The capsule of the knee is a waterproof bladder which surrounds and protects the internal structures of the joint.
The deflated knee capsule has baggy folds at the bottom, which create the 'gutters' that the surgeon can see from the inside.
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.
Water-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.
Effusion and haemarthrosis
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.
Other problems involving the capsule
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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