Size of osteonecrosis of the knee.

In the Swedish Multicenter Study of Knee Arthroplasty, 115 knees in 110 patients with knee osteonecrosis represented all reported cases treated with knee arthroplasty in the period from 1975 to 1986. Primary medial femoral condyle osteonecrosis was the most common osteonecrosis reported (89%). Preoperative roentgenograms were staged according to the type of osteonecrosis and classified according to the grade of arthrosis. Detailed measurements of the size of the lesion were also performed. For primary medial femoral osteonecrosis, the width of the lesion averaged 45% of the condyle. The mean width of the lesion (anteroposterior size) was 21 mm and the mean length (lateral size) was 34 mm. The mean depth on lateral roentgenograms was 6 mm. There was a high correlation of results among the various methods of measurement. Using regression analysis, it was possible to approximate the volume of the lesion even from a single measurement of the width on anteroposterior view. The size of osteonecrosis of the knee can be used to plan resection and select the implant design.