Enhanced in vivo response to silicate-substituted hydroxyapatite

This paper describes an investigation into the effect of silicate ion substitution in hydroxyapatite on the rate of bone healing in vivo. Slotted cylinders composed of stoichiometric hydroxyapatite and 1.2 wt% silicate substituted hydroxyapatite (S-HA) were implanted in the distal end of the femur of 6 month old New Zealand white rabbits for a period of 3 weeks. Tetracycline and alizarine red fluorochrome labels were administered 7 and 14 days post-operatively, respectively. After retrieval the implants were sectioned and examined using optical and fluorescence microscopy. Optical microscopy demonstrated that significant amounts of bone formation was only consistently found at the center of the machined slots of the silicate substituted implants. Moreover, fluorescence microscopy demonstrated that of the new bone laid down around and in contact with the implants, significant quantities of tetracycline labeled bone (i.e. bone laid down seven days after implantation) was only present in S-HA implants. These results indicate that the bioactivity of hydroxyapatite is significantly enhanced by the incorporation of silicate ions into its lattice.