OSTEOCLASTIC RESORPTION OF CA-P BIOMATERIALS IMPLANTED IN RABBIT BONE

The nature of the multinucleated cells involved in the resorption proc esses occurring inside macroporous calcium-phosphate biomaterials graf ted into rabbit bone was studied using light microscopy, histomorphome tric analysis, enzymatic detection of tartrate-resistant acid phosphat ase (TRAP) activity, scanning, and electron microscopy. Samples were t aken at days 7, 14, and 21 after implantation. As early as day 7, oste ogenesis and resorption were observed at the surface of the biomateria ls, inside the macropores. Resorption of both newly formed bone and ca lcium-phosphate biomaterials was associated with two types of multinuc leated cells. Giant multinucleated cells were found only at the surfac e of the biomaterials; they showed a large number of nuclei, were TRAP negative, developed no ruffled border, and contained numerous vacuole s with large accumulation of mineral crystals from the biomaterials. O steoclasts exhibited TRAP positivity and well-defined ruffled border. They were observed at the surface of both newly formed bone and biomat erials, around the implant, and inside the macropores. In contact with the biomaterials, infoldings of their ruffled border were observed be tween the mineral crystals, deeply inside the microporosity. The micro porosity of the biomaterials (i.e., the noncrystalline spaces inside t he biomaterials) increased underneath this type of cell as compared wi th underneath giant cells or to the depth of the biomaterials. These o bservations demonstrate that macroporous calcium-phosphate biomaterial s implanted in bone elicit osteogenesis and the recruitment of a doubl e multinucleated cell population having resorbing activity: giant mult inucleated cells that resorb biomaterials and osteoclasts that resorb newly formed bone and biomaterials.