Structure and porosity of human cervical enamel studied by polarizing microscopy and transmission electron microscopy

Dehydration by alcohols and concentrated solutions of highly soluble salts caused refractive changes in enamel which were enhanced in the cervical regions. Electron microscopy showed crystals in cervical enamel to vary from normal orientation, size and habit to marked disorientation and reduced size. Consequent increases in surface area and inter-crystalline volume could account for the exaggerated optical behaviour. The changes in optical properties of enamel occurred only when the osmotic pressures of the imbibing solutions exceeded 500 atm. It is proposed that the internal pore system may take up water freely but exclude large hydrated molecules and ions which thereby create a negative pressure on the water contained in these pores. Once this negative pressure exceeds the tensile strength of the water-pore system, molecules may be torn from their attachment sites, and possibly from each other, so that a sudden evacuation of the pores is accompanied by a simultaneous reduction in refractive index. It is likely that such sudden evacuation would alter the potential of the pore surfaces to react with small molecules and ions subsequently invading the tissue.