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RESEARCH REPORT |
Paffenbarger Research Center, American Dental Association Foundation, National Institute of Standards and Technology, 100 Bureau Drive, Mail Stop 8546, Gaithersburg, MD 20899-8546, USA;
*corresponding author, Clif.Carey{at}NIST.Gov
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ABSTRACT |
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KEY WORDS: fluoride • fluoride kinetics • glass-ionomer cement • fluoride release • controlled delivery systems
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INTRODUCTION |
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The methods used for the assessment of fluoride release have varied among laboratories, making inter-laboratory comparisons difficult if not impossible (Cranfield et al., 1982). The most utilized method for the assessment of the release rate of fluoride from dental restorative materials has been via batch (static) methods. Typical regimes for the batch methods are to place material samples in a volume of leaching solution for a period of time, followed by an analysis of the solution. Some of these experiments have been extended for as long as 2 yrs (Forsten, 1990).
A continuous-flow cell apparatus and method have been designed that allow for the in vitro assessment of physiologically relevant variables such as salivary flow rate, residual volume, surface area of the restoration, and saliva composition. This method mimics more closely the in vivo situation, because saliva flows continuously through the mouth, and any fluoride released from materials into the saliva is removed from the mouth as the saliva is swallowed. The results garnered via this method are directly relevant to an understanding of the factors that govern the release of fluoride in vivo. The purpose of this paper is to present the flow-cell method and results of studies to determine the kinetics of fluoride release from glass-ionomer cements as a function of pH. The purpose of these experiments was to determine the amount of fluoride that is released from glass-ionomer cement at neutral and cariogenic pH under conditions of continuous flow. The hypothesis tested was that pH significantly affects the rate of release of fluoride from the glass-ionomer cement.
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MATERIALS & METHODS |
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) consists of three basic components: (1) the continuous-flow syringe pump, which can deliver a constant flow of fluids for extended periods of time through the sample chamber (Fig. 2A); (2) the fluoride-measurement cell (Fig. 2B), where the continuous measurement of fluoride concentration in the fluid stream occurs; and (3) the data collection system that records the output of the electrometer as a function of time. There is a pH/reference combination electrode placed in the sample chamber to monitor the pH of the leaching fluid as it passes over the sample. There are pH/reference combination and fluoride-ion-selective (F-ISE) electrodes placed in the waste container. These additional electrodes provide a system check of the cumulative fluoride concentration for the complete experiment, and are used as verification of the experimental run. An eight-channel high-impedance electrometer (Model MS314A, Elchema, Inc., Pottsdam, NY, USA) connected to a data acquisition board (Model VF910, Real Time Device, Inc., State College, PA, USA) in a PC computer running the data program (ELECTRODE.EXE) collected the data.
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The leaching fluid was delivered at a constant rate to the sample chamber, where the sample to be leached was encountered. The volume of the sample chamber can be adjusted, as can the amount of sample placed in the chamber. The volume of the sample chamber divided by the flow rate is defined as the residence time. After the leaching solution passed over the sample and out of the sample chamber, an equal volume of TISAB II was introduced into the sample stream just prior to the measurement cell.
The measurement cell was equipped with a F-ISE and a reference electrode (Fig. 2
). The fluoride concentration was measured continuously as the solution passed through the cell. The solution was then collected in a waste container, where the cumulative fluoride concentration was monitored by another F-ISE/reference-electrode pair. Additionally, a combination pH electrode was placed in the sample chamber so that we could obtain data about the sample’s effect on the pH of the leaching solution. Preliminary experiments showed that the typical drift was less than 2 mV per day and that the drift was both up and down, centering about a mean value. A two-millivolt drift represents a relative standard uncertainty of 3% in the measured value.
Standardization of the F-ISE
Standardization of the F-ISE/reference-electrode pair in the flow cell was done before, during, and after each experimental run. The flow cell was designed such that it could be removed from the system during electrode standardization. With the flow cell removed from the system, fluoride standards were injected into the flow cell with a syringe, and the data were recorded. After standardization, the flow cell was then replaced in the measurement system, and the monitoring program was re-started. This process usually took less than 30 min. The other F-ISE/reference-electrode pairs were standardized in the usual method [Orion] in 10-mL beakers containing the fluoride standards.
Measurement Validation
We validated the measurements by calculating the area under the curve of the flow cell data (kinetic) and comparing that with the cumulative fluoride concentration in the waste container. These data curves overlapped completely throughout the experiment, even though the concentrations measured in the flow cell and the waste container were different at almost all times.
Specimen Preparation
Commercially available glass-ionomer cement samples were made following the manufacturer’s recommended methods (Ketac-Fil®, Espe America, Inc., Norristown, PA, USA). Individual delivery ampoules were activated and placed in a triturater for 10 sec. The mixed cement paste was then quickly dispensed into a Teflon® mold and allowed to cure for 7 min. We made the mold by drilling 2-mm-diameter holes into a sheet of Teflon® 1.75 mm thick. Clean glass slides were used to cover the top and bottom of the mold while the samples were curing. The cylinders (2 mm diameter x 1.75 mm tall) were then removed from the mold and stored at 100% humidity for at least 24 hrs before the leaching experiments began. The surface area of the cylinders was determined by calculation to be 5.33 x 10-3 cm2·cylinder-1 or 0.494 cm2·(g of cylinders)-1 of glass-ionomer cement.
Experimental Procedures
For all the experimental runs, 1 g of cured glass-ionomer cement (Ketac-Fil®) pellets (0.494 cm2/g) was placed in a 4-mL leaching chamber. The sample surface area was approximately equivalent to the exposed surface area of 4 glass-ionomer restorations and was necessary if we were to achieve sufficient fluoride concentrations in the leach fluid to be measurable by the F-ISE. The leaching fluid (distilled H2O at neutral pH or 50 mmol/L KCl titrated to pH 4.0 with concentrated HCl) was passed over the glass-ionomer cement sample at a rate of 0.25 mL/min for up to 10 days at room temperature.
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RESULTS |
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shows that this release was significantly higher throughout the runs at pH 4 than under neutral conditions (p < 0.05 at all time points). At 72 hrs, samples leached under neutral pH conditions released fluoride at a rate of (0.34 ± 0.18) µg F cm-2 h-1 (mean ± SD). Samples leached under pH 4 released fluoride significantly faster at a rate of (1.5 ± 0.55) µg F cm-2 h-1 (p < 0.05, t test). At 5 days, the release rate was much lower at (0.09 ± 0.02, n = 2) µg L-1mm-2 under neutral conditions but still (1.0 ± 0.37) µg F cm-2 h-1 at pH 4.0 leaching.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONDISCLAIMERREFERENCES |
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The continuous-flow method described here allows many different variables to be evaluated either singly or by group so that their synergistic effects on the release of fluoride from timed-release materials can be determined. Examples include the ability of the system to evaluate the effects of saliva composition (pH, calcium, fluoride, ionic strength, etc., as simulated by the leaching fluid) on the dynamic release of fluoride; the flow rate of saliva across the restoration; the surface area of the restoration; and the amount of time required to replace saliva with fresh saliva (residence time). Possibly the most intriguing feature of the continuous-flow method is that it is flexible enough to allow for the evaluation of non-steady-state environments such as the change in pH as plaque generates acids, or intermittent high concentrations of fluoride introduced either by a rinse or from toothbrushing.
During the first 10 hrs, there was a burst of fluoride release that then declined to a slow, steady release of fluoride (Fig. 3). After the initial burst, the release characteristics could be described by the linear relationship between the logarithm of the flux vs. time under either pH 4 or neutral conditions. The logarithm of the fluoride flux from the glass-ionomer cement was significantly correlated to time, as described by the following general equation:
 | (Eq. 1) |
where: JF is the flux of fluoride per unit surface area (µg F·cm-2·h-1); (-1/t) is the rate of change of the flux per unit area (µg F·cm-2·h-2); t is time (h), and A is a constant representing the initial fluoride release rate (µg F·cm-2·h-1), as calculated from the linear regression fit of the data from 15 hrs to 72 hrs. The slope (-1/t) of Eq. 1 is related to the dissolution and diffusion of fluoride through the matrix of the glass-ionomer cement.
The slopes (-1/t) of the fluoride flux from the glass-ionomer resin at pH 4 and neutral conditions were remarkably similar at (0.0153 ± 0.0008) µg F·cm-2·h-2 and (0.0196 ± 0.0005) µg F·cm-2·h-2, respectively. The small size and similar magnitude of the coefficients indicate that the limiting mechanism in fluoride release was the diffusion of fluoride ions through the matrix material and not the effect of the bathing solution composition. The difference in the calculated flux at time zero represents the effect of the bathing solution on the initial dissolution of fluoride ions from the surface of the glass-ionomer cement, where pH 4 liberates much more fluoride initially than does neutral bathing solution. The relationship found in Eq. 1 can be used to predict the length of time the glass-ionomer cement can release defined amounts of fluoride without recharging. For example, assuming that the minimum flux of fluoride release needed to maintain a clinically relevant concentration of fluoride at the restoration is 0.1 µg F·cm-2·h-1, then one can calculate the length of time the glass-ionomer cement will succeed in meeting or exceeding this flux. Under pH 4 conditions, it will proceed for 246 hrs, and under neutral conditions, it will proceed for 133 hrs before the fluoride-release rate reaches the arbitrary minimum rate.
These predictions are based on results obtained when the bathing solutions are much simpler than the composition of whole saliva. It is interesting to speculate on what the effects of saliva would be on the fluoride-release rate from glass-ionomer cements. Because the results of this study indicate that the rate of release of fluoride (after the initial burst) from glass-ionomer cements is diffusion-controlled and not related to the solution chemistry, the only effects that saliva could have would be surface modifications that effect the rate of diffusion from the cement. It is anticipated that salivary protein could form a pellicle over the cement. This pellicle could act as a diffusion barrier as seen in enamel (Zahradnik et al., 1976; Tung and Brown, 1983; Carey et al., 1991) and slow the diffusion of fluoride from the cement, and it could bind fluoride as it passes through the membrane. Thus, it is anticipated that this diffusion barrier would have the effect of slowing the rate of fluoride release.
The glass-ionomer cement released more fluoride when the environment was at lower pH, thus providing the greatest amount of fluoride when it would be most needed to prevent secondary caries. The hypothesis that a greater amount of fluoride is released and at a greater rate at lower pH was partially supported by the observation that the amount of fluoride released was significantly higher throughout the runs at pH 4 by a factor of 3 to 4 times greater than at neutral conditions (p < 0.05 at all time points). However, after the initial burst period, the rate of release was not pH-dependent. More studies are required for a better understanding of this phenomenon and for identification of the rate-limiting step for the release of fluoride into the environment.
In these experiments, the release rate of fluoride after 3 days in continuous flow was much lower than those reported by others after many months’ exposure in batch methods (reviewed by Eichmiller and Marjenhoff, 1998). These findings underscore the necessity for the release of fluoride from dental restorative materials to be evaluated under conditions that model the environment of the mouth if any beneficial effects or limitations that the released fluoride might have are to be fully appreciated.
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DISCLAIMER |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONDISCLAIMERREFERENCES |
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ACKNOWLEDGMENTS |
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Received December 2, 2002; Last revision July 14, 2003; Accepted July 25, 2003
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REFERENCES |
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Carvalho AS, Cury JA (1999). Fluoride release from some dental materials in different solutions. Oper Dent 24:14–19.[ISI][Medline]
Castioni NV, Baehni PC, Gurny R (1998). Current status in oral fluoride pharmacokinetics and implications for the prophylaxis against dental caries. Eur J Pharm Biopharm 45:101–111.[ISI][Medline]
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Forsten L (1990). Short- and long-term fluoride release from glass ionomers and other fluoride-containing filling materials in vitro. Scand J Dent Res 98:179–185.[ISI][Medline]
Haveman CW, Summitt JB, Burgess JO, Carlson K (2003). Three restorative materials and topical fluoride gel used in xerostomic patients: a clinical comparison. J Am Dent Assoc 134:177–184.
Langenbucher F (1969). In vitro assessment of dissolution kinetics: description and evaluation of a column-type method. J Pharm Sci 58:1265–1272.[ISI][Medline]
Randall RC, Wilson NH (1999). Glass-ionomer restoratives: a systematic review of a secondary caries treatment effect. J Dent Res 78:628–637.
Tingstad JE, Riegelman S (1970). Dissolution rate studies. I: Design and evaluation of a continuous flow apparatus. J Pharm Sci 59:692–696.[ISI][Medline]
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Tung MS, Brown WE (1983). Characterization and modification of electrochemical properties of teeth. J Dent Res 62:60–64.
Wilson AD, Nicholson JW (1993). Acid-base cements: their biomedical and industrial applications. Cambridge: Cambridge University Press, pp. 157-161.
Zahradnik RT, Moreno EC Burke EJ (1976). Effect of salivary pellicle on enamel subsurface demineralization in vitro. J Dent Res 55:664–670.
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