The Most Common Disease That Causes Loss of Teeth – Dental Caries Part 8

You can read part 7 of most common disease before reading part 8

Pathology Of Enamel Caries

Enamel is the usual site of the initial lesion unless dentine or cementum becomes exposed by gingival recession. Enamel, the hardest and densest tissue in the body, consists almost entirely of calcium apatite with only a minute organic content. It therefore forms a formidable barrier to bacterial attack. However, once enamel has been breached, infection of dentine can spread with relatively little obstruction. Preventive measures must therefore be aimed primarily at stopping the attack or at making enamel more resistant. The essential nature of the carious attack on enamel is permeation of acid into its substance. The crystalline lattice of calcium apatite crystals is relatively impermeable but part of the organic matrix of enamel which envelops the apatite crystals, has a relatively high water content and is permeable to hydrogen ions. Permeation of enamel by acid causes a series of submicroscopic changes. This process of enamel caries is a dynamic one and, initially at least, consists of alternating phases of demineralization and remineralization, rather than a continuous process of dissolution.

Enamel caries develops in four main phases:

• The early (submicroscopic) lesion

• Phase of nonbacterial enamel crystal destruction

• Cavity formation

• Bacterial invasion of enamel

These stages of enamel caries are distinguishable microscopically and are also clinically significant. In particular, the early (white spot) lesion is potentially reversible, but cavity formation is irreversible and requires restorative measures to substitute for the lost tissue.

The Early Lesion

The earliest visible changes are seen as a white opaque spot that forms just adjacent to a contact point. Despite the chalky appearance the enamel is hard and smooth to the probe.

Early enamel caries, a white spot lesion, in a deciduous molar. The lesion forms below the contact point and in consequence is much larger than an interproximal lesion in a permanent tooth

The microscopic changes under this early white spot lesion may be seen in undecalcified sections but more readily when polarized light is used. Microradiography indicates the degree of demineralization seen in different zones. The initial lesion is conical in shape with its apex towards the dentine, and a series of four zones of differing translucency can be discerned. Working back from the deepest, advancing edge of the lesion, these zones consist first of a translucent zone most deeply; immediately within this is a second dark zone; the third consists of the body of the lesion and the fourth consists of the surface zone.

Early interproximal caries. Ground section in water viewed by polarized light. The body of the lesion and the intact surface layer are visible. The translucent and dark zones are not seen until the section is viewed immersed in quinoline.

These initial changes are not due to bacterial invasion, but due to bacterial lactic or other acids causing varying degrees of demineralization and remineralization in the enamel. The translucent zone is the first observable change. The appearance of the translucent zone results from formation of submicroscopic spaces or pores apparently located at prism boundaries and other junctional sites such as the striae of Retzius.

When the section is mounted in quinoline, it fills the pores, and since it has the same refractive index as enamel the normal structural features disappear and the appearance of the pores is enhanced. Microradiography confirms that the changes in the translucent zone are due to demineralization.

Early interproximal caries. Ground section viewed by polarized light after immersion in quinoline. Quinoline has filled the larger pores, causing most of the fine detail in the body of the lesion to disappear, but the dark zone with its smaller pores is accentuated.

Continue reading Part 9.