Dental silicate cement

Wilson, A. D. (1968). Dental silicate cements VII. Alternative liquid cement formers. Journal of Dental Research, 47, 1133-6. 

Wilson, A. D., Kent, B. E., Clinton, D. Miller, R. P. (1972). The formation and microstructure of dental silicate cement. Journal of Materials Science, 7, 220-38. 

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... 

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). 

Flexural strength and fracture toughness are clinically more significant than compressive strength. The flexural strength of a glass-ionomer cement can reach 39 MPa after 24 hours (Pearson Atkinson, 1991) which is a much higher value than that attained by any dental silicate cement. 

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). 

Another barrier to achieving translucency is mismatch between the refractive indices of the glass and the matrix the refractive index of the glass is greater than that of the matrix, which causes light-scattering. The dental silicate cement tends to be naturally more translucent than the glass... 

The glass polyalkenoate cement was originally intended as a substitute for dental silicate cements for the aesthetic restoration of front (anterior) teeth (Wilson Kent, 1972 Knibbs, Plant Pearson, 1986a Osborne Berry, 1986 Wilson McLean, 1988). It is suitable for restoring anterior cavities in low-stress situations, that is when the restoration is completely supported by surrounding tooth material. These cavities occur on the adjacent surfaces of neighbouring teeth (class III cavities) and at the gum line (class V cavities). 

Paffenbarger, G. C. (1937). Dental silicate cements. In Judd, D. B. Optical specifications of light scattering materials. Rep>ort No. 1026. Journal of the National Bureau of Standards, 19, 314-16. 

Paffenbarger, G. C., Schoonover, I. C. Souder, W. (1938). Dental silicate cements Physical and chemical properties and a specification. Journal of the American Dental Association, 25, 52-87. 

As we have seen in Section 6.2, there is some evidence for supposing that zinc phosphate cements contain an amorphous aluminium phosphate or zinc aluminophosphate phase. Also, as we shall see in Section 6.5, amorphous aluminium phosphate is the binding matrix of dental silicate cement. 

Dental silicate cement was once the most favoured of all anterior (front) tooth filling materials. Indeed, it was the only material available for the important task of aesthetic restoration from the early 1900s to the mid 1950s, when the not very successful simple acrylic resins made their appearance (Phillips, 1975). In the mid sixties there were some 40 brands available (Wilson, 1969) and Wilson et al. (1972) examined some 17 of these. Since that time the use of the cement has declined sharply. It is rarely used and today only two or three major brands are on the market. The reason for this dramatic decline after some 50 years of dominance is closely linked with the coming of modern aesthetic materials the composite resin from the mid 1960s onwards (Bowen, 1962), and the glass-ionomer cement (Wilson Kent, 1971) from the mid 1970s. 

Dental silicate cement was also variously known in the past as a translucent, porcelain or vitreous cement. The present name is to some extent a misnomer, probably attached to the cement in the mistaken belief that it was a silicate cement, whereas we now know that it is a phosphate-bonded cement. It is formed by mixing an aluminosilicate glass with an aqueous solution of orthophosphoric acid. After preparation the cement paste sets within a few minutes in the mouth. It is, perhaps, the strongest of the purely inorganic cements when prepared by conventional methods, with a compressive strength that can reach 300 MPa after 24 hours (Wilson et al, 1972). 

Voelker (1916a) reported three early dental silicate cements which appeared in 1895, 1897 and 1902 all proved inadequate. The first successful material was developed by Steenbock (1903,1904) who explicitly sought and formulated a translucent cement (Voelker, 1916a,b). It was marketed by Ascher in 1904 as New Enamel Richters Harvadid cement followed in the same year. Thereafter development was rapid and eight varieties were reported by Morgenstem in 1905. However, from their chemical composition we doubt whether they were sufficiently translucent. 

Dental silicate cement is used exclusively for the aesthetic restoration of... 

After 1950 some serious attempts were made to improve dental silicate cement. Notable were those of Manly et al. (1951) and Rockett (1968) also, Pendry reported an acid-resistant cement containing indium (Pendry Cook, 1972 Pendry, 1973). None of these experimental materials went into production. These attempts came too late in view of progress made in other directions, and the picture of dental silicate cement remains one of an essentially traditional material which has changed little since the original developments made mainly by German chemists prior to 1914. 

The powders used in dental silicate cement are unusual in being ground opal glasses rather than crushed crystalline clinker. The glassy nature of the powder gives the set cement the unusual property of translucency... 

Effect of glass composition on cement properties As we have indicated previously, two types of glass have been used in dental silicate cements the obsolete oxide glass and the modem fluoride glass. Only four studies on glass composition and its relationship to cement property have been published (Wright, 1919 Crepaz, 1951 Manly et al.. 

Table 6.6. Chemical composition of commercial dental silicate cements (Wilson et al., 1972)...

Figure 6.13 Electron micrograph of a single-stage replica of a dental silicate cement glass, showing phase-separated droplets rich in calcium and fluoride large droplets 400 nm in diameter and small droplets 20 to 30 nm in diameter (Wilson et at., 1972).

Dental silicate cement liquids are concentrated aqueous solutions of orthophosphoric acid generally containing aluminium and zinc (Wilson, Kent Batchelor, 1968 Kent, Lewis Wilson, 1971a,b Wilson et al., 1972). The optimum orthophosphoric acid concentration is 48 to 55 % by mass (Wilson et al, 1970a), although higher concentrations are encountered. Aluminium is present as phosphate complexes and zinc as a simple ion (see Section 6.1.2). Examples are given in Table 6.6. 

Effect of liquid composition on cement properties The properties of dental silicate cements are affected both by the concentration of phosphoric acid and by the presence of metal salts. 

All commercial examples of phosphoric add solutions used in these cements contain metal ions, whose role has been discussed in Section 6.1.2. In the case of the dental silicate cement, aluminium and zinc are the metals added to liquids of normal commerdal cements and have a significant effect on cement properties (Table 6.8) (Wilson, Kent Batchelor, 1968 Kent, Lewis Wilson, 1971a,b). Aluminium accelerates setting for it forms phosphate complexes and is the prindpal cation of the phosphatic matrix. Zinc retards setting for it serves to neutralize the addic liquid - it... 

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