Knee swelling occurs when excess fluid gathers in or around the knee joint. Healthcare professionals sometimes refer to this as knee effusion or “water on the knee.” This fluid accumulation is a symptom, not a disease, and can vary in severity. Understanding its potential reasons is helpful for managing knee health.
The Mechanism of Swelling
Swelling in the knee signals the body’s response to injury or irritation. When tissues within or surrounding the knee joint are damaged, the body initiates inflammation. This involves increased blood flow, delivering immune cells and fluids for repair.
This heightened fluid production, or effusion, results in the noticeable enlargement of the knee. The synovial membrane, which lines the joint, can become inflamed and produce excess synovial fluid. The accumulated fluid can also contain blood or other inflammatory substances. While this fluid attempts to cushion the joint and assist in healing, its increased pressure can also contribute to stiffness and discomfort.
Swelling from Acute Injuries
Acute injuries frequently cause sudden knee swelling due to trauma. Ligament sprains are a common source, notably tears to the anterior cruciate ligament (ACL) or medial collateral ligament (MCL). An ACL injury typically arises from a twisting motion, sudden stops, or direct impact, often producing an audible popping sound, followed by swelling within a few hours. This rapid swelling is often attributed to bleeding within the joint.
Meniscus tears, affecting the C-shaped cartilage that acts as a shock absorber between the thighbone and shinbone, also lead to fluid accumulation. These tears commonly occur from forceful twisting or rotation of the knee while bearing weight. Swelling from a meniscus tear might not manifest immediately, sometimes taking 24 hours or more to develop, and can be accompanied by stiffness, pain, or a sensation of the knee locking.
Fractures of knee bones, such as the patella (kneecap), femur (thighbone), or tibia (shinbone), often cause immediate and substantial swelling. This swelling is primarily due to significant bleeding within the joint capsule and surrounding soft tissues. Even direct impact injuries without a fracture can cause considerable soft tissue damage, triggering an inflammatory response and fluid buildup.
Swelling from Chronic Conditions
Chronic medical conditions are a frequent cause of ongoing or recurrent knee swelling. Osteoarthritis, a common degenerative joint disease, leads to swelling as the protective cartilage cushioning the bones gradually wears away. This breakdown causes bones to rub, leading to inflammation, irritation of surrounding tissues, and increased production of synovial fluid. The body may also form bony growths, called osteophytes, which can further irritate tissues and contribute to swelling.
Inflammatory types of arthritis, such as rheumatoid arthritis and psoriatic arthritis, also cause knee swelling. Rheumatoid arthritis is an autoimmune condition where the immune system attacks the lining of the joints (synovium). This causes the synovium to become inflamed and thicken, leading to excess fluid buildup and painful swelling, often affecting both knees symmetrically. Psoriatic arthritis, another autoimmune condition, can also cause inflammation and swelling in the knee joint, often alongside skin symptoms.
Gout and pseudogout are conditions characterized by the accumulation of crystals within a joint, triggering sudden and severe swelling. Gout results from an excess of uric acid, forming urate crystals, while pseudogout involves calcium pyrophosphate crystals. These crystals deposit in the joint, causing an intense inflammatory reaction that manifests as rapid swelling, warmth, and pain.
Infections within the joint, known as septic arthritis, can also cause significant and rapid knee swelling. Bacteria or other microorganisms enter the joint, leading to a severe inflammatory response with fluid accumulation. This type of swelling is typically accompanied by warmth, redness, and sometimes fever, requiring urgent medical attention.
Other Potential Sources of Swelling
Beyond acute injuries and chronic conditions, other factors can contribute to knee swelling. Bursitis involves the inflammation of bursae, which are small, fluid-filled sacs cushioning bones, tendons, and muscles around the knee. When a bursa becomes irritated, often from repetitive kneeling or direct impact, it produces excess fluid, causing localized swelling and tenderness.
A Baker’s cyst, also known as a popliteal cyst, is a fluid-filled lump that forms behind the knee. This cyst is typically not a primary condition but rather a symptom of another underlying knee problem, such as arthritis or a meniscus tear, which causes an overproduction of joint fluid. The excess fluid then collects in the popliteal bursa, leading to palpable swelling.
Rarely, benign growths or tumors can also cause knee swelling. These non-cancerous growths, if located near or within the joint, may lead to localized fluid accumulation.
When to Consult a Doctor
While some knee swelling resolves with self-care, certain signs indicate the need for professional medical attention. Seek immediate care if swelling occurs rapidly after a significant injury, especially if accompanied by a popping sound, severe pain, or inability to bear weight. A visibly deformed knee or a sudden inability to straighten or bend the knee also warrants urgent evaluation.
Persistent or worsening knee swelling, even without a clear injury, should prompt a doctor’s visit. Signs of infection, such as fever, chills, increased warmth, or redness around the knee, require prompt medical assessment. If the swelling is accompanied by pain that interferes with daily activities or sleep, or if home remedies like rest and ice do not provide improvement within a few days, consult a healthcare provider.
A medical professional can accurately diagnose the underlying cause through examination and, if necessary, additional tests like fluid analysis or imaging. Early diagnosis and appropriate treatment can help prevent long-term complications and improve recovery.