pH- and calcium-dependent solution-phase free fluoride in dental desensitizing gels: a preliminary in vitro study

Dental hypersensitivity (DH) is a common clinical condition characterized by brief, intense pain. This condition is treated with fluoride-containing specialized dental gels. The effect of calcium ions present in saliva and pH of the oral cavity are indicated as factors affecting their clinical performance.

In the presented study, two dental desensitizing gels were evaluated. Fourier Transform Infrared Spectroscopy (FTIR) spectra of the gels were registered and analyzed. Measurement of concentration of measurable free fluoride ions and pH in vitro assessment was performed using selective electrodes. Measurement with addition of TISAB witch CDTA was performed to evaluate the amount of fluoride bound within the gel matrix. Univariate and multifactorial ANOVA tests were used to analyze acquired results of measurable free fluoride in twelve distinct incubation solutions.

The highest values of free, measurable fluoride were obtained after addition of TISAB with CDTA. However, in physiological study without ionic strength adjuster and chelator, the highest free, measurable fluoride observed in the artificial saliva solutions was 1,043 ppm for the SensiKIN desensitizing gel at pH 4.5 in the solution without calcium ion addition. The lowest value (658 ppm) was denoted for the Teeth Desensitiser TD gel at pH 7.0 with calcium ion addition. The presented amounts of fluoride pertain to measurable free fluoride in solution. It has been shown that the presence of calcium in saliva reduces the concentration of measurable free fluoride in solution (by as much as 137 ppm). Although the effect of presence of Ca²⁺ is pronounced, the initial pH remains the most significant variable. Observed levels of measurable free fluoride in solution might reflect initial and preliminary bioavailability under static in vitro conditions. Measurable free fluoride may serve as an indirect analytical proxy for potentially available fluoride under the tested in vitro conditions. However, it does not represent direct biological or clinical bioavailability. The evaluated desensitizing gels showed maximal free, measurable fluoride concentration under acidic conditions. For studies incorporating TISAB/CDTA, the most profound effect on F^- concentration was gel composition. Future formulation studies could investigate whether calcium- complexing or buffering components influence measurable solution-phase free fluoride under standardized in vitro conditions.