The long-term interaction of dental cements with lactic acid solutions

A study of the interaction of dental cements with lactic acid solutions has been carried out in which individual cement specimens were repeatedly exposed to 20 mmol dm(-3) lactic acid for periods of a week. After each week of storage, the mass of the specimens was recorded and the pH of the solution determined. The glass-ionomers showed an initial increase in mass, followed by a decline that became steady from 6 weeks. Zinc polycarboxylate and zinc phosphate cements, by contrast, showed no early gain in mass, but eroded steadily more or less from the start of their exposure to lactic acid. For all cements, acid erosion followed linear kinetics, at rates ranging from 0.5%/week for the zinc phosphate to 0.28%/week for one of the glass-ionomers, Chelonfil (ESPE, Germany). At the end of six months, the zinc phosphate had lost 14.2% of its initial mass, the zinc polycarboxylate 9.9% and the glass-ionomers between 6.2 and 7.2%. Erosion was accompanied on every occasion by neutralization of the acid solution. Both erosion and neutralization continued steadily throughout the experiment. The effectiveness of neutralization was in the following order: zinc polycarboxylate>zinc phosphate>glass-ionomer. The pH change in Week 1 was much greater for the glass-ionomers and the zinc polycarboxylate than in all subsequent weeks.