To understand the concept we can examine this lesion that has been prepared for microscopic examination. We can easily note the outer layer, the surface layer, and the inner demineralized area termed a subsurface lesion.
The calcium and phosphate are dissolved from the crystals in a complex environment. The concept of a "critical pH" is the situation under which net mineral 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.13 Larsen and Pierce14 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.
This process results in the development of an early lesion (begins 10-15 microns beneath the surface) which has a relatively intact surface layer that cannot be penetrated with a dental explorer during the early stages of formation. If the acid challenge continues, greater amounts of subsurface enamel will be demineralized (or as it is sometimes called "decalcified") and the lesion will continue to progress deeper under the intact surface layer.
The caries process is a dynamic situation which occurs as a result of substantial pH fluctuations taking place in the biofilm on the tooth surface. Kidd and Fejerskov15 describe this process and its histopathology and indicate the "Regular removal of the biofilm, preferably with a toothpaste containing fluoride, delays or even arrests lesion progression." The processes of demineralization and remineralization can occur simultaneously or alternately. The high resolution transmission electron microscope has been used to demonstrate how these crystals look in caries lesions.16 In very simple crystallographic terms, demineralization is crystal dissolution and remineralization is the restoration of partially dissolved crystals, growth of surviving crystals and the formation of new crystals within the carious lesion. If net demineralization continues, the outer layer of enamel will eventually become extensively undermined; with the clinical result being an area that is "sticky" or "soft" to the explorer coupled with visual evidence of demineralization. Larsen and Bruun17 describe this process in detail in their textbook chapter on "Caries Chemistry and Fluoride – Mechanisms of Action." They discuss the theory which claims that when "...fluoride is present in the aqueous phase around the tooth, in saliva and in plaque fluid, enamel solubility is low, which tends to prevent it from dissolving."
Remineralization of the subsurface lesion occurs when the dissociated ions of calcium and phosphate recombine to form an even stronger crystal. This reaction is enhanced in the presence of fluoride.
Thus, fluoride changes the remineralization process by acting as a mineral growth catalyst. The action of fluoride accelerates the rebuilding of enamel and can stop or even reverse the progression of dental decay. More importantly, the incorporation of fluoride into the tooth mineral along with calcium and phosphate makes enamel more resistant to acid demineralization then it was originally.
The processes of remineralization and demineralization are similar on any tooth surface upon which they are occurring (enamel pits, fissures, smooth surfaces). However, some surfaces tend to have higher incidence of lesion formation compared to others. This is primarily a function of a combination of tooth morphology and accessibility for cleaning, with pit and fissure along with interproximal sites experiencing the highest rates of lesion formation.