Biologic response to hydroxylapatite-coated titanium hips. A preliminary study in dogs

Hydroxylapatite (HA)-coated and uncoated Ti-6A1-4V alloy femoral endoprostheses were evaluated in adult dogs. The femoral stems had proximal anterior, posterior, and medial pockets of either a commercially pure titanium porous coating or a grooved macrotexture. They also had a medial collar, with an inferior surface pocket of either the porous coating or the grooved macrotexture. HA-coated and uncoated specimens of each type were evaluated. The devices were placed as unilateral hemiarthroplasties in 12 dogs and remained in function for up to 52 weeks. Histologic sections from the uncoated grooved implants showed no direct bone-implant apposition in the proximal regions after up to 10 weeks; the HA-coated grooved implants demonstrated extensive direct bone-coating apposition after 5 weeks. Sections from uncoated porous implants evaluated after 10 weeks demonstrated approximately equivalent in-growth to those sections from the HA-coated devices after 6 weeks. All HA-coated implants demonstrated consistent bone-implant apposition with no fibrous tissue interposition. The HA-coated surfaces were associated with increased bone deposition and proliferation at early implantation periods. In no histologic section examined was there any evidence of deterioration of the HA coating, nor was any separation of the coating from the substrate material observed.