Knee arthroplasty for spontaneous osteonecrosis of the knee: unicompartimental vs bicompartimental knee arthroplasty

Spontaneous osteonecrosis of the knee (SON) is an osteonecrosis that mainly affects the medial femoral condyle. In endstage SON, knee arthroplasty is the therapy of choice. Because of the unicompartimental nature of the knee, unicondylar knee arthroplasty is considered an ideal implant for treatment of this condition. The purpose of this study was to prove that the long-term results of unicondylar implants are better than the results of bicondylar implants for the treatment of SON. All patients treated for SON between 1984 and 2000 have been recorded. Two groups were formed according to the implant used. In all patients the preoperative radiograph was analyzed according to stage and size of the osteonecrotic lesion and the osteoarthritic changes. Postoperatively, the Knee Society Score and the radiograph were recorded. Thirty-nine patients were included in this study, of which 23 patients were treated by a unicondylar implant and 16 by a bicondylar implant. On a short-term basis, unicondylar implants had better clinical results; however, on a long-term basis bicondylar implants were better. In comparison, only unicondylar implants had to be revised. Radiolucency lines were mainly observed in patients with unicondylar impants and large areas of osteonecrosis. Our long-term results suggest that patients with SON are better treated by bicondylar implants. The reasons for the higher failure rate for unicondylar implants are poor bone stock and secondary arthritic changes.