Tissue reaction of hydroxyapatite sol to rat molar pulp

Hydroxyapatite sol was prepared by dispersing unheated hydroxyapatite microcrystals into distilled water. A suspension of hydroxyapatite heated at 800 degrees C (HAp-800 degrees C) in distilled water was used as a comparative material. These were characterized by X-ray diffractometry, SEM and electroconductivity measurements. 0.001 ml of each material was injected into the upper first molar pulp of rats. After 7-14 days post-operatively, the group with hydroxyapatite sol showed slight inflammation in the pulp cavities and a highly calcified tubular dentine bridge was observed. Osteodentine had developed and adhered to the surface of the hydroxyapatite microcrystals. Matrix vesicles were observed in the extracellular matrix between collagen fibrils and electron-dense granules. After 28 days, the inflammation completely disappeared and tubular dentine was observed below the osteodentine. Acid phosphates (ACPase) activity was detected along all biosynthesizing pathways in odontoblast-like cells. Localized dehydrogenate (LDH) activity indicated phagocytosis of hydroxyapatite microcrystals by odontoblasts. In the group with HAp-800 degrees C after 7-14 days post-operatively, the inflammation was moderate. After 28 days, relatively fewer calcified dentine tubules were observed. ACPase activity was detected only along the membranes bounding cytoplasmatic vacuoles. It was concluded that hydroxyapatite sol is a safe biomaterial for use as dental pulp and induces early formation of dentine-bridge more so than HAp-800 degrees C.