Osteointegration of calcium phosphate ceramics in humans and animals

Calcium phosphate ceramics are bioactive, osteoconductive materials used as guides for bone regeneration. We were able to carry out a histological study of ceramics implanted in large mammals and humans. An identical sequence of events apparently occurred at the contact of the ceramics. The osteoblasts differentiated preferentially in close proximity to the ceramic surface in animals. The early stage lasted longer in humans and was characterized by the presence of a monolayer of multinucleated cells containing TRAP+ (tartrate resistant acid phosphatase activity) granules at the surface of the ceramic. The loose connective tissue invading the pores contained almost exclusively fibroblast-like cells with TRAP+ nucleus. Differentiation of the osteogenic cells around the ceramic is diffuse in humans and not always related to the material surface. Ceramics become embedded in immature bone both in humans and animals. The immature bone and the ceramics are then remodeled and progressively replaced by layered bone. The degradation of the ceramic occurs preferentially at the grain boundaries and the grains released are phagocytosed by the macrophages when below a critical size and degraded in the low pH compartments of the cell. In conclusion: the integration of calcium phosphate ceramics is comparable in animals and man. Although in the cases studied the ceramics were unable to induce osteogenic differentiation, they did favor the differentiation of pre-determined osteogenic cells.