THE ROLE OF HYDRATED SILICA, TITANIA, AND ALUMINA IN INDUCING APATITE ON IMPLANTS

Pure soluble silica prepared by a sol-gel method induced bone-like hydroxyapatite formation onto its surface when the silica was immersed in a simulated body fluid (SBF), whereas silica glass and quartz did not. This finding directly supports the hypothesis that hydrated silica plays an important role in biologically active hydroxyapatite formation on the surfaces of bioactive glasses and glass-ceramics, which leads to bone-bonding. Gel-derived titania is also a hydroxyapatite inducer because of its abundant TiOH groups. These results provide further insight into the unique osseointegration of titanium and its alloys. It is suspected that gel-derived titania develops an apatite layer by taking calcium and phosphate from the body fluid, thus producing bone-bonding. Although sufficient A1OH groups may remain in the alumina gel, they do not serve to initiate apatite generation when immersed in SBF. This phenomenon explains the fact that an intermediate fibrous tissue is usually found to separate the alumina implant from bone. One may infer that both abundant OH groups and negatively charged surfaces of gel-derived silica and titania are important for hydroxyapatite induction. Material which possesses and/or develops both a negatively charged surface and abundant OH groups in a physiologically-related fluid is most likely to be an efficient apatite inducer. Such materials are suitable candidates to serve as bone-bonding biomaterials. (C) 1994 John Wiley and Sons, Inc.