The structure of dentin is an arrangement of microscopic ducts, called tubules dentin, that radiate outward from the pulp chamber to the outer border or outer enamel.[79] The diameter of the enteric tubules is largest near the pulp (about 2.5 µm) and smallest (about 900 nm) at the junction of dentin and enamel.[80] The caries process continues through the odontogenic tubules, which are responsible for the triangular patterns resulting from the development of caries deep in the tooth. The tubes also allow tooth decay to progress faster.
In response, the fluid inside the tubules brings immunoglobulins from the immune system to fight the bacterial infection. At the same time, there is an increase in mineralization of the surrounding tubes.[81] This causes the tubes to constrict, which is an attempt to slow the progression of the bacteria. In addition, as the bacterial acid removes the hydroxyapatite crystals, calcium and phosphorous are released, allowing more crystals to fall deeper into the tubule. These crystals form a barrier and slow down the progression of caries. After these protective responses, the dentin is considered hardened.
According to the hydrodynamic theory, fluids within the dental tubules are thought to be the mechanism by which pain receptors are activated within the pulp of the tooth. [82] Because sclerotic dentin blocks the passage of these fluids, pain that may be a warning of invading bacteria may not develop at first. As a result, tooth decay may develop for a long time without any tooth sensitivity, allowing for a greater loss of tooth structure.[citation needed]
