Capsule

Written by Dr Sheila Strover on March 14, 2025

The capsule of the knee is a waterproof bladder which surrounds and protects the internal structures of the joint. Page updated March 2024 by Dr Sheila Strover (Clinical Editor)

capsule of the knee - deflated

The deflated knee capsule has baggy folds at the bottom, which create the 'gutters' that the surgeon can see from the inside.

capsule of the knee inflated with effusion or haemarthrosis

When the joint space fills with fluid the capsule prevents it escaping and the joint can blow up like a balloon.

What does the knee capsule do?

The knee capsule is the water-tight fibrous casing enclosing the important structures inside the joint cavity.

Peer-reviewed paper

  • Quote from peer reviewed paper:

    "...It is a dense fibrous connective tissue that is attached to the bones via specialised attachment zones and forms a sleeve around the joint. It varies in thickness according to the stresses to which it is subject, is locally thickened to form capsular ligaments, and may also incorporate tendons...."

    Citation: Ralphs JR, Benjamin M. The joint capsule: structure, composition, ageing and disease. J Anat. 1994 Jun;184 ( Pt 3)(Pt 3):503-9. PMID: 7928639; PMCID: PMC1259958.

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How does the capsule lubricate the knee?

The inner part of the capsule is lined with special cells - synovial cells - that secrete the fluid that lubricates the knee.

This allows smooth movement of the cartilage surfaces of the bones of the joint. The fluid also nourishes the structures inside the knee. The loose baggy folds also facilitate movement of the knee, but when the knee capsule is tense with fluid the knee cannot move so well and the person will experience stiffness in addition to pain.

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Stiffness from capsular adhesions and contractures

Inflammation of the synovium causes the cellular layer to heap up into folds and little projecting fingers called villi. This is called synovitis.

Sometimes the fragile villi can slowly bleed and give rise to a condition called pigmented villonodular synovitis.

Once the fluid has been aspirated or resorbed, however, it may leave a residual stickiness called adhesions. Without adequate management adhesions may consolidate into scar tissue with marked and painful limitation of movement. This is called arthrofibrosis.

Particularly at the back of the knee the folds can 'glue' together, making it hard to fully extend the knee. The surgeon may choose to release the contracture via a procedure known as 'posterior capsulotomy'.

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