History: 50 year old male with knee pain and swelling. 

This is the appearance of a knee joint effusion on lateral radiographs. Knee joint effusions (sometimes referred to as “water on the knee”) is fluid filling the joint space of the knee. Effusions can be caused by any of the following entities:

1. Arthritis (osteoarthritis or inflammatory arthritides such as rheumatoid arthritis)
2. Crystal arthritides such as gout or pseudogout, now known as calcium pyrophosphate dihydrate deposition disease.
3. Septic arthritis 
4. Lipohemarthrosis – associated with fractures which lead to marrow leaking into the joint
5. Hemarthrosis – associated with trauma or hemophilia

This knee joint effusion was caused by a torn anterior cruciate ligament.