Comparison of Fluoride Ion Release from Fluoride Gel in Various Solvents.

OBJECTIVE: The aim of this study was to determine the concentration of fluoride ions present in various solvent solutions and compare it to the label amount of fluoride concentration.
MATERIAL AND METHODS: The gel containing amine fluoride and sodium fluoride was compared to gel containing sodium fluoride only. The solvents used in this study were distilled water, redistilled water, tap water, 0.9% sodium chloride solution and the artificial saliva. The amount of fluoride ion (F-) was determined by the use of fluoride ion-selective electrode. The ANOVA and the Post hoc Tukey's test were used. The significance level was α = 5%.
RESULTS: The differences between F- ion release in various solvents were statistically significant (p < 0.05). The F- ion concentrations measured in both gels' redistilled water solutions were higher than the label concentration. There was a significant difference in fluoride ion release in sodium fluoride only gel solutions. Fluoride ion release was significantly different in combined fluoride gel solutions.
CONCLUSIONS: Amine fluorides do not interact with other ions present in the solution, whereas F- ion release from sodium fluoride alters when other ions are dissolved in the same solution. Amine fluorides deliver F- ions for caries prevention more efficiently.

Introduction

High concentration fluoride gels are used for caries prevention and therapy (). A variety of fluoride compounds (e.g. sodium fluoride, amine fluoride, stannous fluoride, silane fluoride) are used as topical prophylaxis agents, however, all of them must meet one basic requirement – fluoride ion liberation. It is considered important for fluoride ion concentration to be even slightly increased in the proximity of the tooth surface for the remineralization to be effective (-).

Sodium fluoride, an inorganic and very soluble salt, liberates F- ions readily and its effectiveness has been proven many times. Amine fluorides, on the other hand, contain covalently bonded fluorine. Amphipathic character advantages them to stay in contact with the tooth surface during longer periods. The available data of research are unclear in regard to superiority of one of them in remineralization, but both compounds are proven effective in caries prevention (). Since fluoride ions are very reactive, they become ineffective easily. Eggert and Neubert investigated the liberation of fluoride ions from toothpastes and concluded that fluoride ion release is modified depending on auxiliaries present in the product ().

Topical fluorides are of great importance for cancer patients treated with radiation therapy in the region of head and neck. Irradiated salivary glands deteriorate in function leading to dry mouth, changes in oral environment and radiation caries. A failure in excellent oral hygiene maintenance and topical fluoridation regime leads to rampant caries development (-). Caries can be successfully prevented if high concentration fluoride gels are used in custom trays. Fluoridation protocols are proposed for facilitating the use. Zandim et al. recommended the use of a 0.025% NaF mouth rinse together with artificial saliva for radiation-induced hyposalivation patients, and Abdalla et al. recommended a mouth rinse containing 0.05% NaF and 0.12% chlorhexidine-gluconate twice-a day (, ). The latter combination of fluorides and antimicrobial agent might be more effective due to their effect both on the demineralization-remineralization process and oral microbes as found by Abdalla et al ().
The aim of this study was to determine the concentration of free fluoride ions, using solutions which may be used for xerostomia therapy that served us as solvents – in order that fluoride ion interactions can be assessed as a first step for further investigation of alternative topical fluoridation protocols.

The null hypothesis was that there is no difference in F- ions release from various fluoride gels in different solvents, therefore, there is no deviation from the label amount of F- ion concentration.

Material and methods

Study samples selection

Two fluoride gels containing different active ingredients were used in this study. The information about the products is presented in Table 1.

Table 1

Fluoride gels

Fluoride gel	Product/supplier	Fluoride compound and its amount in 10 g of a gel
sodium fluoride only	Miradent Mirafluorr - gel (Hager & Werken GmbH & Co. KG, Duisburg, Germany) LOT 208702	NaF	0.272 g
amine fluoride and sodium fluoride	Elmexr Gel(CP GABA GmbH, Hamburg, Germany) PZN 01427249	dectaflur	0.0287 g
		olaflur	0.3032 g
		NaF	0.2210 g

Five different solvents were used: distilled water (Iskra d.d., Sv. Ivan Zelina, Croatia), redistilled water (the institution), tap water (the institution), 0.9% sodium chloride solution – saline (Natrii chloridi infundibile Pliva, Pliva Hrvatska d.o.o., Zagreb, Croatia) and artificial saliva Axerostar (Mages Plant Sp. zo.o., Warsaw, Poland)

The samples were measured with the analytical balance ABS 80-4N (Kern & Sohn GmbH, Balingen, Germany) and the automatic pipette Eppendorf Researchr (Eppendorf, Hamburg, Germany) in polystyrene vials (Ratiolab, Germany).

The measuring electrode used was the fluoride-selective electrode type 96 – 09 (Thermo Scientific, Waltham, Massachusetts, USA) with the microprocessor analyzer ORION EA 940r (Orion Research Inc., USA). Samples were stirred at 500 rpm on the magnetic stir plate 
(IKA-Werke GmbH & Co. KG, Staufen, Germany) during the entire measuring process.

Fluoride concentration measurements

The international standard ISO 19448 was used as a reference for the methodology ().

Fluoride gels and solvents are left at the room temperature. To achieve the product homogeneity, a bottle of sodium fluoride only was shaken and the gel tube containing amine fluoride and sodium fluoride was kneaded prior to opening. Subsequently, 1 cm of each product was dispensed and discarded. The samples were obtained from the remaining product.

Five samples were prepared from each fluoride gel to be dissolved in five different solvents. A polystyrene vial was placed on a balance and the balance was tarred. 0.055 g of the gel product was then added to the vial. Distilled water, redistilled water, tap water and saline solution were added to each particular vial with automatic pipette in volume of 5.5 ml. The artificial saliva product was difficult to pipette due to its viscosity. Therefore, the vial containing fluoride gel sample was placed on the balance and the artificial saliva product was added to the vial to the total mass of 5.555 g.

The direct analysis technique was used to determine the amount of fluoride ion concentration.

Prior to the analysis of the solutions, the accuracy of the measuring instrument was checked as well as the electrode inclination according to the manufacturer’s instructions. After that, the measuring instrument was calibrated at room temperature over the range of 19 ppm F- to 190 ppm F-.

Each vial containing the sample was placed on the magnetic stir plate and the magnetic stir bar was added, then the fluoride-selective electrode was immersed in the solution. For each sample 10 measurements were performed and expressed in mgF/l (ppm F-).

Prior to measuring F- ion concentrations in pure solvents, the measuring instrument was once more calibrated at room temperature over the range of 0.19 ppm F- to 1.9 ppm F-.

The direct analysis technique was used as has already been described in the previous paragraph.

Statistical analysis

Since 0.055 g of fluoride gel was dissolved in 5.5 ml of solvent, the dilution factor was 100. Therefore, all the measured values were multiplied by 100 prior to the statistical analysis .

Descriptive statistics were used and a paired two-sample for means t-test was used to determine if there were significant differences between the measured fluoride concentration and the concentration value listed on the label.

The ANOVA was used to determine if there were significant differences between the liberation of fluoride ions from each gel in each particular solvent.

Furthermore, the ANOVA was used to determine if there were significant differences between the liberation of fluoride ions from sodium fluoride only gel and the gel containing amine fluoride and sodium fluoride in the same solvents.

ThePost hoc Tukey’s test was used for multiple comparisons between each of the solutions.

The significance level for all tests was α = 5%.

Results

F- ion concentrations measured in pure solvents are presented in Table 2.

Table 2 F

- ion concentration in pure solvents

Solvent	F- ion concentrationmean value ± SD[ppm F-]
distilled water	0.0206 ± 0.0070
redistilled water	0.0027 ± 0.0019
tap water	0.2595 ± 0.1140
saline solution	0.0023 ± 0.0006
Axerostar	0.0629 ± 0.0059

Mean measured concentrations of fluoride ions in solutions of gel are presented in Tables 3 and 4.

Table 3 F

- ion concentration in sodium fluoride only solutions

Solvent		F- ion concentrationmean value ± SD[ppm F-]	significance
1	distilled water	15060 ± 471.876	*, t, 1/3, 1/4, 1/5, *M/E
2	redistilled water	14770 ± 105.935	#, a, *, t, 2/3, 2/4, 2/5, *M/E
3	tap water	13520 ± 244.040	*, t, 3/4, 3/5, *M/E
4	saline solution	38440 ± 51.640	*, t, 4/5, *M/E
5	Axerostar	33740 ± 1776.513	*, t, *M/E
# t-test yielded a significant result (P<0.05): a when compared to the label amount of fluoride ion concentration*ANOVA test yielded a significant result (P<0.05): difference in fluoride release among the solventst post hoc Tukey’s test (multiple comparison among solvents): 1/3 significant difference between solvents 1 and 3, 1/4 significant difference between solvents 1 and 4, 1/5 significant difference between solvents 1 and 5, 2/3 significant difference between solvents 2 and 3, 2/4 significant difference between solvents 2 and 4, 2/5 significant difference between solvents 2 and 5, 3/4 significant difference between solvents 3 and 4, 3/5 significant difference between solvents 3 and 5, 4/5 significant difference between solvents 4 and 5*M/E ANOVA test yielded a significant result (P<0.05): difference between the liberation of fluoride ions from Mirafluorr gel and Elmexr gel in the same solvents; t except for distilled water solutions

Table 4 F

- ion concentration in gel containing amine fluoride and sodium fluoride solutions

Solvent		F- ion concentrationmean value ± SD[ppm F-]	significance
1	distilled water	15410 ± 324.722	*, t, 1/3, 1/4, 1/5, *M/E
2	redistilled water	16110 ± 331.495	#, a, *, t, 2/3, 2/4, 2/5, *M/E
3	tap water	7892 ± 342.436	*, t, 3/4, 3/5, *M/E
4	saline solution	12570 ± 636.047	*, t, 4/5, *M/E
5	Axerostar	16690 ± 1581.455	*, t, *M/E
# t-test yielded a significant result (P<0.05): a when compared to the label amount of fluoride ion concentration* ANOVA test yielded a significant result (P<0.05): difference in fluoride release among the solventst post hoc Tukey’s test (multiple comparison among solvents): 1/3 significant difference between solvents 1 and 3, 1/4 significant difference between solvents 1 and 4, 1/5 significant difference between solvents 1 and 5, 2/3 significant difference between solvents 2 and 3, 2/4 significant difference between solvents 2 and 4, 2/5 no significant difference between solvents 2 and 5, 3/4 significant difference between solvents 3 and 4, 3/5 significant difference between solvents 3 and 5, 4/5 significant difference between solvents 4 and 5*M/E ANOVA test yielded a significant result (P<0.05): difference in fluoride release between the liberation of fluoride ions from Mirafluorr gel and Elmexr gel in the same solvents; t except for distilled water solutions

A statistically significant difference between the measured fluoride concentration and the concentration value listed on the label was determined both for sodium fluoride only gel and gel containing amine fluoride and sodium fluoride by the use of a paired two-sample for means t-test.

Furthermore, the results suggested that there were some differences in the liberation of fluoride ions in different solvents. The Post hoc Tukey’s test was then used to determine individual differences in the liberation of fluoride ions from each gel in each particular solvent. There was a significant difference in fluoride ion release in every sodium fluoride only gel solution. However, there was not a significant difference between distilled and redistilled water solutions. Fluoride ion release was significantly different in every solution of gel containing amine fluoride and sodium fluoride except between distilled and redistilled water solutions and between redistilled water and artificial saliva solutions.

The ANOVA showed statistically significant differences between the liberation of fluoride ions from sodium fluoride only gel and gel containing amine fluoride and sodium fluoride in the same solvents. The Post hoc Tukey’s test revealed that significant differences exist between all the solvents except for distilled water.

Discussion

In clinical situations it is necessary to recommend reliable therapies with predictable and consistent outcomes. Therefore, it is important to have insight into possible chemical interactions which may influence the effectiveness of topical fluorides ().

Cate et al. proved that the remineralization of advanced enamel lesions benefits from more concentrated fluoride treatment (). Therefore, the first step was to determine whether the label amount of fluorides corresponded with the readily available F- ions in fluoride gels. Depending on which fluoride compound is added to a dentifrice and which fluoride fraction is measured, various analytical methods can be used (). The results suggest that there is a difference between the measured fluoride concentration and the concentration value listed on the label both for sodium fluoride only and the gel containing amine fluoride and sodium fluoride. However, in this study, it has been determined that concentrations of F- ions in redistilled water were higher than the label amount of fluoride ion concentration in both gels, sodium fluoride and amine fluoride gel, which indicates that all the fluoride compounds form both gels dissolved entirely. F- ion concentrations in pure solvent samples, in this study, were negligibly low and could not increase F- ion concentrations in fluoride gels solutions. Fluoride ion concentration measured in tap water corresponded to the previously published data ().

The measured higher concentrations might be a repercussion of Total Ionic Strength Adjustment Buffer (TISAB) (which is prescribed by ISO 19448:2018 standard) not being added to the tested solutions. When fluoride-selective electrode is used, different buffers can be utilized. Those buffers modify the electrode sensitivity and measurement precision (). The TISAB was not used in this study because only ion interactions in various solvents can be observed.

The results showed that there was a significant difference in the fluoride ion liberation from fluoride gels in various solvents.

Fluoride ion concentration in sodium fluoride only and tap water solution was significantly lower than concentration measured in solution of sodium fluoride in distilled and redistilled water. Tap water contains various ions dissolved which interact with F- ions. Calcium ions bind F- ions and tap water contains high amount of calcium ions (). On the other hand, significantly higher fluoride ion concentration was measured in the solution of sodium fluoride only gel in saline than in other sodium fluoride only solutions. Martinez-Mier et al. investigated the methods for measurements of fluoride ions in table salt and possible interferences of the presence of NaCl and other ions in the salt with the measurement of fluoride ions by use of fluoride-selective electrode (). In their study, their results obtained using F standards with NaCl added produced an overall 10% increase in fluoride ion values. Since saline solution contains sodium chloride, Cl- ions could interact with Na+ ions from NaF and make more F- ions free. Furthermore, F- ion concentration measured in the solution of sodium fluoride only gel in artificial saliva was higher than those in all the other sodium fluoride only solution except for the one in saline. The artificial saliva product, according to the label, contains citrates and acetates. According to several studies, citrates liberate fluorides from complex ions (). Acetates, which are part of TISAB, are also responsible for the liberation of F- ions (, ). The aim of this study was not to argue about bioavailability of sodium fluoride in saline and artificial saliva solutions, but this study can be a starting point for further studies.

The gel containing amine fluoride and sodium fluoride contains amine fluorides in a greater amount than NaF. Covalently bonded fluorine in amine fluorides does not dissociate in the same manner as the ionic one from NaF. When observed isolated, the differences in solutions of gel containing amine fluoride and sodium fluoride are observed between all the solutions.

The concentrations of fluoride ions in the solution of gel containing amine fluoride and sodium fluoride in tap water are significantly lower than those of the same gel in other solvents. According to Table 4 and Table 1, the measured concentration of F- ions in tap water solution of gel containing amine fluoride and sodium fluoride corresponds to the amount of amine fluorides in the gel product. As described in the previous paragraph, different ions (primarily Ca2+ ions) present in tap water interacted with NaF, which resulted in lower concentration being measured.

When solutions of the gel containing amine fluoride and sodium fluoride are compared with sodium fluoride only solutions, the stability of amine fluorides is verified. F- ion concentrations in saline and artificial saliva solutions of gel containing amine fluoride and sodium fluoride differ but are much closer to the redistilled water solution of that gel. Those concentrations are almost three times lower than F- concentration in the solutions of sodium fluoride only in the same solvents due to lack of ion interactions between amine fluorides and other ions present in the solutions.

The limitations of this analysis relate to the sample and analysis methods.

The analysis methods were based on a relatively new ISO 19448:2018 standard. ISO 19448:2018 standard was supposed to be a generic guideline, which does not make a distinction between fluoride compounds. The method should be acceptable for the analysis of fluoride in aqueous samples derived from all fluoridated dental products.

We made a modification to the ISO 19448:2018 standard and did not add TISAB to the solutions, which might have influenced the measurement, as mentioned earlier.

In the absence of an additional, well-established, method of analysis being used on the same samples (i.e. liquid gas chromatography ()) it is not possible to completely asses the validity of the ISO 19448:2018 standard.

The study used a small, non-systematic convenience sample of fluoride gels, using only one sample of each type.

Without reliable and comprehensive guidelines on the composition of fluoride gels, it is not possible to extrapolate effectiveness of the gels based on the fluoride content. However, this study observed ionic interactions, which might affect the demineralization-remineralization process in vivo.

Conclusion

Sodium fluoride and amine fluorides differ in fluoride ion delivery mechanisms. Amine fluorides do not interact with other ions present in the solution, whereas F- ion release from sodium fluoride alters when other ions are dissolved in the same solution. Furthermore, the greater amount of F- ions is released from NaF in the solutions which contain chlorides, citrates and acetates. Given that, it should be noted that amine fluorides are more stable compounds as fluoride delivery system and may deliver F- ions more efficiently for caries prevention in patients suffering from xerostomia after radiation therapy.