Knee swelling, medically termed joint effusion or colloquially “water on the knee,” occurs when excess fluid accumulates within the joint capsule. This accumulation is a symptom, signaling that the body is responding to irritation or damage within the joint structure. The fluid buildup is an inflammatory response, where the body sends resources to address an underlying issue, whether from a sudden injury, a long-term disease process, or localized irritation. Understanding the origin of this fluid is the first step toward finding relief.
Acute Swelling: Causes Related to Injury
Acute knee swelling typically follows a traumatic event, appearing rapidly, often within the first few hours. This rapid onset is frequently associated with internal bleeding, known as hemarthrosis, where blood from damaged tissues fills the joint space. Injuries involving sudden twisting, hyperextension, or direct impact are common mechanisms. For example, tearing the anterior cruciate ligament (ACL) often leads to a large, painful effusion almost immediately.
Damage to the meniscus, the C-shaped cartilage that acts as a shock absorber, can also cause acute swelling, though it may take slightly longer to develop. A torn meniscus releases inflammatory mediators into the joint, causing swelling that might be accompanied by mechanical symptoms like locking or catching. Fractures, especially those involving the patella or tibial plateau, result in immediate and severe swelling due to internal bleeding and bone fragments irritating the joint lining.
The volume of fluid produced in acute hemarthrosis can be substantial, often exceeding 50 milliliters, which significantly distends the joint capsule. This rapid distention is the source of intense pain and instability following a significant acute injury. This internal bleeding-related swelling often makes the knee appear taut and globular, obscuring the natural contours of the patella. Recognizing the speed and severity of this swelling indicates structural damage, necessitating urgent medical evaluation.
Chronic Swelling: Causes Related to Disease
Swelling that develops slowly over weeks or months, or that recurs intermittently, often points toward a chronic underlying disease process.
Osteoarthritis (OA)
Osteoarthritis, the most common form of arthritis, causes swelling through the gradual breakdown of joint cartilage, leading to bone-on-bone friction. This mechanical irritation triggers the synovial membrane, which lines the joint, to produce excess fluid. This results in a low-grade, intermittent effusion that worsens with activity. The synovial fluid in OA is typically non-inflammatory, containing few white blood cells.
Rheumatoid Arthritis (RA)
Rheumatoid arthritis is a systemic autoimmune disorder where the body attacks the synovial lining. This causes significant inflammation and thickening of the synovium (synovitis), which secretes large volumes of inflammatory fluid into the joint space. Unlike OA, RA typically presents with symmetrical swelling affecting both knees, accompanied by warmth, redness, and stiffness that is worse in the morning.
Crystalline Arthropathies
Crystalline arthropathies, such as gout or pseudogout, involve microscopic crystals precipitating within the joint fluid. Gout results from uric acid crystals, while pseudogout involves calcium pyrophosphate dihydrate crystals. These crystals trigger an inflammatory cascade, leading to a rapid, painful, hot, and often unilateral episode of swelling. Identifying the specific crystal type through fluid analysis confirms the diagnosis.
Localized Swelling: Non-Joint Causes
Not all swelling around the knee originates from within the joint capsule; fluid accumulation can be localized to external structures.
Bursitis
Bursitis is the inflammation of a bursa, the small, fluid-filled sacs that cushion tendons and bones. Prepatellar bursitis, sometimes called “housemaid’s knee,” causes a distinct, localized swelling directly over the kneecap that feels like a fluid-filled lump. This often results from repetitive kneeling or minor trauma.
Baker’s Cyst
A Baker’s cyst, or popliteal cyst, appears as a noticeable bulge at the back of the knee. This cyst is usually a symptom, forming when excess joint fluid is pushed out of the capsule and into the space behind the knee, often linked to underlying arthritis or meniscal tears. Localized effusions like these often move with the skin and do not cause the internal pressure associated with true joint effusion.
Warning Signs and Next Steps
Determining the cause of knee swelling requires professional medical evaluation, but certain signs indicate the need for immediate attention.
Swelling accompanied by a fever, chills, or spreading redness suggests the possibility of septic arthritis. This serious bacterial infection within the joint requires urgent drainage and antibiotics to prevent cartilage destruction. Septic arthritis often presents with excruciating pain and extreme warmth, distinguishing it from non-infectious causes.
An inability to bear weight on the leg following an injury or a noticeable deformity of the knee joint demands immediate assessment for a possible fracture or severe ligamentous rupture. New swelling or pain in the calf accompanying knee swelling should also be evaluated quickly, as this could indicate a deep vein thrombosis (DVT).
While awaiting medical consultation, temporary management involves the principles of RICE: Rest, Ice applied for 15-20 minutes several times a day, gentle Compression with a bandage, and Elevation of the leg above the heart level. These measures help reduce discomfort until a definitive diagnosis can be made using imaging and joint fluid aspiration.