A Baker's cyst is a tense fluid-filled swelling at the back of the knee.
This area at the back of the knee is called the 'popliteal space', so a Baker's cyst is also called a 'popliteal cyst'. The fluid forms in a lubricating pocket (or 'bursa') that is present in every knee - the gastrocnemius/semimembranosus bursa. This lubricating pocket communicates with the main joint cavity and is lined by the same lubricating cells (synovial cells).
If the knee joint is irritated, for example by osteoarthritis or a torn meniscus, the lining of the joint secretes more joint fluid and the knee may swell. If the joint fluid is under pressure some may pass from the main joint cavity into the bursa. Sometimes the entrance to the bursa may have a valve-like quality, allowing fluid into the bursa from the main joint cavity, but not allowing it to escape again. So a little lump appears at the back of the knee towards the inner aspect.
A Baker's cyst will generally reduce if the underlying condition (eg any arthritis) is managed. If it does get bigger and more tense the cys may rupture, and the fluid may leak down into the calf. This settles the cyst, but may cause pain and swelling in the calf, with bruising around the ankles ('crescent sign').
A cyst can be aspirated (the fluid removed with a syringe), but care must be taken not to introduce infection. Cyst fluid contains a sticky substance called 'fibrin'. Over time the more watery part of the fluid may resorb back into the bloodstream, but the thicker fibrin part may remain in the cyst, causing it to become harder and making it more difficult for the fluid to be aspirated. In problematic cases the cyst can be removed surgically.