A Baker's cyst or popliteal cyst is a bulge at the back of the knee that is filled with fluid and makes fully extending or flexing the knee difficult. The cyst may also cause pain when the leg is in use or when one is standing for a long period. Baker's cysts develop because of arthritis or cartilage tears. They can also be caused by trauma or injury to the knee, which is surrounded by lubricating synovial fluid that keeps the knee moving smoothly and prevents friction, much like the oil in a car engine. Injury or trauma causes the knee to produce too much synovial fluid. The excess is stored in the popliteal bursa behind the knee. [symptom-checker]Treatment for Baker's cysts typically involves treating the underlying condition and allowing the cyst to heal on its own. A Baker's cyst is uncomfortable and may interfere with movement, but it is not particularly dangerous. If it becomes too large and significantly interferes with mobility, a physician may drain the cyst. It is possible for a Baker's cyst to rupture, allowing synovial fluid to leak into the other tissues. This can cause sharp pain in the knee, redness and swelling of the calf, or a feeling that water is running down the calf. These symptoms can resemble a blood clot, so it is appropriate to seek medical attention for confirmation.

Common Symptoms

  • Pain behind knee
  • Swelling behind knee
  • Leg swelling
  • Inability to fully flex knee
  • Inability to fully extend knee


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