The term effusion is a generic term for accumulation of fluid inside a joint. Effusions of the knee joint occur secondary to a variety of causes. The most common is tearing of a ligament or cartilage within the joint. In some instances the effusion is non traumatic and caused by inflammation of the joint lining (synovitis). In cases where a large effusion is present, we may remove the fluid using a needle (arthrocentesis) to help reduce the discomfort from the effusion and to assess the nature of the effusion. Treatment is usually tailored to treating the underlying cause of the effusion.