Answer:

Swelling of the knee capsule extending posteriorly to the popliteal fossa (hence the synonym popliteal cyst). Baker’s cyst is thought to develop as the knee is flexed, producing a significant rise in intra-articular pressure and an outpouching of the synovium posteriorly. The cruciate ligaments may act as a one-way valve, making it hard for the fluid to resorb. Posterior rupture may lead to swelling of the leg below the knee. When rupture occurs, a crescentic hematoma may form beneath one of the malleoli.

Reference: Kraag G, et al: The hemorrhagic crescent sign of acute synovial rupture [letter]. Ann Intern Med 85:477, 1976.

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