Retention of Fluoride in Hydroxyapatite Discs Using Natural and Artificial Saliva for Remineralization

Objective: To determine the fluoride uptake potential by hydroxyapatite (HAP) after NaF exposure followed by remineralization in human or artificial saliva. Methods: HAP discs were pretreated with 0.5N HClO₄, and placed in solutions containing NaF at 0, 250, 500, or 1000 ppm, Fluoriguard mouthrinse, or Crest Tartar Protection dentifrice. After 1 or 5 minutes, discs were placed in artificial saliva or human saliva for up to 4h at 37°C. Discs were treated with KOH, followed by 0.5N HClO₄ for 90 seconds. Artificial saliva was prepared fresh just prior to use and consisted (per liter) of 0.4 g NaCl, 0.4 g KCl, 0.795 g CaCl₂•2H₂O, 0.42 g NaH₂PO₄•H₂O, 0.53 g Na₂HPO₄•7H₂O, 5 mg Na₂S•9H₂O, and 1 g urea; the final pH was 6.5-6.7. Results: Prior to remineralization, the fluoride delivered via treatment became associated with HAP, and this association increased with time; retention was enhanced during remineralization in artificial saliva or human saliva. Fluoride retention by HAP occurred in a dose- and time-dependent manner in both human and artificial saliva with aqueous NaF and dental products. Most of the fluoride was released following HClO₄ treatment, which indicates it was not loosely-bound. Fluoride uptake by HAP discs after 4h in artificial saliva was 1.4, 1.5, and 2.5 µg/square cm, respectively for 250, 500 and 1,000 ppm fluoride, and the process was ongoing at 4h. Remineralization had no effect beyond 1h on KOH-extractable, or “loosely-bound” fluoride, indicating it was tightly sequestered or bound within the HAP. Conclusion: In the absence of remineralization, fluoride was sequestered within HAP by 5 minutes’ exposure, and remineralization solutions (artificial or human saliva) further enhanced fluoride retention by HAP. An in vitro system of defined components can be used as a surrogate for human saliva to monitor fluoride uptake by HAP and eliminate interindividual variability in saliva composition.