Popliteal Cyst

The lining of the knee (synovium) produces a clear liquid (synovial fluid) that lubricates the joint. If the joint produces too much fluid, it can leak out or push through the lining forming a cyst.

A popliteal cyst (also called Baker's cyst) is a fluid-filled sac on the backside of the knee. The cyst creates a bump under the skin that is benign, not cancerous. The cyst can burst and leak fluid into your lower leg. A ruptured cyst is not life-threatening, however the symptoms mimic those of thrombophlebitis, which is a very serious condition in which a blood clot forms inside a swollen vein.