Knee effusion, sometimes called water on the knee, occurs when excess fluid accumulates in or around the knee joint. Common causes include arthritis and injury to the ligaments or meniscus, which is cartilage in the knee. Symptoms Excess fluid will cause puffiness around the bony parts of the knee. One knee may look and feel bigger than the other. It may be difficult to bend or straighten the knee. swelling stiffness a reduced range of motion The following symptoms need urgent medical attention: inability to put weight on the joint no pulse in the feet loss of feeling below the knee fever Osteoarthritis: Pain and stiffness tend to be worse first thing in the morning but often improve within 30 minutes of getting up. Rheumatoid arthritis: Pain and swelling often affects several joints and often symmetrically. The joint may feel hot to the touch. Pain persists longer than 30 minutes after waking. Traumatic injury: There may be bruising and severe pain, and it may be hard to put weight on...