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Effect of CaF2 Content on Rate of Fluoride Release from Filled ResinsDepartment of Dental Biomaterials, College of Dentistry, University of Florida, PO Box 100446, 1600 SW Archer Rd., Gainesville, FL 32610-0446, USA; Correspondence: * corresponding author, kanusavice{at}dental.ufl.edu
Information on the time-dependent release of fluoride from filled resins containing fluoride particles as a function of particle content and solution pH is limited. This study characterized the fluoride ion release from filled resins containing CaF2 particles as a function of filler content and pH. Urethane dimethacrylate and triethylene glycol dimethacrylate resins were used to make filled-resin disks containing 9.09, 23.08, or 33.33 mass% CaF2 filler. Fluoride ion release for the 9.09 mass% concentration was independent of pH. Increasing the filler content from 9.09 to 33.33 mass% increased the fluoride release rate in pH 4.0 buffer solution, because of greater surface degradation. Fluoride ion release from disks stored in pH 6.0 buffer solutions occurred mainly by diffusion from disk surfaces, while fluoride release from disks in pH 4.0 buffers was controlled by diffusion from disk surfaces and degeneration of the resin matrix, which exposed more CaF2 particle surface area.
Key Words: CaF2 • polymer degradation • fluoride leaching • fluoride ion release • filled resin
The effects of fluoride ions on streptococci and lactobacilli are well-known. The mechanisms of fluoride ions in the promotion of remineralization of dental hard tissues are less well-understood (ten Cate, 1990; Rawls, 1991; Eichmiller and Marjenhoff, 1998; Hellwig and Lussi, 2001; van Loveren, 2001; Reich, 2001). The optimal concentrations of Ca2+, PO43–, and F– ions required to prevent demineralization and to ensure remineralization under a variety of oral conditions are not known. Ionic fluoride exhibits antimicrobial activity that can alter the dynamics of the caries process. Restorative materials that have been formulated for fluoride ion release include glass ionomer, resin-modified glass ionomer, compomer, resin composite, and fissure sealant (Lee et al., 1972; Mirth, 1987; Adair et al., 1994; Taylor et al., 1998; Morphis et al., 2000; Asmussen and Peutzfeldt, 2002). Most of these materials exhibit high initial F– release rates and a sharply decreasing rate of release over time. The control of steady-state release rates and the proximity of fluoride ions to areas susceptible to secondary caries are essential to prevent demineralization and to enhance remineralization if significant demineralization has already occurred (Dijkman and Arends, 1992; ten Cate, 1997; Buchalla et al., 2002). Secondary caries can be prevented by the application of a sealing agent that can release fluoride ions to prevent demineralization of marginal enamel areas adjacent to defective restorations. Wei (1998) concluded that NaF was too soluble in water (4.0 g/100 mL) to sustain low F– release rates over long periods of time. CaF2 is only slightly soluble (0.0016 g/100 mL), and it should provide a much slower and longer period of release. However, the effect of solution pH on the fluoride ion release rate from filled resins, the influence of CaF2 content on release rates, and the associated microstructural change of the resin matrix need to be determined. The objective of our study was to test the hypothesis that the release rate of fluoride ions leached from CaF2 particles in a urethane dimethacrylate (UDMA)/triethylene glycol dimethacrylate (TEGDMA) resin matrix (70:30 ratio) is directly proportional to the CaF2 content and inversely proportional to the pH solution.
CaF2 powder (Fisher Co. Inc., Pittsburgh, PA, USA) was ground to a fine particle size (0.04 to 3.0 µm) with the use of a Micronizer (Sturtevant Inc., Hanover, MA, USA). A light-curable resin containing 70 mass% UDMA (Esschem, Inc., Linwood, PA, USA), 30 mass% TEGDMA (Aldrich Co., Milwaukee, WI, USA), and an appropriate amount of light-sensitive initiator and co-catalyst was used to make 3 filled resins. Filler concentrations of 9.09, 23.08, and 33.33 mass% were made by the addition of 0.50, 1.50, and 2.50 g of CaF2 to 5.00 g of the resin, respectively. The rheological properties of resins containing CaF2 particles were measured with the use of a Brookfield Digital Rheometer, LVDV III and CP 115 (Brookfield Engineering Laboratories, Inc., Middleboro, MA, USA).
These filled resin mixtures were poured into a mold 10 mm in diameter and 2 mm thick, and light-cured through the Mylar® matrix for 30 s on each side. Each disk was polished through 2000 SiC abrasive paper under running water. The disks were washed and dried. Sodium acetate-acetic acid buffer solutions were prepared and adjusted to pH 4.0 and 6.0, which are noted as pH4 and pH6 throughout this manuscript. A series of fluoride ion reference solutions was prepared for each buffer solution. Ten disk specimens were prepared for each of the 3 weight loadings and the 2 buffer solutions. Each of 10 disks per group was placed in a 10-mL vial, to which 5 mL of buffer solution was added, and the vial was immersed in a 37°C water bath. A mesh of Teflon cord was placed at the bottom of the vial to prevent disks from coming into contact with the vial wall. The buffer solutions were replaced periodically after exposure for 1, 5, 15, 35, 65, 105, 155, 215, 287, 383, 503, 647, 815, 1007, 1223, 1463, 1727, 2015, 2327, 2663, and 2880 h (approximately 4 mo). The released fluoride ion concentration was analyzed with the use of a fluoride-ion specific electrode and a digital pH/mv meter (Shen and Autio-Gold, 2002). Total Ionic Strength Adjustment Buffer (TISAB) was used as a decomplexing agent for fluoride ion measurement. The values were converted to released mass per unit surface area of the disk. The release rate between 2 sampling periods and the cumulative release for each sampling period were calculated. The times at which solutions were sampled correspond to the data points in Fig. 1
After the specimens were removed from the final solution, 2 specimens were randomly selected from each group, and they were cut in half. Cross-sections of the disks were coated with carbon for scanning electron microscopy (SEM) examination. Energy-dispersive x-ray analysis (EDXA) spectra of the specimens were obtained from cross-sections of disks.
All disks exhibited a typical high initial release rate that decreased rapidly to a much lower but gradually decreasing release rate. The same trend was also observed with the average release rate (n = 10) vs. time for all groups (Fig. 1  ). There was no detectable change in pH of the solutions over time. For the resin containing 9.09 mass% CaF2, the initial mean F– release rates in pH4 and pH6 solutions were 0.6 and 0.4 µg/cm2•h, respectively, and the mean release rates were 0.006 and 0.005 µg/cm2•h after 4 mos of release (Fig. 1). For the resin containing 23.08 mass% CaF2, the mean initial release rates in pH4 and pH6 solutions were 1.6 and 0.4 µg/cm2•h, respectively, and the average release rates were 0.04 and 0.04 µg/cm2•h, respectively, after 4 mo of release. For the resin containing 33.33 mass% CaF2, the initial mean release rates in pH4 and pH6 solutions were 1.8 and 0.8 µg/cm2•h, respectively, and the mean release rates were 0.13 and 0.10 µg/cm2•h, respectively, after 4 mo of release. The cumulative fluoride ion release data (Y in µg/cm2) were fit to the following equation (De Moor et al., 1996):
where a is an estimate of the quantity of short-term ions released, t is the time, t
SEM examination of disk cross-sections that included the outer edge of the specimens revealed microstructural changes (Fig. 2A  ) in disks containing 9.09, 23.08, and 33.33 mass% CaF2 after exposure for 4 mo in ambient air (control), and pH6 and pH4 buffer solutions. The void space caused by dissolution near the disk surface increased as the pH of the storage solution decreased from 6.0 to 4.0. Fluorine and calcium element maps (Figs. 2B, 2C) along cross-sections of outer edges of experimental disks stored in ambient air (control), and pH6 and pH4 buffer solutions, show that the decreasing density of bright spots near the edge resulted from storage in buffered solutions.
The apparent viscosity of filled resins increased sharply with an increasing content of CaF2 filler, and appeared to exhibit non-Newtonian behavior according to the exponent n in the following power-law fluid equation (Benson et al., 1980):
where
The solubility of CaF2 in different buffer solutions at 25°C and the change in pH of these solutions are given in Table 2
Incorporating fine CaF2 particles (0.04 to 3.0 µm) in UDMA-TEGDMA resin provided a nearly stable fluoride ion release rate after 1000 h (Fig. 1 ). The rapid initial release was caused mostly by surface dissolution. The near-stable release period was dominated by Fickian release. As time increased, the difference in the mean release rate between two time periods became so small that a near-steady-state release was achieved. The released fluoride ions are adsorbed onto or integrated within the mineral phases of teeth. Individuals who consume a normal diet and who reside in an area with a water supply containing 0.2 ppm F– are associated with a concentration of 0.6 µmol/L (0.01 ppm) F– in saliva (Oliveby et al., 1987). A CaF2-containing resin that sustains a fluoride ion release rate of 0.1 µg/cm2•h after a period of 4 mo will release 1.2 µg of fluoride ions per day over a treatment area of 0.5 cm2. Based on dynamic salivary flow in a 24-hour period, the total salivary flow is approximately 1250 mL (Carranza, 1979). Therefore, the fluoride ions released should be able to maintain a concentration of 1 x 10–3 ppm over the 24-hour period. Depending on the filler content and the pH of the oral environment, the average concentration can range from 5 x 10–4 ppm for resin containing 9.09 mass% of CaF2 at pH6, to 1.3 x 10–2 ppm for resin containing 33.33 mass% of CaF2 at pH4. Both values may be slightly higher than the release rates found in vivo. This calculated F– concentration over a 24-hour period is lower than the physiological F– concentration in saliva, which can be as high as 1.7 µmol/L (0.03 ppm), based on 1.2 ppm of fluoride in drinking water (Ekstrand and Oliveby, 1999). It is important to note that the goal of this preventive coating agent is not to provide fluoride to the entire oral cavity, but to provide a low steady-state release of fluoride ions locally to inhibit demineralization and to enhance remineralization (if enamel is already demineralized) at the marginal enamel area of defective restorations. Nonetheless, the overall release rates for this group of resins are comparable with the rates reported in the literature. For example, Itota et al.(2004) reported that the F– release rates of a group of materials ranged from 0.55 to 8.55 µg/cm2 between the 21st and 28th days of immersion in de-ionized water. The values are equivalent to 0.003 and 0.05 µg/cm2•h. Using a continuous flow system, Carey et al.(2003) reported the fluoride ion release rate from a glass-ionomer cement to be 1 µg/cm2•hr after 72 hrs in pH4 buffer solution.
Compared with the control disks that had not been exposed to an aqueous environment (Fig. 2A
The density of white spots within the cross-section of specimens (Figs. 2B
There was no statistically significant effect of pH on the long-term release coefficient for the disks with 9.09 mass% filler. The effect of pH on the long-term release coefficient increased as the filler content increased (Table 1 CaF2-filled resins exhibit non-Newtonian behavior according to Eq. (2). Although the addition of fillers up to 33.33 mass% increased the viscosity by a factor of 2.5, the apparent viscosities are comparable with the published values for commercial products. Fluoride ion release from the CaF2-filled resin occurs primarily by diffusion from the surface region for disks stored in pH6 buffer solution. For disks stored in pH4 buffer solution, the release of fluoride ions occurred by diffusion from the surface of the filled resin and degradation of the resin matrix, which exposed more surface area of the CaF2 particles. A filler content of 33.33 mass% of CaF2 can almost double the release rate compared with that for a 23.08 mass% filled resin. In this situation, greater surface degradation can occur that could shorten the service life of the coating. The resins containing 23.08 mass% CaF2 exhibited moderate viscosity, a range of potentially useful fluoride ion release rates, and minimal surface degradation after 4 mo in pH4 and pH6 buffer solutions. This type of filled resin represents a model for developing optimal filler contents that will exhibit adequate steady-state fluoride ion release to inhibit or prevent demineralization of tooth enamel and enhance the remineralization process if demineralization has already occurred.
This study was supported by NIH/NIDCR Grant DE13412. We appreciate the assistance of Esschem, Inc. in providing the UDMA monomer. We also acknowledge the SEM-EDS support of Mr. Wayne Acree and Mr. Bradley Willenberg. Received for publication August 4, 2004. Revision received January 19, 2005. Accepted for publication January 19, 2005.
Journal of Dental Research, Vol. 84, No. 5,
440-444 (2005) This article has been cited by other articles:
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ABSTRACT
INTRODUCTION
is the time at which 50% of a has been released, and b is the coefficient for long-term Fickian release. When the b value is divided by twice the square root of a given time, it yields the mean release rate of the filled resin at that given time. Therefore, only the value of b is of interest in this study. The mean values of b varied between 0.7 and 11.0 µg/cm2•h1/2 (Table 1
is the shear stress, 
is shear rate, n is flow index, and K is the consistency index. The mean values of the exponent n (± SD) based on 6 measurements were 0.99 ± 0.01 for resin with no filler, 0.87 ± 0.08 for 9.09 mass% CaF2, 0.51 ± 0.05 for 23.08 mass% CaF2, and 0.46 ± 0.04 for 33.33 mass% CaF2. The apparent viscosity, at a shear rate of 1 rpm (0.38/sec), was 314, 333, 1093, and 2670 mPa•s for unfilled resin, and resin containing 9.09, 23.08, and 33.33 mass% CaF2 particles, respectively. 
