Fluid on the knee, also known as knee effusion, occurs when excess fluid accumulates in or around the knee joint. A small amount of fluid exists in normal joints, but in some cases, excess fluid can build up, causing the knee to become puffy and swollen. Knee effusion can be caused by a variety of factors, including:

• Trauma or injury to the knee joint, such as a sprain, broken bone, or ligament tear
• Arthritis, including osteoarthritis, rheumatoid arthritis, and gout
• Infections
• Overuse injuries
• Systemic causes

Symptoms of fluid on the knee include swelling, stiffness, and reduced range of motion. Depending on the cause of the fluid buildup, the knee may also be very painful, to the point that its impossible to bear weight on it. Treatment for fluid on the knee depends on the underlying cause and may include rest, ice, compression, elevation, over-the-counter pain relievers, and aspiration to remove excess fluid.