Surface replacement of the hip. Experimental studies.

Surface replacement of the hip has been performed in 25 rabbits. Twenty animals were chosen for evaluation of the result of the operation after an average period of five months postoperatively. Tetracycline injections were given for fluorochrome labeling and radiographic controls have been made in vivo to check the position of the cup before the animals were sacrificed. Histological examinations were made from slides, which were gained after the metal cup had been removed with the bone cement still in place. It was found that the bone underneath the cup survived the surface replacement and that there was a normal metabolism further on. The trabeculae even grew into the surface structures of the inner layer of the bone cement and there was no interposition of membranes of fibrous tissue. It could be demonstrated that fibrous tissue between bone and bone cement is found only in cases of loosening of the cup and the cement or in cases of insufficient contact between this interface from the moment of the operation on. This is possible when air bubbles arise in the bone cement and get in contact with the trabeculae of the femoral head. Fluorochrome labeling proved the formation of new bone underneath the cup and the survival of the osseous structures of the coxal end of the femur. We believe, that in clinical cases it is necessary to remove all predamaged cancellous bone from the head of the femur (i.e. in cases of avascular necrosis) before the cup is attached. Close contact between the bone cement and the healthy spongiosa is one of the conditions for success in surface replacement of the hip.