Knee effusion, informally known as water on the knee, occurs when excess synovial fluid accumulates in or around the knee joint. It has many common causes, including arthritis, injury to the ligaments or meniscus, or fluid collecting in the bursa, a condition known as prepatellar bursitis.

Signs and symptoms of water on the knee depend on the cause of excess synovial fluid build-up in the knee joint. While important in lubrication, shock absorption, and nutrient transportation, too much can often be the culprit of a variety of symptoms. Some of which include:

Osteoarthritis knee pain usually occurs while the joint is bearing weight, so the pain typically subsides with rest; some patients experience severe pain, while others report no discomfort. Even if one knee is much larger than the other, pain is not guaranteed.

One knee may appear larger than the other. Puffiness around the bony parts of the knee appear prominent when compared with the other knee.

When the knee joint contains excess fluid, it may become difficult or painful to bend or straighten. Fluid may also show under the knee when straightened.

If an individual has injured their knee, they may note bruising or deformity on the front, sides or rear of the knee. Bearing weight on the knee joint may be impossible, and the pain may be unbearable. Bruising may be observed first as red and later transitioning from bluish-purple to a green or yellow as it heals.

Causes of the swelling can include arthritis, injury to the ligaments of the knee, or an accident after which the body's natural reaction is to surround the knee with a protective fluid. There could also be an underlying disease or condition. The type of fluid that accumulates around the knee depends on the underlying disease, condition or type of traumatic injury that caused the excess fluid. The swelling can, in most cases, be easily managed.

Underlying diseases may include