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

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. 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.

Excessive synovial fluid inside the capsule is called an effusion. Blood inside the capsule is called an haemarthrosis.

The inner aspect of the knee capsule is lined by cells called synovial cells that secrete a lubricating and nourishing fluid into the joint. The whole layer of synovial cells, which is quite complex in sutructure, is called the synovium.

collapsed knee capsuleWater-tight

This illustration of the knee from the side shows how the knee capsule is attached to the margins of the bones. Here we show the normal situation of a collapsed capsule, with wrinkly folds, and containing very little fluid. Note the extent of the joint space, reaching a handsbreadth above the joint and right round to the back of the knee.

Within the capsule are those structures that the surgeon can see and operate on when he inserts an arthroscope into the knee. To see them well, the knee surgeon introduces extra irrigating fluid under pressure, and the cavity blows up, allowing the surgeon a much better view. The space can accept up to about 180ml. By allowing some flow of fluid during the procedure, the debris can be irrigated away so that visibility inside the joint remains optimum.

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Effusion and haemarthrosis

joint capsule of knee inflated

The space can also become distended of its own accord if problems inside the knee trigger excessive secretion of synovial fluid, called an 'effusion'.

The capsule can also be distended also by blood if there has been an injury or if blood vessels damaged during surgery are not properly cauterised during the procedure. When this happens the knee can look very swollen and feels tense and painful. This is called an 'haemarthrosis'.

Pain from the increased fluid pressure of both an effusion and an haemarthrosis may be relieved by withdrawing the fluid via a needle placed into the joint. This is called joint aspiration.

The loose baggy folds of the normal capsule facilitate movement of the knee, but when the knee capsule is inflated and tense the knee cannot move so well and the person will experience stiffness in addition to pain.

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Other problems involving the capsule


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.

Loose bodies

Sometimes loose fragments of meniscus, joint cartilage or even bone my float in the synovial fluid as a loose body. These can shoot around the back and be concealed amongst the capsular folds, and be missed during arthroscopy.

Capsular adhesions & Contractures

Where there has been inflammation of the synovium over some time, sticky strands may form and gum the folds together. Eventually these may mature into scar tissue and contract, so that joint range of motion is affected.

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The capsule and arthrofibrosis

The capsule is often involved in the disorder known as 'arthrofibrosis', when adhesions and scar tissue lead to stiffness in the joint. 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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joint capsule
synovial capsule

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