Molecular Sieve Behaviour of Dental Enamel

NORMAL mammalian tooth enamel is highly calcified, containing 97 per cent mineral and 3 per cent organic material and water. The mineral is hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 , and it is associated with a protein constituent of the keratin type 1 . From histological studies it is well known that normal enamel is made up of structural units which are prismatic in shape and contain mainly crystallites of hydroxyapatite. The organic constituent is concentrated at the margins of these prisms but it is also present in smaller amounts within them. In the prisms the crystallites are highly oriented, with their optic (fibre) axes approximately parallel with the longitudinal axes of the prisms. This submicroscopic structure of normal enamel is shown diagrammatically in Fig. 1. The fully formed enamel can be broken down either in vitro by organic acids such as lactic acid, or in vivo by the processes concerned in carious decay. In each case the mineral is gradually dissolved out until, eventually, the structure of the enamel is destroyed 2 .