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RESEARCH REPORT |
1 Oral Health Research Institute, Indiana University School of Dentistry, Indianapolis, IN, USA;
2 Department of Operative Dentistry and Periodontology, University of Freiburg, Germany;
3 Department of Operative Dentistry and Periodontology, University of Göttingen, Germany; and
4 Institute for Medical Biometry, University of Freiburg, Germany;
*corresponding author, present address, Georg-August-Universität Göttingen, Zentrum Zahn-, Mund- und Kieferheilkunde, Abteilung für Zahnerhaltung, Präventive Zahnheilkunde und Parodontologie, Robert-Koch-Str. 40, 37075 Göttingen, Germany, buchalla{at}med.uni-goettingen.de
| ABSTRACT |
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KEY WORDS: fluoride carious enamel remineralization clinical study salivary flow rate
| INTRODUCTION |
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| MATERIALS & METHODS |
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Three narrow strips of adhesive tape (Tesa, Beiersdorf, Hamburg, Germany) were applied parallel to each other on the surfaces of 2 of the 4 enamel specimens originating from 1 individual tooth, leaving 2 equally sized windows. All 360 specimens were immersed in 9 L of unstirred demineralizing solution at 37°C for 6 days. This solution contained 3 mmol/L CaCl2 x 2 H2O, 3 mmol/L KH2PO4, 50 mmol/L C2H5COOH, 6 µmol/L methyldiphosphonate, amounts of KOH to adjust the initial pH to 5.0, and traces of Thymol (Buskes et al., 1985). The adhesive tape was removed from the respective specimens (n = 180), and 1 of the 2 demineralized windows was covered with a dentin-bonding agent (Syntac classic, Vivadent, Schaan, Liechtenstein) for control. The specimens with the control area (n = 180) were intended for mineral gain; specimens without control area (n = 180) were intended for fluoride measurements. Specimens were stored at 100% relative humidity at 8°C in a refrigerator until use.
A removable appliance was fabricated for each volunteer's lower jaw, with a buccal resin wing on each side (Koulourides et al., 1974). Five specimens were mounted in each wing flush with the buccal surface.
The volunteers received the intra-oral appliance (at the beginning of period I) after a one-week wash-out phase. Elmex fluid or a placebo (both GABA International AG, Münchenstein, Switzerland) was applied to the specimens in the appliances of nine volunteers each. Volunteers were assigned to a treatment regimen by means of a computer-generated randomization list. Elmex fluid (pH 3.9) contained 10,000 ppm fluoride from amine-fluoride (9250 ppm fluoride from Olaflur and 750 ppm from Dectaflur), sweetener, flavor, and water. The placebo fluid contained sweetener, flavor, PEG-40 hydrogenated castor oil as emulsifier, potassium parabene and polyaminopropyl biguanide as preservatives, color, and KOH to adjust pH to 6.9. The fluid (0.2 mL) was applied for 20 sec before the appliances were inserted into the mouth. After 5 min in situ, one specimen each for fluoride and mineral content determination was removed, rinsed with distilled water, and stored in 100% humidity. The appliances were kept in situ except during meals and toothbrushing, when kept in 100% humidity.
The recommended toothbrushing procedure included gentle brushing of the intra-oral appliances twice daily without toothpaste to prevent plaque growth. Another set of specimens was removed after 1, 2, 3, and 4 wks in situ. Appliances were refilled with a new set of specimens during a one-week wash-out phase. Period II was carried out in the same way as Period I, but the elmex® fluid-placebo application scheme was performed in reverse.
To determine the KOH-soluble fluoride (Caslavska et al., 1975), we exposed the oral surface of each respective specimen to 1 mL KOH solution (1 mol/L) and agitated it for 24 hrs at 23°C. The solution was neutralized (1 mL 1 mol/L HNO3) and buffered (1 mL TISAB II, Orion Research Corporation, Cambridge, MA, USA). Fluoride content was measured with a fluoride-sensitive electrode (Orion Research Corporation) and calculated in µg/cm2 enamel surface.
The same specimens were used to determine the structurally bound fluoride (Hellwig et al., 1989). A 100-µm layer was ground off, dissolved (0.5 mL 0.5 mol/L HClO3), agitated at 23°C for 1 hr, and buffered with 2.5 mL TISAB II. Fluoride content was measured and calculated in µg/cm3 enamel.
A 90 ± 10-µm-thick slice was prepared perpendicularly to the exposed surface of the specimens dedicated for mineral content analysis. A semi-contact microradiograph of each slice together with an aluminum calibration step wedge was taken. High-speed holographic film (SO 253, Kodak AG, Stuttgart, Germany) was exposed with Cu K
x-rays at 20 kV and 20 mA for 12 sec. Mineral content
Z and lesion depth ld were calculated (Angmar et al., 1963). Sound enamel mineral content was considered 87% by volume. Lesion depth was defined as depth from the specimen surface where 95% of sound mineral content was reached. A Stereo Microscope (Axioplan, Zeiss, Oberkochen, Germany) with CCD-Camera (XC-77CE, Sony, Tokyo, Japan) and a PC with framegrabber and data acquisition and calculation software were used (TMR 1.25e, Inspector Research BV, Amsterdam, The Netherlands). Mineral gain (
Z) was calculated as the difference in mineral loss (
Z) between the remineralized and the protected demineralized (control) area: 
Z =
Z (control)
Z (remineralized). Lesion depth reduction was calculated accordingly,
ld = ld (control) - ld (remineralized).
A repeated-measurement ANOVA model was fitted to the data, and transformations were made where necessary (p
0.05). Multiple comparisons were made by sequential analysis; therefore, an
-adjustment was not necessary. Two hypotheses each were tested for the therapy effect and the time effect of the investigated parameters. For the therapy effect, the hypotheses were: There is no difference between the average values of both therapies (elmex fluid and placebo, H0), and, there is a difference (H1). The hypotheses for the time effect were: There are no differences in changes with time between both therapies (H0), and, there are differences (H1).
| RESULTS |
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Fluoride Content
The KOH-soluble fluoride (Fig. 2A
) decreased continuously in the elmex fluid group, from 11.5 to 1.5 µg/cm2 over 4 wks but always remained significantly higher than the placebo control. The fluoride level of the placebo group remained almost stable, around 0.6 µg/cm2. The global therapy effect was significant (p
0.001). The time effect (p
0.001) showed a significant decrease of KOH-soluble fluoride following treatment.
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0.001). The time effect was also significant (p
0.001), showing that following treatment, structurally bound fluoride was acquired with time.
Correlation of Fluoride Uptake with Salivary Flow Rate
An inverse relation was found between stimulated salivary flow rate and fluoride content of enamel for all times. For example, the correlations of the volunteers' stimulated salivary flow rate with KOH-soluble fluoride after 14 days in situ (Fig. 3A
) and structurally bound fluoride after 14 days in situ (Fig. 3B
) are given, indicating a logarithmic relation.
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0.001). Sixteen specimens in the placebo group demineralized further, but only 2 specimens in the elmex fluid group did. One volunteer in the placebo group produced a high amount of mineral loss after 21 days (
Z = -1771 vol% x µm, n = 1), which was out of range. This value was not included for calculation of the placebo's 21-day mean (Fig. 2C
0.05).
Means for the elmex fluid group were larger and standard deviations smaller than for the control (Figs. 2C, 2D
). The calculated average coefficient of variation (average of [(SD/mean) x 100]) for 1 to 4 wks decreased from 158 (placebo) to 57 (elmex fluid) for mineral gain and from 222 (placebo) to 94 (elmex fluid) for lesion depth reduction. Hence, a single application of elmex fluid on initial enamel lesions not only increased the remineralization but also led to a more stable mineral accumulation with less fluctuation.
Correlation of Mineral Gain with Salivary Flow Rate
A positive relationship was found between stimulated salivary flow rate and mineral gain for both fluoridated and non-fluoridated specimens beginning with 7 days, as can be seen in Figs. 3C and 3D
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| DISCUSSION |
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As in other in vitro and in situ studies, KOH-soluble fluoride in the present investigation was highest following fluoride application, then decreased to a minimum amount after a certain period, thereby serving as a fluoride reservoir (Rølla et al., 1993; Attin et al., 1995). After 4 wks in situ, KOH-soluble fluoride was still significantly higher than in the fluoride-free control group. It can be assumed that KOH-soluble fluoride present after 4 wks still plays an important role, due to its remineralizing capacity (Øgaard et al., 1990).
Structurally bound fluoride increased with time following treatment, which is also in accordance with reports from previous studies (van Rijkom et al., 1998). Although many suggest that firmly incorporated fluoride is not as important as KOH-soluble fluoride with respect to its cariostatic efficacy (Helfenstein and Steiner, 1994), there is evidence that enamel resistance against further demineralization increased with an increase in structurally bound fluoride (Takagi et al., 2000). Interestingly, structurally bound fluoride cannot be increased in intact human enamel simply by increasing the amount of fluoride in a fluoridation product. Calcium phosphates like dicalcium phosphate dihydrate, present in the demineralized enamel of an initial caries lesion, react readily with fluoride to form fluoro-hydroxyapatite (Chow and Brown, 1973), which is less soluble compared with hydroxyapatite. This mechanism may be one reason for the relatively high amount of structurally bound fluoride already present after 5 min. The greater surface area of the lesion due to porosities may be a more important factor for the development of structurally bound fluoride. It is still not fully understood, and the present study does not provide an answer, to what extent these processes contribute to the build-up of structurally bound fluoride and what other processes are involved.
Unexpectedly, a small mineral gain was measurable after only 5 min in situ. Mineral redistribution due to the storage of the specimens in 100% humidity for some days before being sectioned is a possible cause. Mineral gain and reduction of lesion depth increased most during the first 2 wks. It is likely that fluoride made available from the KOH-soluble fluoride reservoir contributed to this remineralization. It is also most likely that fluoride released from the CaF2-like precipitates was incorporated into the gained mineral and accumulated structurally bound. This was not only because the KOH-soluble fluoride fraction decreased with time, but also because no other source of fluoride was involved in the experiment. With KOH-soluble fluoride decreasing and structurally bound fluoride increasing with time, structurally bound fluoride may become the more important cariostatic fluoride fraction by enhancing demineralization resistance and thereby impeding lesion progression rather than promoting remineralization. Although not studied here, this may explain the long-lasting efficacy of highly concentrated fluoride products applied only 2 or 3 times annually. Further studies must evaluate whether a second application, e.g., after 3 months, can reproduce similar mineral gain. It should also be determined whether a lesion will progress under more severe cariogenic conditions. Clinically, there is evidence that, in these circumstances, fluoride application provides only limited caries protection (Ekstrand and Oliveby, 1999).
Both KOH-soluble and structurally bound fluoride was lower in volunteers with higher stimulated salivary flow rates, which may explain interindividual differences in fluoride uptake. This indicates that topically applied fluoride solutions are diluted and washed away by saliva during their tissue-interaction, and that application forms with higher retention rates could be advantageous. Despite a lower fluoride uptake, individuals with higher salivary flow rates gained more mineral. Therefore, salivary flow is the important factor for remineralization, while fluoridation is an effective supportive measure.
| ACKNOWLEDGMENTS |
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Received August 22, 2001; Last revision February 11, 2002; Accepted March 18, 2002
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