Reactions of the Dentin-Pulp Complex to Calcium Hydroxide Paste in Rats

In this study, we performed animal experiment using calcium hydroxide paste, a root canal filling material (Vitapex (R), Neo Dental Chemical Products Co., Tokyo), on dentin-pulp complex and observed several tissue reactions. While rats were under general anesthesia, pulp exposure was done by drilling a cavity on the occlusal part of the maxillary molar using 1/2 round bur. Thereafter, calcium hydroxide paste was injected into the cavity, temporarily sealed with composite resin and photographed using m_CT. After 4 weeks, the experimental part was surgically excised as a whole and examined histologically. A thick tertiary dentin was formed in the area where iodoform calcium hydroxide paste was directly applied. The newly formed hard tissue is composed of extremely irregular dentinal tubules. Although only few samples confirmed the formation of the so-called 'dentin bridge', the hard tissue could not be absolutely classified as a 'reparative dentin' but it can be recognized as a hard tissue connecting the dentin walls as it filled or covered the exposed pulp. The tertiary dentin formed underneath was thick. No necrotic layer was observed. Photographs of m_CT of the experimental side showed that the hard tissue formed in the root canal was radiopaque. However in the part of root canal where no hard tissue was formed, a radiolucent image was observed in the center. In this regard, a thick tertiary dentin was formed rather than a dentin bridge filling the gap. Since this is uncommon for calcium hydroxide, it is regarded as a distinct characteristic of Vitapex when applied to an exposed pulp. This phenomenon was thought to be due to the silicone oil component. The results suggest that the silicone oil reduced the alkalinity of the material thereby making it more amiable as a pulp capping agent. Furthermore, it is believed that the odontoblasts were promptly activated which led to the formation of large amounts of dentin.