A radiolucent lesion in the medial femoral condyle was observed in 40 knees in 39 patients over age 60. Mast patients had had spontaneous onset of severe knee pain. Twelve knees were radiographically normal within 2 months following onset of pain. Seven knees proceeded to osteoarthritis whereas others became rather asymptomatic. Strontium-85 scintimetry of symptomatic knees showed N THE COURSE of prospective studies I of osteoarthritis of the knee we became aware of a peculiar radiolucent lesion of subchondral bone in the medial femoral condyle. Radiographically, this lesion differed from osteochondritis dissecans in appearance and location. It afEicted aged rather than adolescent individuals and was associated with spontaneous pain in the knee. This particular lesion has received only minimal attention; previous descrip-

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Summary. The distribution and excretion of Sr90and Ca45was studied at varying intervals after simultaneous intraperitoneal injection of the isotopes to young rats. Sr90was retained to a lesser extent than Ca45owing to the preferential excretion (or failure of reabsorption) of the former element by the kidneys as well as by the gastrointestinal tract. With regard to skeletal accretion and exchange there was no marked difference between the two elements. The importance of the accretion process for the fixation of the isotopes in the skeleton is emphasized. The computation of the rate of Ca accretion in the whole skeleton is described.