A hydroxyapatite graft substitute reduces subsidence in a femoral impaction grafting model

Synthetic bone graft substitutes have been developed for impaction grafting revision hip surgery in response to concerns about the availability and potential infection risk of allograft. We performed an in vitro experiment to determine whether a synthetic porous hydroxyapatite material might improve resistance to cumulative subsidence and cyclic (elastic) movement on loading. We impacted different ratios of hydroxyapatite and allograft (0%, 50%, 70%, and 90% hydroxyapatite) into composite femoral models (eight per group) and implanted tapered, polished stems with cement. Models were loaded for 18,000 cycles on a servohydraulic materials testing machine. Migration of the femoral prostheses and the distal cement was determined using radiostereometric analysis, and cyclic movement was measured mechanically by the testing machine. Hydroxyapatite decreased overall subsidence and subsidence at the prosthesis-cement interface, but there was no difference at the cement-femur interface. This decrease may have occurred because the hydroxyapatite-containing graft was more rigid, or because it resulted in a thicker cement mantle. Cyclic prosthesis movement also was lower with hydroxyapatite. While the biologic response to porous hydroxyapatite remains to be determined, its mechanical properties appear promising.