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).

baker's cyst or popliteal cyst baker's cyst or popliteal cyst

Bakerzyste2 MRT sag

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 tense Baker's cyst may rupture and cause sudden painful swelling of the calf, mimicking a thrombosis.


An explanation by Dr Lars Blond.

Enclosing the articulating part of the knee joint is a water-tight capsule made of a strong fibrous material lined on the inside with a thin layer of secretory cells.

Pain in the back of the knee

A short eBook discussing those conditions that may cause pain at the back of the knee.

A short explanation of the very common problem of a painful lump at the back of the knee due to a Baker's cyst.

Pain in the back of the knee

An overview with illustrations.



Baker's Cyst. Demange MK., Rev Bras Ortop. 2011 Nov-Dec; 46(6): 630–633.

Popliteal cysts: a current review. Herman AM, Marzo JM. Orthopedics. 2014 Aug;37(8):e678-84

Baker's Cyst Filled with Hematoma at the Lower Calf Yoo M-J, Yoo J-S, Jang H-S, Hwang C-H. Knee Surg Relat Res. 2014 Dec; 26(4): 253–256.

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