Component motion in bipolar cemented hemiarthroplasty.

A total of 17 patients with a cemented bipolar Hastings hemiarthroplasty were examined clinically and radiologically between 1 and 3 years after being treated for a displaced femoral neck fracture. Only a minor part of the motion in the Hastings prostheses took place between the metal femoral head and the polyethylene liner (intraprosthetic). In seven cases, no "true" intraprosthetic motion weightbearing was observed (including five cases with intraprosthetic motion caused by impingement). The relative amount of intraprosthetic motion did not decrease with weightbearing. The functional result was not correlated to the relative amount of intraprosthetic motion with weightbearing. In conclusion, the short-term success of the bipolar prostheses is probably not accomplished by a reduction in extraprosthetic motion caused by the biarticular design but by other factors, including the optimal indications for hemiarthroplasty (high age, low activity level), the consistent use of acrylic bone cement, and the shock-absorbing effect of the polyethylene liner.