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 (hydroxyapatite–tricalcium phosphate) were prepared and implanted in dorsal muscles of dogs. The purpose was to study the biological processes prior to and during the morphogenesis of bone in extraskeletally implanted porous calcium phosphate ceramics. Specimens were harvested after implantation for 7, 15, 30, 45, 60, 90 and 120 days. Decalcified and undecalcified sections were prepared for alkaline phosphatase (ALP) histochemical localization and comparative histological analysis. The results show that bone morphogenesis in the pore regions of the extraskeletally implanted ceramics follows a complex process involving clot formation, vascular invasion, granulation-like tissue formation, polymorphic 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 capillaries or small venules. These cells were of various sizes and shapes, and some of them were positive for ALP activity. ALP-positive cell aggregates were more numerous where capillaries or venules were close to the pore inner surface. Osteoblast differentiation occurred within the cell clusters aggregated on the pore inner surface and bone matrix was secreted in direct contact with the ceramics. During bone formation, capillaries or small venules were always found close to the developing fronts 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 into osteoblasts may be interrelated in some way.