Effusion

An acute knee effusion is one that comes on suddenly, very often after the knee has been injured or has been subject to overuse. An acute effusion may also occur in an arthritic or gouty knee. The joint fluid is produced by the cells lining the joint. This lining is called the 'synovium' and fluid is 'synovial fluid'. Synovial fluid is normal and helps to lubricate the knee - it is only when it is produced in excess that it becomes abnormal and is then called an effusion or 'water on the knee'.

If a knee is filled with blood, rather than joint fluid, then this is called an haemarthrosis. If the blood does not fully break down and be taken up by the body tissues, then it can irritate the joint and cause an effusion, too.

A chronic effusion is one which is there all the time, although sometimes if the synovium becomes very irritated (synovitis) the effusion may suddenly get worse.