Tribology of Prosthetic Joints - Validation of Wear Simulation Methods

In hip and knee arthroplasty, loosening of prosthetic components is the principal reason for expensive and complicated reoperations. Loosening is believed to be caused mainly by tissue reactions to wear particles removed from ultrahigh molecular weight polyethylene bearing surfaces. The tissue reactions, leading to bone loss, depend on the size and amount of the particles. Wear simulators are important in the evaluation of prosthetic joints and their materials. The validation of a simulation is based on the comparison of wear rates and wear mechanisms with clinical findings. Polyethylene wear particles from two different hip simulators, a pin-on-disk hip wear device, and a knee wear simulator, were examined using digital image analysis. The average equivalent circle diameter of the particles was 0.10-0.69 μm, which corresponded well with published clinical studies. The particle size distribution was influenced by crosslinking, counterface roughness, and type of simulation (hip or knee). The kinematics of the hip joint in walking, and of ten contemporary hip simulators was analyzed. To this end, software for computing tracks drawn on the counterface by arbitrary points of the bearing surface, the so-called slide tracks, was developed and experimentally verified. The slide track pattern, produced by many points, illustrated the relative motion. Walking was shown to result in open slide tracks on the center of contact, implying continually changing direction of sliding. This phenomenon, known to be crucial for valid wear simulation, was reproduced by simulators having abduction-adduction motion in addition to flexion-extension motion. The most widely used hip simulator, commonly referred to as biaxial, was found to be actually three-axial. The new method can be further utilized, e.g., for computing an improved wear factor involving the variation of relative motion and contact pressure with location.