Ion-release, Dissolution, and Buffering by Zinc Phosphate Dental Cements

Objectives: The interaction of zinc phosphate dental cement with aqueous solutions has been studied to elucidate the relationship between pH change and ion release/dissolution. Methods: For each storage medium (deionised water, lactic acid at pH 2.7 and lactate buffer at pH 2.2) 5 cylindrical specimens of zinc phosphate cement (6 m diameter x 12 mm high) were prepared and weighed. They were stored individually in 8 ml of solution for a week, then the pH was determined and the specimens reweighed. The solutions were renewed, specimens stored for another week, then the pH and weight were again measured. This was repeated for four weeks. For each storage solution, the concentration of ions (Na, Mg, Al, Zn and P) were determined using ICP-OES. Results: Lactate buffer reduced specimens to 4.1 % (S.D. 0.9%) of their original mass after 4 weeks, lactic acid reduced them to 98.3%, water had no significant effect. In all media Zn and P ions predominated, and Na, Al, and Mg were also released. Mole ratios varied at each time interval, but Zn was always the highest. In all cases, solution pH was increased. Conclusions: The high Zn concentration shows dissolution/ion release involves mainly the filler. Ion release influences the pH of storage medium and the presence P in water suggests that the set cement does not contain completely insoluble Zn3(PO4)2, as previously assumed.