HISTOPATHOLOGY OF CARIES OF DENTIN
From the histopathological point of view, caries in dentin involves two steps: irst there is no visible defect. However, the presence of dentin tubules provides rapid penetration of acids and microorganisms. Therefore, even upon a slight lesion, dentin demineralization occurs (fig. 21.1).
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Fig. 21.1. Demineralized enamel and dentin
After dentin demineralization, degeneration and dissolution of the organic matrix begin, leading to disruption of its structural integrity.
Microscopically, there are ive areas of carious dentin (from inside to outside):
Zone 1 - fatty degeneration of Tom's fibers (odontoblasts processes) (also called normal dentin). Dentin here has a normal structure. A layer of fat globules provides impermeability of the dentinal tubules (DT) trying to prevent further invasion of carious lesion. Also in this zone tertiary dentin formation can be observed.
Zone 2 - dentinal sclerosis (subtranslucent dentin). Calcification of DT aims to protect vital pulp and vital dentin from further penetration of microorganisms. Formation of this zone is minimal in rapidly progressing caries, and prominent in slow caries.
Zone 3 - zone of decalcification (demineralization) (translucent dentin). Here there is initial decalciication, and a small number of bacteria can be found in individual DT. This zone could be large in rapidly progressing caries or thin if progression is slow.
Zone 4 - zone of bacteria invasion (turbid or muddy dentin). Presence of a large amount of microorganisms.
Zone 5 - zone of decomposed dentin (infected dentin). No recognizable structure in decomposed dentin, a great number of bacteria dispersed in necrotic mass.
An important aspect of the pathogenesis of dental caries is formation of substitution dentin protecting the pulp from penetration of microorganisms and acids. Formation of substitution dentin is the result of odontoblast activity. Newly formed substitution dentin usually has an irregular structure. Fig. 21.2 demonstrates a cross section of molar tooth with carious lesions of varying degrees.