Dental phosphate cements

Wilson and Nicholson [3] give an extensive overview of dental cements in their book. Because such an excellent review already exists, we will only provide milestones in the development of dental phosphate cements and will add other developments that were not covered in the earlier review. 

Crisp, S., O Neill, I. K., Prosser, H. J., Stuart, B. Wilson, A. D. (1978). Infrared spectroscopic studies on the development of crystallinity in dental zinc phosphate cements. Journal of Dental Research, 57, 245-54. 

Scanning electron microscopy shows the cement to consist of zinc oxide particles embedded in an amorphous matrix (Smith, 1982a). As with the zinc phosphate cement, a separate globular water phase exists since the cement becomes uniformly porous on dehydration. Porosity diminishes as the water content is decreased. Wilson, Paddon Crisp (1979) distinguish between two types of water in dental cements non-evaporable (tightly bound) and evaporable (loosely bound). They found, in the example they examined, that the ratio of tightly bound to loosely bound water was 0-22 1-0, the lowest for all dental cements. They considered that loosely bound water acted as a plasticizer and weakened the cement. 

Unlike other aqueous dental cements, the zinc polycarboxylate retains plastic characteristics even when aged and shows significant stress relaxation after four weeks (Paddon Wilson, 1976). It creeps under static load. Wilson Lewis (1980) found that the 24-hour creep value for one cement, under a load of 4-6 MPa, was 0-7 % in 24 hours, which was more than that of a zinc phosphate cement (0-13 %) and a glass-ionomer cement (0-32%), but far less than that of the zinc oxide eugenol cement (2-2%). 

Anzai, M., Hirose, H., Kikuchi, H., Goto, J., Azuma, F. Higasaki, S. (1977). Studies on soluble elements and solubility of dental cements. I. Solubilities of zinc phosphate cement, carboxylate cement and silicate cement in distilled water. Journal of the Nihon University School of Dentistry, 19, 26-39. 

The most important of the phosphate bonded cements are the zinc phosphate, dental silicate and magnesium ammonium phosphate cements. The first two are used in dentistry and the last as a building material. Copper(II) oxide forms a good cement, but it is of minor practical value. In addition, certain phosphate cements have been suggested for use as controlled release agents. The various phosphate cements are described in more detail in the remainder of this chapter. 

In vivo studies have indicated that zinc phosphate cements erode under oral conditions. Also, cements based on zinc oxide, including the zinc phosphate cement, are less durable in the mouth than those based on aluminosilicate glasses, the dental silicate and glass-ionomer (Norman et al., 1969 Ritcher Ueno, 1975 Mitchem Gronas, 1978,1981 Osborne et al., 1978 Pluim Arends, 1981, 1987 Sidler Strub, 1983 Mesu Reedijk, 1983 Theuniers, 1984 Pluim et al., 1984, Arends Havinga, 1985). However, there is some disagreement on whether the zinc phosphate cement is more durable than the zinc polycarboxylate cement. 

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 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). 

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. 

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... 

The setting reaction of dental silicate cement was not understood until 1970. An early opinion, that of Steenbock (quoted by Voelker, 1916a,b), was that setting was due to the formation of calcium and aluminium phosphates. Later, Ray (1934) attributed setting to the gelation of silicic acid, and this became the received opinion (Skinner Phillips, 1960). Wilson Batchelor (1968) disagreed and concluded from a study of the acid solubility that the dental silicate cement matrix could not be composed of silica gel but instead could be a silico-phosphate gel. However, infrared spectroscopy failed to detect the presence of P-O-Si and P-O-P bonds (Wilson Mesley, 1968). 

In neutral solution when fully hardened, dental silicate cements are resistant to aqueous attack. Before they have fully hardened, set cements contain soluble reaction intermediates - soluble sodium salts, acid phosphates and fluorides - which render them vulnerable to attack even by neutral solutions including saliva (Wilson, 1976). 

The flow properties are not as good as those of zinc phosphate cement (Eames et al., 1978 Hembree, George Hembree, 1978) and film thickness is greater (Table 6.11). Moreover, it does not have the translucency of dental silicate cement (Wilson, 1975c). 

Darvell, B. W. (1984). Aspects of the chemistry of zinc phosphate cements. Australian Dental Journal, 29, 242-4. 

On the properties of the hydraulic zinc phosphate cement during and after the setting process. Journal of the Osaka University Dental School, 3, 77-88. 

Paffenbarger, G. C., Sweeney, S. J. Isaacs, A. (1933). A preliminary report on zinc phosphate cements. Journal of the American Dental Association, 20, 1960-82. 

Tuenge, R., Sugel, I. A. Izutsu, K. T. (1978). Physical properties of a zinc phosphate cement prepared on a frozen slab. Journal of Dental Research, 57, 593-6. 

Watts, A. (1979). Bacteria contamination and the toxicity of silicate and zinc phosphate cements. British Dental Journal, 146, 7-13. 

Wei, S. H. Y. Sierk, D. L. (1971). Fluoride uptake by enamel from zinc phosphate cement containing stannous fluoride. Journal of the American Dental Association, 83, 621—4. 

Wilson, A. D., Kent, B. E. Lewis, B. G. (1970). Zinc phosphate cements chemical study of in vitro durability. Journal of Dental Research, 49,... 

The senior author first became interested in acid-base cements in 1964 when he undertook to examine the deficiencies of the dental silicate cement with a view to improving performance. At that time there was much concern by both dental surgeon and patient at the failure of this aesthetic material which was used to restore front teeth. Indeed, at the time, one correspondent commenting on this problem to a newspaper remarked that although mankind had solved the problem of nuclear energy the same could not be said of the restoration of front teeth. At the time it was supposed that the dental silicate cement was, as its name implied, a silicate cement which set by the formation of silica gel. Structural studies at the Laboratory of the Government Chemist (LGC) soon proved that this view was incorrect and that the cement set by formation of an amorphous aluminium phosphate salt. Thus we became aware of and intrigued by a class of materials that set by an acid-base reaction. It appeared that there was endless scope for the formulation of novel materials based on this concept. And so it proved. 

In parallel to the work on zinc phosphate cements, porcelain dental cements also were developed. Steenbock [23] was the first to produce silicophosphate dental cement using 50 wt% concentrated phosphoric acid solution and an aluminosilicate glass. Schoenbeck [24] introduced fluoride fluxes in these glasses and vastly improved the dental cements. Fluorides lower the temperature of fusion of the glasses used in forming these cements. The same fluorides impart better translucency to the cement, and have some therapeutic effects. As a result, fluorides have become a part of modern dental cements. 

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