SUBSTITUTION OF NATURAL CORAL BY CORTICAL BONE AND BONE-MARROW IN THERAT FEMUR .2. SEM, TEM, AND IN-SITU HYBRIDIZATION

Natural coral consisting of calciumcarbonate (CaCO3) in the crystal fo rm of aragonite was investigated after implantation into the cortex an d marrow cavity of rat femur at 7, 14, 21, and 28 days by means of sca nning and transmission electron microscopy (SEM and TEM) as well as in situ hybridization (ISH) in order to better understand its mechanism of bone bonding, which is somewhat different to that of other slowly d egradable bioactive materials, e.g. hydroxyapatite or glass ceramics. Bone bonding was shown to be closely coupled with implant degradation. Degradation by dissolution started as early as the insertion of the i mplant leading to a pronounced surface rugosity. Additionally, at late r stages, degradation by multi-nucleated osteoclast like cells was enh anced. Bone bonding was mediated by flat cells settling in groups on t he implant surface. These cells produced mineralizing globules and col lagen that anchored directly to the implant surface, i.e. on the tips of the surface rugosity and inside the pores. Through ISH the cells we re shown to produce procollagen al (I) transcripts. The calcification at the interface was enhanced by matrix vesicles similar to woven bone formation. Therefore, the calcification on the implant surface resemb led woven bone formation and no distinct afibrillar intervening layer resembling a cement-line, as in other bioactive implants or as in bone -bone interfaces, e.g. in lamellar bone was observed in bone bonding a reas. The mineralization in deeper micropores which did not include di rect cell activity depended on other processes, e.g. dissolution and r eprecipitation.