Instability and wear of total hip prostheses determined with roentgen stereophotogrammetry.

A roentgen stereophotogrammetric analysis of hip prostheses was made in four patients with rheumatoid arthritis during a postoperative period of 2 years. Implanted tantalum balls, prosthetic femoral head, and ends of the wire in the acetabular socket were used as measurement points. The migration of the prosthetic head and the acetabular socket in relation to the pelvic bone was determined. The difference between the cranial migration of the head and the socket is a measure of the postoperative instability of the hip joint and deformation of the prosthetic components. This deformation occurs in the plastic acetabular socket and is mainly due to wear. The roentgen stereophotogrammetric method can be applied to standard hip orostheses with a metal femoral head and a plastic or metal acetabular socket to study migration or loosening of the prosthesis in relation to the supporting bone, prosthesis instability, and deformation of the prosthetic components.