- gical conditions of the articulation and hosts its effusions. Studies in cadaveric knees have shown the apex of the suprapatellar pouch to be the site most vulnerable to rupture. Rapid changes in the position of the knee cause instantaneous high intraarticular pressure, enhance the already elevated pressure of the previously accumulated synovial fluid, and seem to be responsible for the rupture of the pouch or extension into the thigh. This unusual condition has been described in a variety of diseases such as trauma, hemophilia and rheumatoid arthritis. The clinical features, apart from location, are similar to those of ` pseudothrombophlebitis,'' a syndrome caused by rupture or extension of a popliteal (Baker's) cyst into the calf, namely diffuse swelling and tenderness, local heat and erythema, severe pain aggravated by movements of the knee joint, and marked functional disability. The diagnosis is based on the chronological link between a previous knee effusion, partially alleviated after an acute episode of knee and thigh pain, and the sudden appearance of the already described inflammatory changes in the ipsilateral thigh shortly after this dramatic event [1±3]. It is confirmed by imaging techniques: arthrography now replaced by ultrasonography and magnetic resonance imaging ± both accurate methods for detecting and monitoring the course of the disease [4,5]. Differential diagnoses include deep or superficial vein thrombophlebitis, muscle rupture, infectious or tumor conditions.