Temporal and spatial distributions of directional counterface motion at the acetabular bearing surface in total hip arthroplasty.

The motions of counterface articulation against the bearing surface of the acetabular liner strongly influence polyethylene wear debris production in contemporary total hip arthroplasty. However, the available body of relevant articular force and motion information is largely confined to resultant load excursions measured relative to instrumented femoral components, and/or to global angular motions (flexion, adduction, endorotation) of the joint. Analytical frameworks are here developed to transform such information into temporal and spatial variations of the resultant load and of the local counterface sliding velocity relative to an ordered set of discrete locations (e.g., finite element nodes) on the acetabular bearing surface. Whole-duty-cycle time histories of acetabular resultant load and counterface velocity distributions are presented for two important practical situations: human level walking gait, and a 23 degrees biaxial rocking hip simulation machine. The local counterface motions occurring in the simulator are characterized by higher velocities, smoother motion patterns, and wider directional variation than those occurring in human gait.