A hip wear simulator for the evaluation of biomaterials in hip arthroplasty components

A multistation (8-station) hip simulator has been designed and tested that provides a practical imitation of the motions and loads seen by the hip joint during a typical walking cycle. A biaxial rocking motion of +/- 23 degrees is synchronized with the respective resultant forces of the extension-flexion (heel-strike to toe-off) movements of the leg. This particular simulator provides a practical engineering in vitro implementation of the walking cycle. It also provides a realistic and practical compromise between general wear screening devices (such as pin-on-disk systems) and the intensive research accomplished through full scale simulation (full 6 degree-of-freedom systems) and modeling. Evaluation of system performance shows that control of rpm (revolutions-per-minute) for the desired axial rotation of 1 Hz was kept to 60 cpm +/- 1 cpm for axial loads (per actuator) as high as 4500 Newtons. Although loading error was 2% in the peak load areas of interest (3000 Newton), station-to-station load control variability was less than .6%. Baseline wear studies with this simulator using ultra-high-molecular-weight polyethylene and Cobalt-Chromium (UHMWPE/CoCr) hip systems indicate an average specimen-to-specimen wear variability of less than 7% range after 5 million test cycles. Testing was performed in a calf serum environment at an equilibrium temperature of 33 degrees C.