[The ultrastructural aspects of the carious lesion]

The aim of the present study was to analyse under SEM the ultrastructural alterations in tubular content and peri- and intertubular dentin in teeth subject to active carious lesions. The samples used were teeth with grade 2 active carious lesions without signs of abrasion, wear, erosion and conservative reconstruction. The coronal fragments obtained by fracture were fixated, dehydrated and metallized before SEM analysis. The study was performed by analysing samples of amelo-dentinal junctions towards the pulpo-dentinal edge. By correlating histopathological lesions to their topographical distribution and to the physiopathological mechanisms involved, it was possible to subdivide dentin underlying a carious lesion into three main zones: demineralisation zone, sclerosis zone, subsclerotic dentin zone. The individualization of these sectors was based on the identification of basic lesions linked to changes in tubular lumen and peri and intertubular dentin. The demineralisation zone was in fact characterised by: dentinal infection and reversal of normal tissue morphology in the outermost layers; the fibrous appearance of intertubular dentin and disappearance of peritubular dentin; modifications in the tubular content secondary to demineralisation, such as sheaths, hollow rods and intratubular crystals. The sclerosis zone was characterised by: fully occluded tubules, tubules fully occluded by rods, tubules partially occluded by hollow rods and by thickening of peritubular dentin. The third zone was the result of tissue-forming activity on the pulpodentinal edge in response to carious lesions and was represented by regular and irregular tertiary dentin. From an analysis of all data collected it is possible to hypothesize that changes in the demineralisation zone are the results of a passive tissue response exclusively linked to physicochemical mechanisms; whereas those in the sclerosis zone are due to the realisation of a lithogenic environment of multifactorial genesis at a intratubular level. The site in which there is the greatest probability of observing a cellular response is the subsclerotic dentinal zone.