The major channels for solute diffusion across dentin are the dentinal tubules. Since dentin permeation is proportional to the product of tubule number and diameter, both of which increase as the tubules converge on the pulp, we find that dentin permeability increases rapidly as the pulp chamber is approached. The presence of a smear layer of cutting debris on top of cut dentin decreases dentin permeability, especially when permeability is measured by fluid filtration. Further, intratubular material--such as mineral deposits, collagen fibrils, proteoglycan linings, bacteria, etc.--can greatly reduce dentin permeability. Although the presence of irregular or irritation dentin has been thought to greatly reduce dentin permeability, recent in vivo experiments in dogs indicate that the dentin permeability of freshly cut cavities prepared in sound dentin falls very rapidly (i.e., 50-60% in the first six hours) before any histologic changes can be detected, either in the pulp or the dentin. When dogs were depleted of their plasma fibrinogen, this rapid decline in dentin permeability following cavity preparation failed to take place. The results implicate leakage of plasma proteins from the underlying pulpal vessels. The proteins subsequently permeate the tubules, where they are either adsorbed to the tubule walls or physically trapped in such a way as to reduce dentin permeability.
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