Bone implants--a challenge to materials science.

The review paper discusses the conditions for obtaining in vivo a composite from a synthetic, inorganic material and collagen fibres. Bone itself is a composite containing collagen fibres and hydroxoapatite crystals. The crystal size is, however, far smaller than can be made using conventional methods of ceramics technology. The paper discusses therefore the possibilities to synthesize the apatite in a gel. The requirements of initial load bearing can be met by forming the gel on the surface of glass particle, sintered to a porous body. In the pores calcium ions react with the gel to form (= SiO)Ca+ complexes. When the phosphate concentration in the pore is high enough to exceed the solubility product of apatite, the calcium-gel complexes release the calcium and highly dispersed apatite crystallites are precipitated in the gel. These give the gel the osteoconductive properties observed for bioactive glasses. Glass compositions, which give a gel surface enough hydrated to form calcium complexes, are discussed.