyalkenoate cements when placed on a black and white striped background.

The first glass polyalkenoate cement had a C, of 0-76, which was far too high, but improved modem materials are more acceptable and a value as low as 0-52 has been reported for one of these (Crisp, Abel Wilson, 1979). Knibbs, Plant Pearson (1986b) have found that most glass polyalkenoate cements have a good optical match with tooth enamel. 

The glass polyalkenoate cement has the important property of adhering to untreated enamel and dentine as many workers have shown (Wilson McLean, 1988 Lacefield, Reindl Retief, 1985). It also appears to adhere to bone and base metals (Hotz et al., 1977). 

Figure 5.21 The laminate restoration, showing the glass polyalkenoate cement as a dentine substitute and a composite resin as an enamel substitute.

Although the glass polyalkenoate cement is the most durable of all dental cements it is susceptible to attack by aqueous fluids under certain conditions. There are three related phenomena to consider erosion, ion release and water absorption. 

When fully hardened, the cement is resistant to erosion provided the solution has a pH above 4. However, the glass polyalkenoate cement is susceptible to erosion immediately after set because some of the matrixforming cations and anions are still in soluble form. In fact, the hardening process is one where these cations and anions continue to precipitate. For this reason these cements have to be protected, temporarily, by a varnish. 

When immature glass polyalkenoate cements are exposed to neutral solutions, such as normal saliva, they release ions and absorb water. The... 

Figure 5.22 Effect of acid-etching on the surface of a glass polyalkenoate cement (McLean et al, 1985).

As the cement ages, absorption of water and loss of aluminium ions ceases (after 7 days). Other species - sodium and fluoride ions and silicic acid - continue to be eluted. The release of fluoride is important, for the glass polyalkenoate cement can be seen as a device for its sustained release. 

Crisp, Lewis Wilson (1980) made a chemical study of the erosion of a glass polyalkenoate cement under acid attack. They found that the chief species eluted were sodium and fluoride ions and silicic acid suggesting that attack occurred mainly on the glass particles rather than on the matrix. 

McKinney, Antonucci Rupp (1987) found that the clinical wear of the glass polyalkenoate cement compared favourably with that of the composite resin, but they noted that it was prone to brittle fracture and chemical erosion. 

The biocompatibility of the glass polyalkenoate cement is good (Wilson McLean, 1988 Nicholson, Braybrook Wasson, 1991) and its capacity to release fluoride in a sustained fashion makes it cariostatic (Hicks, Flaitz Silverstone, 1986 Kidd, 1978). Its ability to provide an excellent seal (Section 5.9.9) is an important attribute because in recent years it has... 

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