The clinical relevance of hip joint simulator testing: In vitro and in vivo comparisons

Hip joint simulators are widely used for evaluation of new bearing designs and materials. Although test methodology has been refined and accuracy improved over time, establishing clinical relevance remains the ultimate goal of this testing. Three criteria were set to establish clinical validation; simulator wear rates, qualitative and quantitative wear rate rankings and wear debris morphology. All must duplicate those reported clinically. Several material models with known and published clinical performance can be used to validate any hip simulator and test methodology. These include polytetrafluoroethylene (PTFE) versus ultra-high molecular weight polyethylene (UHMWPE) (or the Charnley model), Hylamer® versus conventional UHMWPE and ethylene oxide (EtO) or gas plasma sterilized UHMWPE versus gamma-irradiated in air UHMWPE. Studies on these materials are retrospective, however, a fourth model involving Duration® stabilized UHMWPE versus gamma-irradiated in air UHMWPE was used prospectively. Generic or commercially available acetabular liners of these materials were fabricated or obtained and tested in a multi station hip simulator under physiologic loading, cross-path motion, anatomical positioning and biological lubrication conditions. All liners were articulated with CoCr femoral heads (Ra = 0.02 μm). Literature reported clinical wear rates and wear rate ratios were precisely reproduced for the retrospective models. Methodology was found to have significant influence. Clinical performance of the fourth material model was first predicted and then corroborated.