Abstract
We investigated the effects of an unequal ratio of de- and remineralization balance on the acid resistance of human enamel. Human enamel premolars were demineralized and subsequently remineralized with 3 ppm fluoride. Sixty enamel samples were exposed to demineralizing and remineralizing solutions [D1R3, D1R6, D3R1, D6R1 ; the number means daily demineralization (D) and remineralization (R)], then addition to a 0.1 M lactic acid solution (pH : 4.5) for 1,2, or 3 days. The dissolved Ca, P, F and the mineral loss values were measured by means of an atomic absorption spectrophotometer, a double beam spectro-photometer, a fluoride electrode and transversal microradiography, respectively. The amount of dissolved Ca increased linearly in proportion to the periods of the acid resistant test (ART). After 1 and 2 days of ART, samples were classified into the two groups of long (D1R3 and D1R6) and short (D3R1 and D6R1) terms of remineralization (p<0.05). However, there was no difference between the D1R3 and D6R1 groups after 3 days of ART. The longest remineralization D1R6 group had the maximum fluoride concentration (approximately 4,000 ppm) and a statistically significant higher fluoride value than the other groups (p<0.01). After 3 days of ART, a clear lamination type of mineral distribution was found in all groups. The microradiograph pictures and the mean mineral profiles for the long-term remineralization groups showed that mineral loss was not pronounced after 1 or 2 days of ART and no subsurface lesions were formed. However, typical subsurface lesions were found in the short-term remineralization groups. The present and previous studies indicate that at least 3 days of remineralization are necessary to form acid resistance regardless of the demineralization period. Laminations themselves may not have a direct connection with acid resistance unless a significant amount of fluoride is incorporated. From the Ca/P weight ratio and fluoride concentrations in remineralized enamel, the formation of partially fluoridated hydroxyapatite Ca10(P04)6(OH, F) is suggested.
