Effect of initial demineralization on the permeability of human tooth enamel to iodide.

Blocks of intact human enamel were exposed for four successive periods of 5 min to 2 ml of 10(-2) M K-acetate at pH 4.0, 4.5 and 5.0. The permeability to iodide (Ip) and the amount of Ca dissolved were determined after each exposure. The Ip values varied considerably, but for each block there was a consistent increase in Ip with increase in acid exposures, and the Ca-Ip relationship was linear at all pH levels. The slope and intercept of the regression lines varied widely, but the correlation coefficients showed close fit of the data. The mean Ip values after the same period of exposure at pH 4.0 and 4.5 were statistically different from those obtained at pH 5.0, but the differences between the pH 4.0 and 4.5 values were not. This discrepancy is attributed to dissolution of surface enamel by the pH 4.0 buffer. Because loss of surface enamel is minimized by partial saturation of the buffer with Ca and phosphate, and oral fluids contain high concentrations of these ionic species, the iodide method should give valid measurements of the demineralization produced intarorally by acid and cariogenic foods.