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J Dent Res 84(5):440-444, 2005
© 2005 International and American Associations for Dental Research

RESEARCH REPORT
Biomaterials & Bioengineering

Effect of CaF2 Content on Rate of Fluoride Release from Filled Resins

K.J. Anusavice*, N.-Z. Zhang, and C. Shen

Department of Dental Biomaterials, College of Dentistry, University of Florida, PO Box 100446, 1600 SW Archer Rd., Gainesville, FL 32610-0446, USA;

* corresponding author, kanusavice{at}dental.ufl.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
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. 1Go. The pH of the replaced solutions was measured. The solubility of CaF2 in the buffered solutions and water was also determined.



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  		Figure 1. Fluoride ion release rate as a function of time in pH4 and pH6 buffer solution for filled resin specimens containing (A) 33.33 mass%, (B) 23.08 mass%, and (C) 9.09 mass% CaF2 (n = 10).

 
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.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
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. 1Go). 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. 1Go). 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):


(1)

where a is an estimate of the quantity of short-term ions released, t is the time, t1/2 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 1Go). Because of the wide variance among the groups, non-parametric statistical analyses were used. The Kruskal-Wallis test was used to examine differences among filler content (level = 3) for each pH, and the Wilcoxon rank-sum test examined effects of pH (level = 2) for each filler content. The results indicate that filler content significantly influenced the mean b values for each pH value, and the pH exhibited a significant influence on the mean b values, except for the 9.09 mass% group (p = 0.5587).


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  		Table 1. Mean (n = 10) and Standard Deviation of Long-term Release Coefficient of CaF2-filled Resin in pH4 and pH6 Buffer Solutions
 
SEM examination of disk cross-sections that included the outer edge of the specimens revealed microstructural changes (Fig. 2AGo) 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. 2BGo, 2CGo) 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.



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  		Figure 2. SEM images (A) and x-ray elemental maps of fluorine (B) and calcium (C) of the cross-section of the outer edge of composite 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 white vertical bar in A represents 10 µm, and the white vertical bars in B and C represent 50 µm

 
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):


(2)

where {tau} is the shear stress, {gamma} 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.

The solubility of CaF2 in different buffer solutions at 25°C and the change in pH of these solutions are given in Table 2Go.


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  		Table 2. Solubility of CaF2 in Different pH Acetate Buffer Solutions and Distilled Water
 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
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. 1Go). 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. 2AGo, top row), disks immersed in pH6 (Fig. 2AGo, middle row) and pH4 (Fig. 2AGo, bottom row) buffer solutions for 4 mo exhibited surface degradation. The degree of dissolution increased directly with increasing filler content. It is clear that significant agglomeration of filler particles (bright spots) occurs within specimens containing 33.33 mass% CaF2 filler. One might ascribe the higher release rate of F– ions to higher acetate levels, but this does not appear to be the case. If acetate played a significant role, the release rate would be greater for the 9.09% group, but it did not occur.

The density of white spots within the cross-section of specimens (Figs. 2BGo, 2CGo) is lower for the specimens stored in the buffered solutions. There are fewer white spots along the disk surfaces when the CaF2 filler content increased from 9.09 to 33.33 mass% for the specimens immersed in pH6 buffer solution. For the specimens stored in pH4 solution, the continuous white area indicates significant surface degradation for the more highly filled specimens (23.08 and 33.33 mass%). This indicates that the release rates as measured for the resin with 23.08 and 33.33 mass% filler contents were strongly influenced by surface degradation.

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 1Go). Surface degradation likely occurred through a combination of diffusion and subsequent dissolution of filler within the surface region. The greater the degradation, the greater the surface area of the fillers exposed to the solution. Both processes complement each other and result in greater release rates.

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.


   ACKNOWLEDGMENTS
 
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 August 4, 2004; Last revision January 19, 2005; Accepted January 19, 2005


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Adair SM, Whitford GM, McKnight-Hanes C (1994). Effect of artificial saliva and calcium on fluoride output of controlled-release devices. Caries Res 28:28–34.[ISI][Medline]

Asmussen E, Peutzfeldt A (2002). Long-term fluoride release from a glass ionomer cement, a compomer, and from experimental resin composites. Acta Odontol Scand 60:93–97.[ISI][Medline]

Benson AM, Martin GQ, Son JS, Sternling CV (1980). Fluid mechanics. In: Kirk-Othmer encyclopedia of chemical technology. Mark HF, Othmer DF, Overberger CG, Seaborg GT, editors. New York: John Wiley & Sons, pp. 582–629.

Buchalla W, Attin T, Schulte-Monting J, Hellwig E (2002). Fluoride uptake, retention, and remineralization efficacy of a highly concentrated fluoride solution on enamel lesions in situ. J Dent Res 81:329–333.[Abstract/Free Full Text]

Carey CM, Spencer M, Gove RJ, Eichmiller FC (2003). Fluoride release from a resin-modified glass-ionomer cement in a continuous-flow system: effect of pH. J Dent Res 82:829–832.[Abstract/Free Full Text]

Carranza FA Jr, editor (1979). Glickman’s clinical periodontology. 5th ed. Philadelphia: Saunders, pp. 405–432.

De Moor RJ, Verbeeck RM, De Maeyer EA (1996). Fluoride release profiles of restorative glass ionomer formulations. Dent Mater 12:88–95.[ISI][Medline]

Dijkman GE, Arends J (1992). Secondary caries in situ around fluoride-releasing light-curing composites: a quantitative model investigation on four materials with a fluoride content between 0 and 26 vol%. Caries Res 26:351–357.[ISI][Medline]

Eichmiller FC, Marjenhoff WA (1998). Fluoride-releasing dental restorative materials. Oper Dent 23:218–228.[ISI][Medline]

Ekstrand J, Oliveby A (1999). Fluoride in the oral environment. Acta Odontol Scand 57:330–333.[ISI][Medline]

Hellwig E, Lussi A (2001). What is optimum fluoride concentration needed for the remineralization process? Caries Res 35(Suppl 1):57–59.

Itota T, Carrick TE, Rusby S, Al-Naimi OT, Yoshiyama M, McCabe JF (2004). Determination of fluoride ions released from resin-based dental materials using ion-selective electrode and ion chromatograph. J Dent 32:117–122.[ISI][Medline]

Lee H, Ocumpaugh DE, Swartz ML (1972). Sealing of developmental pits and fissures: II. Fluoride release from flexible fissure sealers. J Dent Res 51:183–190.[Abstract/Free Full Text]

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