It is normal for the knee to contain a small amount of yellowish liquid or ‘synovial fluid’.  The normal volume (3 to 5mls) of fluid does not cause visible swelling.  This complex fluid plays an essential role in lubricating the joint and provides nourishment to the cartilage surfaces covering the bone.

The synovial lining of the knee often produces excess amounts of fluid in response to inflammation or direct injury.  This leads to swelling, an effusion, and a sensation of tightness within the joint.  If the swelling gets tense, the range of movement in the knee becomes restricted.

If the knee sustains a direct injury it may fill with blood, called a haemarthrosis.  A rapidly forming haemarthrosis, forming in minutes to a few hours, may be associated with a ligament tear or an underlying fracture.  Classically the haemarthrosis associated with an acute meniscal tear develops over many hours.

Occasionally an acute haemarthrosis may be drained with a needle and syringe to make the knee more comfortable.