Water on the knee, sometimes referred to as knee effusion, is a term used to describe excess fluid that has accumulated around or within the knee joint. The knee joint lining produces a fluid that helps to nourish the cartilage and lubricate the moving components of the joint. Certain injuries and diseases can cause an over-production of this fluid. When this fluid accumulates it can cause stiffness and pain in the knee joint.
Injuries
If any part of the knee is injured, the damage can result in fluid accumulation. The injuries that can cause fluid on the knee include a meniscus tear, overuse injuries, broken bones and a ligament tear, according to the Mayo Clinic.
Rheumatoid Arthritis
Rheumatoid arthritis can cause water on the knee. This chronic condition is characterized by joint lining inflammation. The initial symptoms of this condition include joint lining swelling causing joint stiffness, joint swelling, joint pain and redness and warmth on and around the joint, according to the Arthritis Foundation.
Osteoarthritis
Osteoarthritis, the most common type of arthritis, can cause water on the knee. Approximately 27 million Americans are affected by this type of arthritis, according to the Arthritis Foundation. This chronic condition is most often associated with wear and tear of the joints. As the wear and tear progresses, the joint's cartilage breaks down. Loss of joint mobility, joint stiffness and joint pain are the most common symptoms.
Knee Bursitis
The bursa is a small sac that is filled with fluid. It provides cushioning between the muscles, tendons and bones, and helps joints move freely. Knee bursitis occurs when the bursa close to the knee joint becomes inflamed. In addition to causing water on the knee, this condition can cause pain, swelling, limited mobility and warmth of the affected knee, according to the Mayo Clinic.
