The calcium and phosphate are dissolved from the crystals in a complex environment. The concept of a "critical pH" is the situation under which this net loss occurs. Previously this was thought to be a fixed value, but it is now accepted that it is rather a value that is inversely proportional to the calcium and phosphate concentrations in solution in the localized environment.4 Larsen and Pierce5 developed a computer program for examining the solubility of enamel. Small pH changes around a pH of 4 were demonstrated to significantly impact the demineralization potential of enamel.
What has actually happened? Acids diffuse through the interprismatic rod substance and travel along the rod margin to an area of lowered fluoride content. (Note: the outer 10 microns of enamel holds a higher concentration of fluoride.)
As the process of demineralization continues, the edges of the enamel crystal demineralize . . . that is, calcium and phosphate become dissociated in the small subsurface area.