OSTEOGENIC RESPONSES TO EXTRASKELETALLY IMPLANTED SYNTHETIC POROUS CALCIUM-PHOSPHATE CERAMICS - AN EARLY-STAGE HISTOMORPHOLOGICAL STUDY IN DOGS

In this experiment, synthetic porous calcium phosphate ceramics (hydro xyapatite-tricalcium phosphate) were prepared and implanted in dorsal muscles of dogs. The purpose was to study the biological processes pri or to and during the morphogenesis of bone in extraskeletally implante d porous calcium phosphate ceramics. Specimens were harvested after im plantation for 7, 15, 30, 45, 60, 90 and 120 days. Decalcified and und ecalcified sections were prepared for alkaline phosphatase (ALP) histo chemical localization and comparative histological analysis. The resul ts show that bone morphogenesis in the pore regions of the extraskelet ally implanted ceramics follows a complex process involving clot forma tion, vascular invasion, granulation-like tissue formation, polymorphi c cell aggregation, osteoblast differentiation and bone formation. The characteristic feature preceding bone formation was polymorphic cell aggregation on the pore inner surface and near the invading capillarie s or small venules. These cells were of various sizes and shapes, and some of them were positive for ALP activity. ALP-positive cell aggrega tes were more numerous where capillaries or venules were close to the pore inner surface. Osteoblast differentiation occurred within the cel l clusters aggregated on the pore inner surface and bone matrix was se creted indirect contact with the ceramics. During bone formation, capi llaries or small venules were always found close to the developing fro nts of the osseous nidi. It is suggested that those cells which first appeared near the invading vasculature, the cells which aggregated on the pore inner surface and those cells which finally differentiated in to osteoblasts may be interrelated in some way.