The Effects of Fluoride Treatment Time and Concentration on In Vitro Caries Lesion Demineralisation and Remineralisation

Purpose: To investigate the possible interaction between fluoride treatment time and concentration on enamel caries lesion de-/remineralisation. Materials and Methods: The study design followed a three (fluoride concentration: 0, 275, 1250 ppm as sodium fluoride) x four (treatment time: 10, 30, 60, 120 s) factorial design. Caries lesions were created in bovine enamel and the extent of demineralisation determined using Vickers surface microhardness (VHN). Lesions were pH cycled (18 days) with the daily schedule consisting of two fluoride treatments, a 4-h demineralisation period and exposure to artificial saliva at all other times. VHN was determined again after pH cycling and changes to baseline values calculated (∆VHN). Enamel fluoride uptake (EFU) was determined using the microbiopsy technique. Data were analyzed using two-way ANOVA. Results: The concentration x treatment time interaction was significant for ∆VHN (p 0.0001) and EFU (p = 0.0298). Dose-response relationships were observed for both variables for fluoride concentration and treatment time. ∆VHN: higher fluoride concentration compensated for shorter treatment time (e.g. ∆VHN [mean ± SD] = 85.5 ± 60.6 for 30 s with 1250 ppm fluoride vs ∆VHN = 84.3 ± 26.9 for 120s with 275 ppm fluoride). EFU data were similar but highlighted a greater ability to discern between fluoride concentrations (e.g. EFU = 4364 ± 1166 ppm vs 8538 ± 9531 ppm; above examples). Although ∆VHN and EFU correlated well (r = 0.723; p 0.001), lesion demonstrated a greater ability to acquire fluoride than to remineralise. Conclusions: Behavioural aspects relating to caries can be studied in vitro, although model limitations must be considered. Adequate exposure times to cariostatic concentrations of fluoride are important in maximising caries prevention. Keywords: caries, enamel, fluoride, pH cycling, remineralisation