You can read part 8 of most common disease before reading part 9
The dark zone is fractionally superficial to the translucent zone. Polarized light microscopy shows that the volume of the pores in this zone has increased to 2-4% of the enamel volume. This change is due mainly to formation of additional small pores. Two different-size pores thus coexist in the dark zone. The small ones are so minute that molecules of quinoline are unable to enter and the tissue has become transformed into a molecular sieve. The small pores therefore remain filled with air — this appears to produce the zone’s dark appearance. Microradiography confirms that the dark zone has suffered a greater degree of demineralization. However, when the lesion is exposed to saliva or synthetic calcifying solutions in vitro, the dark zone actually extends further.
This may indicate that the formation of the dark zone may be due not merely to creation of new porosities but possibly also to remineralization of the large pores of the translucent zone so that they become micropores impermeable to quinoline. It is widely believed therefore that these changes in the dark zone are evidence of remineralization, as discussed later. The body of the lesion forms the bulk of the lesion and extends from just beneath the surface zone to the dark zone. By transmitted light the body of the lesion is comparatively translucent compared with normal enamel and sharply demarcated from the dark zone. Within the body of the lesion the striae of Retzius appear enhanced, particularly when mounted in quinoline and viewed under polarized light. Polarized light examination also shows that the pore volume is 5% at the periphery but increases to at least 25% in the centre.
The same lesion in early section of the article viewed dry under polarized light to show the full extent of demineralization
Microradiography, which will detect demineralization in excess of 5%, shows that the area of radiolucency corresponds closely with the size and shape of the body of the lesion, in contrast to the surface zone which appears relatively radiopaque.
Early interproximal caries. A microradiograph of the same section, showing radiolucency following the same pattern, the intact surface zone and accentuation of the striae of Retzius
Alternating radiopaque and radiolucent lines, about 30 µm apart, can also be seen passing obliquely through the subsurface region. The radiolucent lines show an apparently preferential demineralization and probably represent the striae of Retzius.
At higher magnifications, still finer lines running at right angles to the enamel surface, and others parallel to the surface, may be discerned. These lines may represent preferential demineralization along the junctional sites mentioned earlier, and represent the prism boundaries and the cross striations, respectively.
The surface zone represents one of the most important changes in enamel caries in terms of prevention and management of the disease. It shows the paradoxical feature that it has not merely remained intact during this stage of the attack but remains more heavily mineralized and radiopaque than the deeper zones. It has a pore volume of only 1%. When the surface zone is removed and the enamel is exposed to an acid buffer, the more highly mineralized surface zone reappears over the deeper changes described earlier. The surface zone therefore appears to form partly by remineralization.
The remineralizing salts may come either from those concentrated in the plaque or from precipitation of calcium and phosphate ions diffusing outwards as the deeper zones are demineralized. In pit and fissure caries the same changes take place but as acid diffuses out from the pit the lesion forms a ring round it. However, in a two-dimensional view, the same zones as in smooth surface caries are seen on either side of the fissure.
Undecalcified section showing early enamel lesions in the enamel surrounding and deep to an occlusal pit
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