Fluid on the knee, commonly known as “water on the knee” or knee effusion, occurs when excess fluid collects in or around the knee joint. This condition is a symptom of an underlying issue, not a diagnosis itself. While healthy joints contain a small amount of synovial fluid for lubrication, abnormal accumulation suggests a problem. The type of fluid—synovial fluid, blood, or pus—can help determine the cause.
Injuries and Overuse
Physical trauma and repetitive stress are frequent reasons for fluid accumulation in the knee. When knee structures are damaged, the body often responds by increasing fluid production or by bleeding into the joint space, a condition called hemarthrosis. The amount of fluid can vary based on the injury’s type and severity.
Meniscus tears, involving the C-shaped cartilage, can cause fluid buildup, sometimes with a gradual onset. Ligament sprains, particularly to the anterior cruciate ligament (ACL) or medial collateral ligament (MCL), commonly cause rapid knee swelling due to bleeding. Fractures, even hairline ones around the knee, can also lead to fluid accumulation from bleeding.
Patellar dislocation, where the kneecap slips out of its normal position, frequently results in knee effusion, often with blood in the joint, and is common in active adolescents. Overuse injuries, such as patellar tendinitis (inflammation of the tendon connecting the kneecap to the shinbone) or bursitis (inflammation of the fluid-filled sacs that cushion the knee), can also cause fluid to collect. Bursitis typically presents as a localized swelling, often feeling squishy, and can result from continuous pressure or repetitive movements.
Inflammatory Joint Conditions
Chronic inflammatory processes within the knee joint are a significant cause of persistent fluid buildup. These conditions irritate the synovial lining, which then produces excess fluid. This fluid accumulation often accompanies symptoms like pain, stiffness, and warmth around the joint.
Osteoarthritis, a degenerative joint condition involving cartilage breakdown, can lead to fluid on the knee as the joint attempts to lubricate itself. Irritation from cartilage fragments or exposed bone triggers the synovial membrane to produce more fluid. Rheumatoid arthritis, an autoimmune disorder, causes the immune system to attack synovial tissue, resulting in chronic inflammation and significant fluid accumulation.
Gout and pseudogout are types of arthritis caused by crystal accumulation in the joint, triggering an acute inflammatory response and fluid effusion. Gout involves monosodium urate crystals, while pseudogout is associated with calcium pyrophosphate dihydrate crystals. Psoriatic arthritis, an inflammatory condition linked to psoriasis, can also cause knee effusion.
Infections and Other Causes
Infections and certain systemic conditions are among other causes of knee fluid. Septic arthritis, a joint infection, causes rapid onset of severe pain, warmth, redness, and significant fluid accumulation containing pus, requiring urgent medical attention. Bacteria can enter the joint through a wound, injection, or spread from other body parts.
Bleeding disorders, such as hemophilia, can cause spontaneous bleeding into the knee joint (hemarthrosis). Recurrent bleeding episodes can contribute to long-term joint damage. Tumors, both benign and malignant, are rare causes of knee effusion, but they can irritate the joint or lead to fluid accumulation.
A Baker’s cyst, also known as a popliteal cyst, is a fluid-filled lump that forms at the back of the knee. It is often a consequence of an underlying knee problem, such as arthritis or a meniscus tear, which causes excess joint fluid to bulge into a bursa.
When to Seek Medical Attention
Prompt medical evaluation is advisable for fluid on the knee, especially with certain symptoms. Severe pain making it impossible to bear weight is a warning sign. Any accompanying fever, significant redness, or warmth around the joint suggests a possible infection, needing immediate attention.
Sudden and substantial swelling, particularly after an injury, warrants medical consultation. If you have difficulty bending or straightening your knee completely, or if swelling persists despite rest, seek professional medical advice.