Failure of one design of surface replacement knee arthroplasty due to loosening deformation and wear of the plastic femoral component.

Twenty four knee prosthesis femoral and tibial components of the 'load angle inlay' design, removed for loosening and pain were examined in the scanning electron microscope, light microscope, and volume loss measured using a simple gauge. In all cases the deformation of the plastic (femoral) component was seen both by increase in the curvature of the inner surface and wear usually on the edge of the bearing surface. Two pairs were subluxed prior to removal from the patient and these not only produced gross deformation of the plastic components, but maximum volume loss of the components examined; the knees exhibited gross instability. The tibial (metal) components stood up remarkably well in all cases, both in wear and deformation which was minimal but sinking and rotation of the plateau did occur for the most part on the external side. Scratch patterns seen on the tibial components were clearly seen and gave an indication of the direction and amount of sliding between the components; in some cases these scratch lines were in more than one direction probably indicative of loosening and instability. Short deep scratches were usually indicative of bone and/or cement particles embedded in the plastic components, abrasive wear was seen on 92 per cent of the femoral components, and cracks were seen on two-thirds, usually parallel and close to the sides.