Klaas de Groot

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 AlOH 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.

Calcium phosphate bioceramics are widely used in orthopedic and dental applications and porous scaffolds made of them are serious candidates in the field of bone tissue engineering. They have superior properties for the stimulation of bone formation and bone bonding, both related to the specific interactions of their surface with the extracellular fluids and cells, ie, ionic exchanges, superficial molecular rearrangement and cellular activity.

The combination of the high mechanical strength of metals with the osteoconductive properties of calcium phosphates make hydroxyapatite coatings on titanium implants widely used in orthopedic surgery. However, the most popular coating method, plasma spraying, exhibits some important drawbacks: the inability to cover porous implants and to incorporate biologically active agents, delamination, and particle release. The aim of this study was to elaborate a dense, strong, and thick calcium‐phosphate coating on titanium and porous‐tantalum implants using a two‐step biomimetic procedure. In the first step, the implants were soaked in a solution that was 5 times more concentrated than regular simulated body fluid (SBF‐A solution). A thin but uniform amorphous calcium‐phosphate coating was deposited on the metal. Then, the implants were immersed in the SBF‐B solution, which had a similar composition as the SBF‐A solution, but with decreased contents of crystal growth inhibitors (i.e., Mg 2+ and HCO 3 − ). This resulted in the fast precipitation of a 30 μm thick crystalline calcium‐phosphate coating. The pH of the SBF‐B solution and the thickness of the crystalline coating layer were studied as a function of time. The Fourier transform infrared spectra and X‐ray diffraction patterns showed that this new coating closely resembles bone mineral. Our biomimetic coating should facilitate rapid bone formation around the implant, reducing therewith the patient's recovery time after surgery.

CRC Press, Inc., Boca Raton, FL, 1983, ISBN 0–8493-6456–6, 146 pp., $49.00 (U.S.), S56.00 (outside U.S.)