Baker's cyst
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A Baker's cyst, otherwise known as a popliteal cyst, is a benign swelling found over the knee joint. It is named after the surgeon who first described it, Dr. William Morrant Baker (1839-1896).
The synovial sack of the knee joint can, under certain circumstances, produce a posterior bulge, into the popliteal space, the space behind the knee. When this bulge becomes large enough, it becomes palpable and cystic. Most Baker's cysts maintain this direct communication with the synovial cavity of the knee, but sometimes, the new cyst pinches off. A Baker's cyst can rupture and produce acute pain behind the knee and in the calf and swelling of the calf muscles. It can be confused with a deep vein thrombosis. An ultrasound examination of the calf can easily distinguish between a ruptured Baker's cyst and a DVT.
Baker's cysts usually arise from arthritic knee joints. Almost any form of knee arthritis can cause it.