Cross-section of tooth showing enamel, dentin and pulp.

Healthy tooth enamel is not a solid piece of impermeable material.  It is porous, consisting of millions of enamel rods, each measuring about the width of a human hair. It is surrounded by a matrix containing inorganic material, small amounts of water, protein and lipids.

Each rod is made up of millions of these extremely fine crystals, composed primarily of calcium and phosphate.

The acid from sugar metabolism in plaque bacteria will diffuse onto and into the enamel via the intercrystalline spaces and partially dissolve the enamel rod.

The dissolved calcium and phosphate ions can diffuse through to the surface of the enamel and may be lost into the saliva if not trapped by the pellicle or plaque.

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