Abstract
Objectives
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.
Methods
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.
Results
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).
Significance
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.
1
Introduction
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 ,
where q e is the equilibrium uptake and q t the uptake at time t . Pseudo-first order kinetics can be shown by plotting ln( q e − q t ) against t , where the result is a straight line of slope − k (rate constant). For fluoride uptake by glass-ionomer cements, values of k were found to vary from around 6 × 10 −5 s −1 from the 100 ppm solution to around 3 × 10 −5 s −1 for the 1000 ppm solution .
Other ionic species have been found to undergo adsorption onto solids by pseudo-first order processes, including uptake of chloride by solid double hydroxides and uptake of both Cr(VI) and Cd(II) ions by activated carbons . Pseudo-first order kinetics has been observed several times for adsorption of ionic species onto biological materials though overall pseudo-second order kinetics appears more common in adsorption processes .
To date, all reported studies of fluoride uptake have used specimens that are relatively young. In the most extreme case, they had been cured at 37 °C for only 10 min prior to being exposed to fluoride solution . Other studies have employed specimens cured for only short periods, typically from 1 h to 24 h . However, glass-ionomers are known to undergo changes on maturation. Properties such as compressive strength may change gradually over the first 4 weeks or so of a cement’s life , and other properties, such as translucency and proportion of tightly bound water also change. Solid state NMR spectroscopy has revealed changes in the co-ordination state of aluminum as maturation proceeds, with an increase in 6-co-ordinate Al at the expense of 4-co-ordinate Al .
The current study was undertaken to examine whether these changes influence the capacity of glass-ionomer cements to take up fluoride. In order to examine this possibility, four different commercial glass-ionomer cements have been employed and matured for varying lengths of time. Specimens have then been exposed to solutions of sodium fluoride and changes in concentration determined over time.
2
Materials and methods
Four commercial conventional glass-ionomer cements were used in this work, namely Fuji IX Fast and Fuji IX Extra (both ex GC, Japan), Chemflex and Ketac Molar Quick (both ex Dentsply, Germany). Three of these (Fuji IX Fast, Fuji IX Extra and Ketac Molar Quick) were capsulated and prepared on an auto-mixer (Linea Tac Mixer, ex Kent Express, UK) with 10 s vibration time to ensure mixing, followed by extrusion form the capsule into molds. The other materials, Chemflex, was hand mixed and prepared at a powder:liquid ratio of 3.8:1 using a metal spatula on a ceramic tile.
Freshly prepared pastes were placed in silicone rubber molds between glass microscope slides to produce sets of five discs of dimensions 6 mm diameter by 2 mm thickness. They were left in their molds and cured at time intervals of 10 min, 24 h or 1 month at 37 °C at ambient humidity in an incubator. They were then removed from their molds and placed in 5 cm 3 volumes of aqueous sodium fluoride (GPR grade, ex BDH, Poole, UK) at a nominal concentration of 1000 ppm with respect to fluoride ion.
Fluoride ion concentration was measured at time intervals of 30 min, 1, 2, 3, 4, 5 and 24 h, 1, 2, 3 and 4 weeks, using a calibrated fluoride ion selective electrode (type 309/1050/03 combination electrode, ex BDH Poole, UK).
Means and standard deviations of fluoride concentration were determined and, where appropriate, first and second order uptake graphs plotted from the data and best fit lines determined by least squares regression. Data were compared for significance using the Student t -test.
2
Materials and methods
Four commercial conventional glass-ionomer cements were used in this work, namely Fuji IX Fast and Fuji IX Extra (both ex GC, Japan), Chemflex and Ketac Molar Quick (both ex Dentsply, Germany). Three of these (Fuji IX Fast, Fuji IX Extra and Ketac Molar Quick) were capsulated and prepared on an auto-mixer (Linea Tac Mixer, ex Kent Express, UK) with 10 s vibration time to ensure mixing, followed by extrusion form the capsule into molds. The other materials, Chemflex, was hand mixed and prepared at a powder:liquid ratio of 3.8:1 using a metal spatula on a ceramic tile.
Freshly prepared pastes were placed in silicone rubber molds between glass microscope slides to produce sets of five discs of dimensions 6 mm diameter by 2 mm thickness. They were left in their molds and cured at time intervals of 10 min, 24 h or 1 month at 37 °C at ambient humidity in an incubator. They were then removed from their molds and placed in 5 cm 3 volumes of aqueous sodium fluoride (GPR grade, ex BDH, Poole, UK) at a nominal concentration of 1000 ppm with respect to fluoride ion.
Fluoride ion concentration was measured at time intervals of 30 min, 1, 2, 3, 4, 5 and 24 h, 1, 2, 3 and 4 weeks, using a calibrated fluoride ion selective electrode (type 309/1050/03 combination electrode, ex BDH Poole, UK).
Means and standard deviations of fluoride concentration were determined and, where appropriate, first and second order uptake graphs plotted from the data and best fit lines determined by least squares regression. Data were compared for significance using the Student t -test.
3
Results
Table 1 shows the values of fluoride concentration for storage solutions after all four materials cured for the three different time periods had been stored for 24 h. The initial solution was prepared as 1000 ppm in fluoride ion, but measured as 978.0 ± 5.7 ppm. Results show that fluoride concentrations for three of the four materials cured for 1 month prior to exposure to aqueous sodium fluoride did not differ significantly from this initial value. The one exception was Fuji IX Extra. It showed a small reduction in fluoride concentration in the storage solution, indicating a small uptake of fluoride by the material.
| Material | Cure time | Fluoride concentration/ppm |
|---|---|---|
| Fuji IX Extra | 10 min | 862.0 ± 4.9 |
| 24 h | 880.0 ± 17.6 | |
| 1 month | 965.2 ± 4.7 | |
| Fuji IX Fast | 10 min | 854.0 ± 4.5 |
| 24 h | 902.4 ± 4.7 | |
| 1 month | 983.6 ± 2.3 | |
| Ketac Molar Quick | 10 min | 861.4 ± 11.9 |
| 24 h | 892.4 ± 7.8 | |
| 1 month | 982.2 ± 5.1 | |
| Chemflex | 10 min | 792.8 ± 23.7 |
| 24 h | 911.1 ± 3.1 | |
| 1 month | 965.4 ± 8.8 |
Specimens cured for the shorter time periods all showed reductions in fluoride concentration in the storage solution at 24 h, indicating greater fluoride uptake by these cements. These were all significant (to p < 0.01). For all four materials, specimens cured for 10 min took up the most fluoride, and the differences between the uptake in the specimens cured for 10 min and the specimens cured for 24 h was significant in all cases (to p < 0.01).
More detailed data for fluoride concentration are shown in Tables 2 and 3 for 24 h and 10 min cure times respectively. These data have been used to determine the equivalent fluoride uptake figures shown in Tables 4 and 5 .
| Time | Fuji IX Extra | Fuji IX Fast | Ketac Molar Quick | Chemflex |
|---|---|---|---|---|
| 30 min | 969.8 ± 5.4 | 965.0 ± 3.0 | 942.6 ± 3.6 | 929.8 ± 8.0 |
| 60 min | 947.4 ± 4.4 | 969.8 ± 3.4 | 934.6 ± 11.4 | 913.4 ± 9.9 |
| 90 min | 944.6 ± 4.2 | 969.2 ± 1.6 | 929.6 ± 8.2 | 901.0 ± 11.7 |
| 120 min | 934.6 ± 3.9 | 963.8 ± 6.9 | 927.2 ± 7.9 | 892.4 ± 14.9 |
| 180 min | 916.4 ± 4.6 | 959.8 ± 4.6 | 916.8 ± 14.4 | 864.2 ± 15.7 |
| 240 min | 862.0 ± 4.9 | 926.2 ± 8.4 | 918.2 ± 13.3 | 850.8 ± 15.1 |
| 24 h | 808.0 ± 16.7 | 854.0 ± 4.5 | 861.4 ± 11.9 | 792.8 ± 23.7 |
| Equilibrium (3–4 weeks) | 638.8 ± 13.5 | 840.0 ± 8.8 | 816.8 ± 12.3 | 653.8 ± 18.2 |
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