Bone attachment to hydroxyapatite coated polymers

Bone attachment to two classes of hydroxyapatite (HA) coated polymers was evaluated mechanically and histologically. Particulate HA was molded into the surface of thermoplastic implants and cast into the surface of thermoset implants. Coated and uncoated implants of both types were implanted in the distal femurs of rabbits for four and twelve weeks. After sacrifice, the bone/implant interface was evaluated mechanically via a push-out test with a servohydraulic test system. Paired statistical analysis revealed significantly greater shear strengths for coated vs. uncoated implants for both polymer systems at both time periods. Mixed mode of failure occurred with particle removal from the polymer surface and from the bone. One animal at each time period was saved for histology. Histology at both time periods indicated direct bone apposition to the HA coating as compared to a fibrous encapsulation about the uncoated implants. This was confirmed with backscatter scanning electron microscopy and scanning acoustic microscopy. Hydroxyapatite coatings significantly improve the bond strength between polymers and bone by allowing direct bone apposition and some mechanical interlocking with the bone.