Uptake and release of fluoride by saliva-coated glass ionomer cement

The 'recharging' of aged glass ionomer cement restorations with fluoride by exposing them to a concentrated NaF solution has been suggested to improve their performance as devices with slow fluoride release. We have studied the interference of salivary coatings on glass ionomer cement with the uptake of fluoride in vitro. Freshly prepared glass ionomer discs were leached in distilled water for 14 weeks, then incubated in human saliva for 2 h, 24 h or 1 week before exposure to 0.53 M NaF (1% fluoride) for 5 min. Fluoride uptake by the cement was measured indirectly as the subsequent increase in fluoride release. A 2-hour incubation in saliva reduced the fluoride uptake by half, a 24-hour incubation by 74%. The major part (93-95%) of the fluoride that was taken up was released on the first day. A 1-week salivary coating reduced the uptake by 49%, but also caused some retardation of the fluoride release: 80% of the total release was on the first day. It was concluded that despite the considerable interference by salivary coating, the amount of fluoride that can be taken up by aged glass ionomer cement remains significant. However, the fast release will limit the usefulness of such recharging in vivo.