Variations in the compressive strength of dental cements stored in ionic or acidic solutions

The compressive strengths of various dental cements (a zinc polycarboxylate, a zinc phosphate, a glass-ionomer and two resin-modified glass ionomers, RMGICs) have been determined following storage in pure water, 0.9% sodium chloride solution or 20 mmol dm(-3) lactic acid solution for periods of time ranging from 24 h to 3 months. The glass-ionomer cement showed no differences between different storage solutions or at different storage times, whereas the zinc polycarboxylate, zinc phosphate and the resin-modified glass ionomer cements showed significant differences following storage in the solutions for 24 h compared with pure water. The zinc polycarboxylate cement was significantly weaker at 24 h in 0.9% NaCl and lactic acid than in pure water, whereas most of the other cements were significantly stronger in both 0.9% NaCl and lactic acid. One of the RMGICs (Vitremer luting, ex. 3M), however, was significantly stronger only in the NaCl solution, not in the lactic acid. In general, by 1 week, the strengths all reverted to being essentially the same as for specimens stored in pure water for most subsequent storage times, and did not change significantly on storage for up to 3 months. This effect of storage medium on the early strength has not been reported previously and since the media were chosen to model certain characteristics of natural saliva, the changes observed seem likely to occur in vivo. It is concluded that pure water is not the best medium for storing these cements if they are to behave as they do under clinical conditions.