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Thirty-two patients in whom Legg-Perthes disease apparently involved only one hip were examined with venography, measurement of intraosseous and intra-articular pressures, arthrography, and dynamic triphasic bone-imaging with 99mTc methylene diphosphonate. The arterial flow of blood in the affected femoral head was slightly decreased, but the difference from that on the normal side was not statistically significant. However, there was marked disturbance of the venous drainage in the diseased hip, elevated intraosseous pressure in the affected femoral neck, and increased intra-articular pressure in the involved hip compared with the normal side. An animal model was then created in twenty immature dogs, venous drainage was obstructed, and intraosseous pressure of the femoral head and neck was elevated by injection of four milliliters of semiliquid silicone into the femoral neck. In eleven of the dogs, areas of avascular necrosis resembling those associated with Legg-Perthes disease developed in the femoral head.