Sports injury. Increased fluid in the right knee joint cavity (left on image), known as knee effusion, seen here two days after a sports injury. There is swelling of the joint and loss of the bony contours around the knee cap compared with the patient's left knee. The diagnosis is made by taking a good history of the injury and providing support and rest to the joint. Direct inspection of the knee joint through the surgical investigation of arthroscopy may be needed to make a final diagnosis and provide treatment.