The calcium hydroxide is most commonly used material in dental practice for pulp capping.
The mechanism by which calcium hydroxide initiates the reparative process is unclear. It has been suggested that a rise in pH as a result of the free hydroxyl ions may initiate or favour mineralization (Tronstad et al. 1981).
calcium hydroxide may act as a local buffer against the acidic reactions produced by theinflammatory process (Heithersay 1975). An alkaline pH may also neutralize the lactic acid secreted by osteoclasts, and this may help to prevent further destruction of mineralized tissue.
It has been speculated that the material exerts a mitogenic and osteogenic effect, the high pH combined with the availability of calcium and hydroxyl ions having an effect on enzymatic pathways and hence mineralization (Torneck et al. 1983).
The high pH may also activate alkaline phosphatase activity which is postulated to play an important role in hard tissue formation (Guo & Messer 1976). The optimum pH for alkaline
phosphatase activity is 10.2 (Gordon et al. 1985), a level of alkalinity which is produced
by many calcium hydroxide preparations.
Heithersay (1975) suggested that calcium ions may reduce the permeability of new capillaries, so that less intercellular serum is produced, thus increasing the concentration of calcium ions at the mineralization site.
The presence of a high calcium concentration may also increase the activity of calcium dependent
pyrophosphatase, which represents an important part of the mineralization process.
The reduced capillary permeability following the increase in the number of calcium ions could reduce serum flow within the dental pulp, and consequently the concentration of the inhibitory pyrophosphate ion would be reduced.
This would coincide with an increase in levels of calcium-dependent pyrophosphatase as promulgated by Heithersay (1975), and would result in uncontrolled mineralization of the pulp tissue (Fig. 1). This could possibly explain the high incidence of mineralized canals observed following pulpotomy and direct pulp capping (Langeiand et al. 1971, Seltzer & Bender 1984)
SOME VARIATION in the way in which a dentine bridge is formed, depending on the pH of the material that is used to dress the tooth.
high pH material such as pulpdent
Necrotic zone is formed adjacent to the material, and the dentine bridge then forms between this layer and the underlying vital pulp. The necrotic tissue eventually degenerates and disappears, leaving a void between the capping material and the bridge.
lower pH, such as Dycal
The necrotic zone is similarly formed but is resorbed prior to the formation of the dentine bridge, which then comes to be formed directly against the capping material.
Dentine bridges formed by the high pH materials are histologically identical to those produced by lower pH materials, but are easier to distinguish on a radiograph because of the space between the bridge and the calcium hydroxide.
reference
International Endodontic Journal ,1990,23,283-297
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