Aseptic loosening of hip prostheses. A histologic and enzyme histochemical study.

Twenty-one total hip arthroplasties were revised because of aseptic loosening. The bone and soft tissue membrane bordering the cement in a total of 37 biopsies from the acetabulum and femur were studied histologically and by enzyme histochemistry. The soft tissue membrane was infiltrated with macrophages and had high acid phosphatase activity. Other inflammatory cells were rare or absent. Concomitant bone formation and bone resorption were usually present at the bone-soft tissue border. The bone generally consisted of viable, remodeled lamellar bone. The host bone seemed to have been resorbed by a soft tissue front moving outward from the cement. Reactive new bone formation began at the bone-soft tissue border and appeared to constrain the resorptive process. Articular wear seemed to play no part in initiating the loosening process. The role of bone cement in this respect is obscure, because there were no obvious local signs of cytotoxicity. These observations support the theory of mechanical instability as the primary cause of the tissue reaction.