Zone of enamel caries: development of enamel caries
Ground sections of teeth have been used extensively in histopathological studies of enamel caries and have been examined by transmitted and polarized light, and by microradiography. Electron microscopy and biochemical analysis of microdissected pieces of carious enamel have also been carried out. Most research has concentrated on smooth surface caries to avoid the problems of interpretation of histological features imposed by the anatomy of pits and fissures. However, the pathological features are essentially similar in both sites. The established early lesion (white spot lesion) in smooth surface enamel caries is cone-shaped, with the base of the cone on the enamel surface and the apex pointing towards the amelodentinal junction. The shape is modified in pit and fissure caries. In ground sections it consists of a series of zones, the optical properties of which reflect differing degrees of demineralization (Fig.). These zones are described below.
This is the first recognizable histological change at the advancing edge of the lesion. It is more porous than normal enamel and contains 1 per cent by volume of spaces, the pore volume, compared with the 0.1 per cent pore volume in normal enamel. The pores are larger than the small pores in normal enamel which approximate to the size of a water molecule. Chemical analysis shows that there is a fall in magnesium and carbonate when compared with normal enamel, which suggests that a magnesium- and carbonate- rich mineral is preferentially dissolved in this zone. Dissolution of mineral occurs mainly from the junctional areas between the prismatic and interprismatic enamel. The prism boundaries, which are relatively rich in protein, allow ready ingress of hydrogen ions and the magnesium- and carbonate-rich mineral that is preferentially removed may represent the surface layers of crystallites at the prism boundaries. The translucent zone is sometimes missing, or present along only part of the lesion.
This zone contains 2-4 per cent by volume of pores. Some of the pores are large, but others are smaller than those in the translucent zone, suggesting that some remineralization has occurred due to reprecipitation of mineral lost from the translucent zone. It is thought that the dark zone is narrow in rapidly advancing lesions and wider in more slowly advancing lesions when more remineralization may occur.
Body of the lesion
This zone (Fig.) has a pore volume of between 5 and 25 per cent, and also contains apatite crystals larger than those found in normal enamel. It is suggested that these large crystals result from the reprecipitation of mineral dissolved from deeper zones. However, with continuing acid attack there is further dissolution of mineral both from the periphery of the apatite crystals and from their cores. The lost mineral is replaced by unbound water and to a lesser extent by organic matter, presumably derived from saliva and microorganisms. There is increased prominence of the striae of Retzius in the body of the lesion, the explanation for which is unknown.
This is about 40um thick and shows surprisingly little change in early lesions (see Fig. above). The surface of normal enamel differs in composition from the deeper layers, being more highly mineralized and having, for example, a higher fluoride level and a lower magnesium level, and so interpretation of possible chemical changes in this zone is difficult. The surface zone remains relatively normal despite subsurface loss of mineral, because it is an area of active reprecipitation of mineral derived both from the plaque and from that dissolved from deeper areas of the lesion as ions diffuse outwards.