Influence of phase purity on the in vivo response to hydroxyapatite

Two batches of calcium phosphate implants, termed HA1 and HA2 were prepared with calcium to phosphorus ratios of 1.67 and 1.70 respectively. XRD demonstrated that the HA1 implants were composed of phase pure hydroxyapatite, while the phase composition of the HA2 implants contained approximately 5% calcium oxide with the balance being hydroxyapatite. Sintered cylindrical implants, 3 mm in diameter, were prepared from each of the powders, containing slots I mm wide x 2 mm deep in one circular face. The implants were implanted laterally in the distal femur of 6 month old New Zealand white rabbits for a period of 3 weeks. Subsequent histological evaluation demonstrated that the presence of the CaO impurity phase significantly reduced the osteoconductivity, as demonstrated by minimal penetration of new bone within HA2 implant slots and a significant reduction in direct bone apposition on any of the HA2 implant surfaces as compared with HA1 implants. This study has confirmed the need for precise control of the phase purity of hydroxyapatite based bioceramics for optimal biological performance.