# Fluoride Ion Release Characteristics of Fluoride-Containing Varnishes--An In Vitro Study
## 4 Discussion
The fluoride concentration was measured after successful calibration of the electrode between two set values using previously determined calibration solutions. By repeatedly measuring these calibration values before and between the series of measurements, whether the measuring range of the electrode had shifted during the measurement series could be detected. The measurement of each product was repeated three times to verify the fluoride concentration. In order to prevent the accumulation of fluoride in the examination medium, the solution was replaced by a new solution after each measurement time. The product Bifluorid 12 showed the highest fluoride ion release capacity in our study. The interpretation of the global Kruskal-Wallis H test and the descriptive statistics, in addition to the pairwise Mann-Whitney U test, led to the conclusion that Bifluorid 12 outperformed all other tested products at all measurement timepoints, T0, T1, T2, and T3. Our results on the fluoride release characteristics of Bifluorid 12 match the results of other studies showing that the fluoride release decreases over time [30]. [PERSON] et al. [31] also found Bifluorid 12 to have the highest fluoride release in solution when compared to other product groups. This shows that the results are consistent with those of other studies. It can be assumed that Bifluorid 12 is able to release fluoride for longer than the other products tested in our study. The product Fluor Protector S showed similar characteristics to Bifluorid 12, but with lower median values of fluoride release at any measurement point. The results of our study show that Fluor Protector S had highly significant fluoride release until T1. After this time, fluoride release was detectable only at a low value. [PERSON] et al. investigated Fluor Protector S using a fluoride ion electrode after storage of the test specimen in an artificial saliva solution over a period of 4 h. Fluor Protector S showed the highest amount of fluoride uptake into enamel compared to other product groups, but its fluoride release was not the highest [32]. A peak fluoride release within the first hour following application [32] could not be confirmed by our data, as Fluor Protector S showed a peak fluoride release at T1 in our study (meaning after 20 days of thermal cycling). It can be assumed that Fluor Protector can deliver a high initial level of fluoride release to the surrounding oral cavity environment, but thereafter the fluoride levels fall to a minimum. For orthodontic treatment, which typically lasts about two years, continuous long-term fluoride release would be desirable. It is well documented that the fluoride concentration in the test medium is increased immediately after the use of various fluoride compounds in the denitrifice [33; 34]. Showing a peak fluoride concentration in the tested medium after 20 days of thermocycling, our data suggest a different kinetic behavior for fluoride varnishes. Protector CaF\({}_{2}\) nano one-step seal showed high fluoride release at T1 and T2 during the test period. Immediately after application of the product (T0) and after the first thermocycling at timepoint T1, peak fluoride release was detectable in the solution. When compared to the values of the control group at measurement timepoint T2, the difference is 0.001 ppm, though this is statistically insignificant, bordering on the significance level. At T3, the difference from the control group was 0.002 ppm. These values might not show clinical relevance; therefore, clinical effectiveness in long-term use and after a single application of Protector CaF\({}_{2}\) nano is to be questioned. It should be noted that the measured values of fluoride ion concentrations are so low that accuracy cannot be guaranteed, even with the use of a highly sensitive fluoride ion electrode in this study. This could also be one of the reasons why a minimal amount of fluoride release was measured in the control group with hydroxyapatite discs. A study by [PERSON] et al.