Surface replacement hip arthroplasty with the Freeman double cup.

The authors report their clinical experience with 43 hip arthroplasties using Freeman's double femoral cup. The patients were followed up from 3 to 6 years after operation. At that time, only 44% were totally free from pain, while migration of the femoral cup, which was severe and progressive, was observed radiographically in almost half the cases. In 3 cases it was possible to study histologically the stump of the neck of the femur and the zone of contact between femoral bone and cement. Biomechanical studies were also performed with photo-elastic models, using the technique of finite elements. These studies indicated overloading on contact between the edge of the femoral cup and the cortical bone (especially medially), but without any load over the whole of the spongy bone. Consequently there was bone resorption in the overload zone and osteoporosis in the spongy bone. The femoral cup rapidly became unstable, and this produced progressive migration, thus constituting a vicious circle.