Four commercial glass-ionomer cements have been studied for their ability to take up fluoride from aqueous solution following variable maturation times in the dry at 37 °C.
Sets of five specimens of four different materials were cured for times of 10 min, 24 h and 1 month, then transferred to a neutral solution of NaF at approximately 1000 ppm in fluoride. Fluoride concentration was then measured at regular time intervals up to 1 month using a fluoride ion selective electrode.
Specimens cured for 1 month showed little or no uptake over 24 h, specimens cured for 24 h showed fluctuating uptake over time, and specimens cured for 10 min showed the greatest fluoride uptake. For the latter specimens, simple kinetic models were found to apply in two cases (pseudo-first order and pseudo-second order for Chemflex and Ketac Molar Quick respectively).
The ability to take up fluoride has been shown to decline with age of cement which suggests that in clinical use glass-ionomers may become less effective at undergoing fluoride recharge than has been previously assumed.
Glass-ionomer cements are used in clinical dentistry as liners and bases and as full restorative materials . An important advantage of these materials is their ability to release fluoride , a release that can be sustained for many years . Glass-ionomers have also been shown to be capable of taking up fluoride from their surroundings , and this has led to the suggestion that they may be able to act as fluoride reservoirs when used clinically .
Most studies of fluoride uptake have been indirect. This means that cements have been exposed to high concentrations of fluoride, typically potassium fluoride at around 1000 ppm, then the specimens placed in pure water and the subsequent fluoride release studied. In such experiments, it has been shown that fluoride release is enhanced by exposure to high concentrations of fluoride . It has also been shown that experimental cements formulated from fluoride-free glass can be made to release fluoride following exposure to fluoride solutions in this way .
However, to date there have been few studies of fluoride uptake using direct measurement, only one of which has determined uptake kinetics. This study employed various commercial cements, and exposed specimens to potassium fluoride solutions at concentrations of 100 and 1000 ppm. Fluoride in solution was measured directly using a fluoride-ion selective electrode, and reductions in fluoride concentration were observed with time. Significant reductions were apparent within 15 min, and experiments were continued for up to 24 h, with measurements made mainly at earlier time intervals.
Fluoride uptake was found to follow pseudo-first order kinetics . The integrated form of the appropriate first order adsorption equation is
ln ( q e − q t ) = ln ( q e ) + k t ,