In vivo measurements of relative motion between an uncemented femoral total hip component and the femur by roentgen stereophotogrammetric analysis.

Roentgen stereophotogrammetric analysis (RSA) was used to study rotation and subsidence of an uncemented polyethylene-coated femoral component for total hip arthroplasty in ten patients younger than 55 years of age. Three years postoperatively, the total subsidence averaged 2.44 mm and the internal rotation was 7.4 degrees. The rate of subsidence decreased between the observation periods, whereas the rate of rotation, after an initial significant decrease, became almost constant. One patient, who was a clinical failure and required revision arthroplasty, had a significantly higher rate of migration after six months. The pattern of subsidence in the absence of pain might be the normal reaction of bone to an uncemented prosthesis, but the continuing rotation suggests the potential for failure of the arthroplasty. The type of analysis made possible in RSA may allow early prediction of the success of arthroplasties and significantly speed the design cycle for arthroplasty components.