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A Baker cyst, also known as a popliteal cyst, is a fairly common disorder in preadolescents and adolescents. It is a benign, soft and "squishy" swelling in the bend behind the knee - the "popliteal fossa."
The cyst forms as a collection of joint fluid in a pocket formed by distended knee joint lining tissue called synovium or synovial membranes of the knee joint or the tendon sheaths.
These cysts are not associated with any sort of other disease of the knee joint, and x-rays are normal. An ultrasound test can confirm that it is a hollow fluid-filled cavity, or the cyst can be tapped with a needle (aspirated), again to confirm what it is.
Ordinarily we just watch these cysts. Over a period of up to a couple of years, they usually disappear on their own. Orthopedic surgeons are reluctant to operate on them for this reason, but if the cyst progressively enlarges or causes discomfort it can be removed surgically.