Methods: HAp disks of 2mm thickness and 20% porosity were placed in individual SMR flow cells through which demineralisation solution (1.0% citric acid, pH 3.8) was circulated. The rate of demineralisation (RDHAp) was measured at 20 points in each disk for 100h using SMR. The disks were flushed with deionised water for 1h, before either test or control fluoride solutions were circulated for 2mins. Control solutions contained concentrations of 0, 0.2, 1, 5, 25, 100, 300, 1000 and 5000 ppm F. Test solutions were supernatants from dentifrices containing 0, 250, 1150 ppm F. Demineralisation was continued for 100h. SMR was used to measure subsequent demineralisation at the same points after as before exposure to test and control F solutions, eliminating the effects of specimen variability. This data was used to calculate the mean percentage reduction in RDHAp following exposure to test and control solutions.
Results: The control studies showed a logarithmic relation between percentage reduction in RDHAp (y) and fluoride concentration in ppm (x), (y=3.92ln(x)+3.2, R2=0.97) in the concentration range used. The test supernatants conferred reductions in the rate of demineralisation commensurate with their F concentration, and consistent with the previously obtained fluoride dose-response curve.
Conclusion: A quantitative in vitro method suitable for determining the anti-erosion efficacy of fluoride toothpastes has been developed.