All caries treatment and preventive procedures are based on understanding of enamel histology and composition, hydroxyapatite properties in different pH conditions, histopathology of the carious spot. That is why it is important for a clinician to understand it.
ENAMEL STRUCTURE
Enamel is an acellular tissue comprised mostly of crystals of substituted calcium hydroxyapatite (HA).
The apatite crystals are long (possibly up to 1 mm), 50 nm wide by 25 nm thick. They are arranged parallel in bundles of approximately 1000 crystals, the so-called enamel prisms or rods. Enamel rod is a fundamental unit of tooth enamel. The cross sectional profile of enamel rods varies from circular to keyhole shaped.
The HA crystals are arranged with their long axes parallel to the long axis of the rods. At the periphery of each rod, however, the crystals deviate from this orientation, producing an interface between rods. Areas between rods are called interprismatic spaces or interrod spaces. In these spaces crystals are packed not so tight and have more intercrystalline gaps compared with crystals in rods. It permits easier diffusion of acids and protons into the tissue and mineral ions out of it.
The boundary between rods and interrod crystals is marked by a narrow space filled with organic materials - rod sheath.
Because of bigger amount of intecrystalline gaps, HA in interrod spaces demineralizes faster than rods.
Demineralized interred spaces become tunnels leading into subsurface area of enamel and acting as pathways for acids,proteolytic enzymes and microorganisms migrating deeper and destroying HA and organic matrix.
ENAMEL COMPOSITION
Composition of enamel comprises approximately 96% of inorganic material and 2.8% water and of 1.2% organic material by weight. Despite the fact that Bodecker isolated an organic material from enamel in 1906, the existence of organic material in enamel was not accepted by the dental community for a long time because of its small quantity. To explain the organic to inorganic ratio in enamel he gave an example of a stone and a sponge of the same size; the stone represented inorganic components of enamel, and the sponge represented organic components. Although the stone weighed about 34 grams and the sponge - only 0.31 grams, they were of the same size.