Determination of Fluoride Penetration in Enamel by Dynamic-SIMS

Objective: Determine fluoride incorporation into demineralised human enamel by DSIMS following treatment with toothpastes containing fluoride from different sources. Methods: Erosive lesions were prepared from human enamel by immersion in 1.0% citric acid, pH 3.8 for 5 min. Specimens were subsequently treated for 2 min with agitated 1:3 w/w toothpaste:deionised water slurries. Three of the five toothpastes investigated contained NaF. Paste 1 was a marketed anti-caries paste containing 1350ppm fluoride; paste 2 contained sodium pyrophosphate plus 1450ppm fluoride; paste 3 contained 5% KNO3 plus 1450ppm fluoride. Paste 4 contained SnF2 (1100ppm fluoride) and paste 5 contained amine fluoride (AmF, 1250ppm fluoride). Treated specimens were vertically sectioned and the fluoride concentration in the first few microns of their surface determined using secondary ion mass spectrometry (SIMS) in depth profiling mode. The remaining half of each specimen was employed to generate SIMS images of the cross-sections. Results: Fluoride depth profiles of the enamel surface showed that fluoride uptake was higher in the first 2μm for specimens treated with pastes 1, 3 and 5 compared to specimens treated with pastes 2 and 4. DSIMS cross-sectional imaging of enamel treated with either paste 1 or 3 revealed a continuous 8-10μm thick fluoride-rich surface layer. Enamel treated with pastes 2, 4 and 5 resulted in the formation of fluoride-rich surface layers penetrating 3-4μm, 3-5μm and 5-7μm below the surface respectively. Conclusion: This DSIMS study demonstrates that treatment of human enamel erosive lesions with fluoride-containing toothpastes leads to penetration of fluoride into the surface of the enamel. Fluoride penetration was between 40 and 150% greater for the marketed anti-caries paste and the paste containing KNO3 compared to those based on AmF, SnF2, or NaF plus pyrophosphate.