The capsule of the knee is a waterproof bladder which surrounds and protects the internal structures of the joint.

The knee capsule is usually baggy, with folds in its lower part, but when the joint space fills with fluid it can blow up like a balloon.

capsule of the knee - deflated capsule of the knee  inflated with effusion or haemarthrosis The inner part of the capsule is lined with special cells - synovial cells - that secrete the fluid that lubricates the knee.
The deflated knee capsule has baggy folds at the bottom, which create the 'gutters' that the surgeon can see from the inside. The inflated capsule - filled with fluid under tension. Clear fluid inside the capsule is called an effusion. Blood inside the capsule is called an haemarthrosis.  

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.

Knee capsule

Quick overview of the outer casing of the knee joint.



An anatomic study on the attachment of the joint capsule to the tibia in the lateral side of the knee. Nasu H, Nimura A, Sugiura S, Fujishiro H, Koga H, Akita K. Surg Radiol Anat. 2018 May;40(5):499-506.

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