An in vivo study of microhardness and fluoride uptake in partially demineralized human enamel covered by plaque.

In this investigation, microhardness changes in partially demineralized human enamel were studied after in vivo use of fluoridated toothpaste systems. Flattened enamel specimens were demineralized in vitro and subsequently positioned in approximal positions in the prostheses of 27 participants for three weeks; the samples were plaque-covered in vivo. After brushing for one week with a non-fluoridated paste to achieve an in vivo equilibrium, participants brushed with an assigned product for a two-week period. This test was a six-way cross-over design which used randomization of subjects. Five fluoridated and one non-fluoridated paste were tested. Knoop hardness measurements were carried out on sound and on in vitro demineralized enamel, and after in vivo exposure of the enamel to the dentifrice treatments. The results showed that during the two weeks of in vivo exposure to the fluoride products, net rehardening of the demineralized enamel did not occur, and no correlation was observed between fluoride uptake into the demineralized enamel and changes in microhardness. That we failed to observe rehardening may be due to the fact that the duration of this study was too short for any net remineralization to have occurred, especially because the samples were constantly covered with plaque. Other possibilities, such as the type and/or severity of the lesions used in this study, may account for the lack of rehardening.