3.6.3.1 Linear relationship of fluoride release with time ½
The linear relationship between the fluoride release and square root of time is evident from Table 3.5 with very good correlation. This relationship occurs in both water and artificial saliva. The fluoride release is considered to be driven by the diffusion mechanism (Fick, 1855).
3.6.3.2 Fluoride release from AH2
Relative to the LG series, the AH2 showed the highest fluoride release which may be associated with the high fluoride content of the glass. From figure 3.5 a “burst release” can be observed initially which is more marked in case of the pseudocement similar to the real cement (Forsten, 1990). Such a burst release pattern is associated with sodium containing glasses and has been attributed with the release of fluoride in the form of highly soluble sodium fluoride. However, it was interesting to note that in comparison to deionised water, fluoride release from raw glass and pseudocement
increased when stored in artificial saliva. Considering the fact that glasses used in commercially available GIC have a similar composition to AH2, this finding comes as a contrast with the results of Mallakh and Sarkar (1990) and Williams et al. (1997 and 2001) which reported a decreased fluoride release from glass ionomer cements in artificial saliva similar to the one used in this experiment. This may suggest different fluoride release behaviour for fluoro-alumino-silicate glasses and cements.
Although earlier studies have shown that fluoride release from glass ionomer cements increases in an acidic medium due to an erosive mechanism (De Moor et. al 1998; Czarnecka et al. 2002)., it would be inappropriate to relate the acidic pH of artificial saliva with the increase in fluoride release from AH2 raw glass and pseudocement. This is because the linear relationship between fluoride release and time1/2 (Table 6) provides more evidence for the presence of a diffusion controlled fluoride release rather than erosion based one. Furthermore, if the acidic pH of the artificial saliva resulted in the increase in fluoride release, an increase from cements would also have been reported by Mallakh and Sarkar (1990) and Williams et al. (1997 and 2001)
Reviewing the AH2 composition (Table 3.1), a possible reason for the increase in fluoride release may be related to its sodium content which disrupts the glass network making it prone to phase separation and hence providing fluoride rich domains. However the decrease in fluoride release from acidwashed AH2 contradicts this
On a comparison of free and complex fluoride in deionised water and artificial saliva, it was noted that a substantial amount of fluoride is released as complexed fluoride in both the media. For the raw glass and pseudocement, there is a considerable increase in complexed fluoride in artificial saliva possibly due to the presence of ions in artificial saliva. However, once again the decrease in complexed fluoride from acid- washed glass contradicts the complex ionic composition of the artificial saliva to be responsible for this. This behaviour of the acid-washed glass in artificial saliva is not clear and requires some further work.
3.6.3.2 Fluoride release from LG26SR, LG125 and LG26
The fluoride release from LG series glasses was substantially lower than the AH2 (Table 3.4) in both deionised water and artificial saliva and the release pattern did not show the burst release. There was little difference in the fluoride release for each elution media from the LG series glasses which suggests that the substitution of calcium with strontium in the glass composition does not have a marked effect on the fluoride release. It appears that the absence of sodium is a more significant contribution to the reduction in fluoride release.
On comparison of the fluoride release in deionised water (Table 3.4) and artificial saliva it was noted that except for raw LG26 there was a substantial decrease in the fluoride release in artificial saliva. This decrease in fluoride release is similar to that observed by Mallakh and Sarkar (1990) and Williams et al. (1997 and 2001).
Williams et al. (1990) suggest that this reduce may be due to the presence of calcium in the artificial saliva which forms insoluble calcium fluoride. This may account for a distinctive white material clearly distinguishable from the glass after centrifugation of all glasses. However, the role of calcium fluoride in decreasing the fluoride release seems unlikely considering the increase in fluoride release by AH2 and that the raw LG26 release an almost equal amount of fluoride in artificial saliva and deionised water. It seems that since it is essential that there should be a balance in the release of cations and anions from the aluminosilicate glass in order to maintain its electrical neutrality (Nicholson, 1998), the low solubility of calcium and strontium ions in the artificial saliva may have affected the release of fluoride.
Another observation with the LG series was that it showed virtually no difference between free fluoride and complex fluoride in both the elution media. Comparing this with the high difference observed with AH2, it seems that the ions in the artificial saliva are not forming complexes with the fluoride but it is rather due to the inherent ions in AH2 composition which can account for this.
3.6.3.3 Fluoride release from raw glass, acid washed glass and pseudocement
In deionised water it was observed that for all glasses the raw glass released the lowest amount of fluoride, followed by acid washed and then pseudocement. This suggests that the presence of a depleted glass layer enhances the fluoride release from
fluoroaluminosilicate glasses. However, the results of release in artificial saliva show that except for AH2 there was not an obvious difference in the fluoride release from raw glass, acidwashed glass and pseudocement. Although this suggests that the presence of a depleted glass layer does not affect the release, a possible reason behind this is not clear.