Effect of monovalent ions in glass ionomer cements on their interaction with sodium fluoride solution

The effects on surface morphology of glass ionomer cements following exposure to 0.2% NaF solution were studied. The effect of cement on the solution was also evaluated. The four cements were chosen to contain Na and F, Na alone, F alone and neither Na nor F to show any interactions produced by having the same ion in both the cement and solution. Four glass ionomer cements were formulated so that they differed only in respect of the glass component. AH2 (a glass used in dental restorative cement) contained both Na and F, MP4 (a glass used in orthopaedic cement) contained Na only, LG26 (a glass used in surgical cement) contained F only and LG30 (an experimental control glass) contained neither F nor Na. Discs of cement were set in moulds at 37 degrees C for 1 h, then matured in water for 3 d. Each test disc was then immersed in 10 ml 0.2% NaF for 24 h at 37 degrees C whereas control discs remained in water. The test and control disc surfaces were assessed qualitatively using electron microscopy and quantitatively by linear profilometry generating roughness values (Ra). Test solution pH was measured before and after cement immersion. Inspection of the electron micrographs showed considerable disruption of AH2 and LG26 test surfaces compared to their controls whereas MP4 and LG30 showed similar surfaces for test and control. Statistical analysis of the Ra values showed that AH2 and LG26 test surfaces were significantly rougher than their controls as well as LG30 and MP4 test surfaces, which were not significantly different from their controls. All NaF solutions show pH increases; those for AH2 and MP4 were significantly higher than those for LG26 and LG30. The F-containing cements were subject to surface disruption whereas F-free cements were not. The Ra values of test surfaces correlated strongly (r = 0.998) with the F uptake of the cements (data from a previous study) but it was not possible to ascribe the causality to this association. The pH changes appear to be influenced by whether or not Na is present in the cement. The resultant pH values are too near to neutral for pH alone to explain the surface disruption observed. In addition, it is concluded that the changes in OH ion concentration are too low to permit F-/OH- interchange as a possible explanation for F uptake by these cements.