Stiffness of bipolar hip prostheses

Following the removal of a Bateman bipolar hip arthroplasty for displaced intracapsular fracture, the inner bearing was found to be stiff. After dismantling the acetabular component, high density polyethylene (HDP) debris was found in the inner bearing in association with wear and distortion of the HDP and fibrous tissue. To explore the hypothesis that movement does not occur at the inner bearing of bipolar prostheses because of absent lubrication, a mechanical model of the hip joint was devised incorporating a Hastings bipolar arthroplasty. The dynamic frictional forces at the inner bearing were found not to be significantly reduced when the joint was lubricated with either synovial or serosanguinous fluid. It is suggested that if the initial range of movement of the inner bearing is reduced because of debris, then loads are concentrated by the femoral component on a smaller area of the acetabular HDP component. This leads to increased shear forces, creep and distortion of the HDP, which further contributes to inner bearing stiffness.