The effect of traction on the hip in osteonecrosis. A comment on the "radiolucent crescent line".

Abstract Roentgenograms obtained during traction in 3 patients with necrosis of the femoral head confirm that a vacuum phenomenon is responsible for the clarity of the “radiolucent crescent line” or “rim sign” characteristic of this condition. This indicates the release of gas in the zone of subchondral separation with the induction of a vacuum by traction. This method of examination may be of value in the early diagnosis of osteonecrosis and in evaluating the cartilage and articular space in other conditions.