Bioactive ceramics are known, which can bind bone tissue chemically. The authors tested bone-bonding strength of biomaterials using detaching test and observed the interface between bone and bioactive ceramics with transmission electron microscopy. An intervening apatite layer was observed at the interface of bone and bioactive ceramics. This layer was distinguished from bone apatite or ceramic. This apatite layer was formed within several days after implantation before bone was observed on the materials. Bisphosphonate is well known to inhibit apatite formation. The injection of bisphosphonate to rabbits concentration-dependently decreased bone-bonding strength of ceramics. The apatite layer was formed on bioactive ceramics in vitro by immersing them in simulated body fluid that contained similar concentrations of inorganic ions as plasma did. Using this apatite layer formed in vitro, it is possible to characterize the apatite layer. This apatite layer enhanced the differentiation of rat bone marrow cells to bone cells in vitro. When osteoclasts were cultured on this layer, they absorbed the apatite layer. These results suggested this apatite layer not only plays a key role for bone bonding but also behaved as bone-like tissues.
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