[*primary osteonecrosis of the medial femoral condyle. Unicompartmental or total replacement?].

We have inserted 18 Mod II unicondylar knee replacements for idiopathic necrosis of the medial femoral condyle. The age of the patients at operation averaged 72 years, with a range of 49 to 90. The average length of follow up at review was 4 years 6 months, with a range of 2 to 8 years. The only notable complication was deterioration of the lateral compartment if operation had produced overcorrection of the tibiofemoral angle. The stability, level of pain, range of movement and function was satisfactory in 95% of the knees. The average flexion obtained was 115 degrees, and was deliberately restricted to this level to diminish stress on the lateral side. It did not result in aseptic loosening. Most knee flexion deformities were corrected at operation. Laxity and instability before operation were usually due to bone loss rather than ligament insufficiency, and were corrected by the prosthesis. A varus deformity of more than 10 degrees, a flexion deformity greater than 15 degrees or femorotibial subluxation were not considered to be contraindications to this operation.