A calcified barrier may be induced when calcium hydroxide is used as a pulp-capping agent or placed in the root canal in contact with healthy pulpal or periodontal tissue. Because of the high pH of the material, up to 12.5, a superficial layer of necrosis occurs in the pulp to a depth of up to 2 mm. Beyond this layer only a mild inflammatory response is seen, and providing the operating field was kept free of bacteria when the material was placed, a hard tissue barrier may be formed. However, the calcium ions that form the barrier are derived entirely from the bloodstream and not from the calcium hydroxide.
The hydroxyl group is considered to be the most important component of calcium hydroxide as it provides an alkaline environment which encourages repair and active calcification. The alkaline pH induced not only neutralises lactic acid from the osteoclasts, thus preventing a dissolution of the mineral components of dentine, but could also activate alkaline phosphatases which play an important role in hard tissue formation.
The calcified material which is produced appears to be the product of both odontoblasts and connective tissue cells and may be
termed osteodentine. The barrier, which is composed of osteodentine, is not always complete and is porous. In external resorption, the cementum layer is lost from a portion of the root surface, which allows communication through the dentinal tubules between the root canal and the periodontal tissues. It has been shown that the disassociation coefficient of calcium hydroxide of 0.17 permits a slow, controlled release of both calcium and hydroxyl ions which can diffuse through dentinal tubules. Tronstad et al. demonstrated that untreated teeth with pulpal necrosis had a pH of 6.0 to 7.4 in the pulp dentine and periodontal ligament, whereas, after calcium hydroxide had been placed in the canals, the teeth showed a pH range in the peripheral dentine of 7.4 to 9.6.
The hydroxyl group is considered to be the most important component of calcium hydroxide as it provides an alkaline environment which encourages repair and active calcification. The alkaline pH induced not only neutralises lactic acid from the osteoclasts, thus preventing a dissolution of the mineral components of dentine, but could also activate alkaline phosphatases which play an important role in hard tissue formation.
The calcified material which is produced appears to be the product of both odontoblasts and connective tissue cells and may be
termed osteodentine. The barrier, which is composed of osteodentine, is not always complete and is porous. In external resorption, the cementum layer is lost from a portion of the root surface, which allows communication through the dentinal tubules between the root canal and the periodontal tissues. It has been shown that the disassociation coefficient of calcium hydroxide of 0.17 permits a slow, controlled release of both calcium and hydroxyl ions which can diffuse through dentinal tubules. Tronstad et al. demonstrated that untreated teeth with pulpal necrosis had a pH of 6.0 to 7.4 in the pulp dentine and periodontal ligament, whereas, after calcium hydroxide had been placed in the canals, the teeth showed a pH range in the peripheral dentine of 7.4 to 9.6.
Tronstad et al. suggest that calcium hydroxide may have other actions; these include, for example, arresting inflammatory root resorption and stimulation of healing.
It also has a bactericidal effect and will denature proteins found in the root canal, thereby making them less toxic. Finally, calcium ions are an integral part of the immunological reaction and may activate the calcium-dependent adenosine triphosphatase reaction associated with hard tissue formation.