Knee effusion. Coloured X-ray of the swelling of an injured knee caused by effusion. Effusion is a painful condition whereby fluid accumulates in the joint space, limiting movement. Joints such as the knee contain small amounts of synovial fluid for lubrication. Injuries which damage or inflame the area around these joints often result in excessive production of this fluid, resulting in effusion. Painkillers and anti-inflammatory drugs are used to reduce the swelling. In rare cases the fluid may need to be withdrawn using a hypodermic needle.