[Ultrastructural study on the developmental process of the dentin bridge following direct capping using hydroxyapatite ceramic].

Hydroxyapatite ceramics (HAP), which have been used in the endodontics or periodontics because of their bioafinitive characteristics, were applied directly as the capping of the wound surface after the experimental exposure of pulp in versional teeth to be extracted on orthodontics ground. In order to investigate the developmental process of dentine bridge induced by HAP capping, the pulp was studied ultrastructurally, 1, 3, 6 months after the treatment. One month after the capping, the multi-nuclear giant cells being rich in mitochondria and lysosomes were in contact with the HAP granules directly by extending their projections into the granules, which appeared to absorb the HAP granules. At the same time, the osteoblast-like cells with remarked rough endoplasmic reticulum (rER) were also observed just behind the giant cells. Only a few matrix vesicles and fibers were recognized around these osteoblastic cells. The HAP granules embeded in the pulp for 3 months were surrounded by fine matrix fibers which seemed to be produced by adjacent osteoblastic cells. The surrounding of these cells were filled with abundant matrix vesicles and collagen fibers, which would show the production of osteodentin matrix and the minerilization were active in this region. Six months after the treatment, the osteodentin matrix in upper dentin bridge became compact; matrix fiber surrounding the HAP granules became thick and a part of fibers inserted into the granules, which should mean that the HAP granules had bonded to the osteodentin matrix. In the deeper region of the dentin bridge, tubular dentin was formed newly. Both osteodentin and tubular dentin fused tightly without any organelles in the boader between them. The covering of hard tissue of dentin bridge then bonded to primary and reparative dentin closed the wound surface to protect the inner pulp.