A swollen knee, or knee effusion, occurs when excess fluid accumulates within the knee joint or surrounding tissues. This fluid buildup, sometimes called “water on the knee,” signals the body’s response to irritation, injury, or disease. While many instances are minor and temporary, swelling can indicate underlying conditions that threaten joint function or overall health. Understanding the cause is essential, as the type of fluid—synovial fluid, blood, or pus—points directly to the problem’s severity.
Common Causes of Non-Urgent Swelling
Many cases of knee swelling result from minor mechanical issues or overuse that resolve with self-care. Overexertion or repetitive strain from activities like running can trigger inflammation in the joint lining, leading to a mild, temporary effusion. This swelling is often associated with conditions such as patellar tendonitis or “runner’s knee.”
Bursitis, the inflammation of the small fluid-filled sacs that cushion the knee, is another frequent cause of swelling, typically localized outside the joint. For these less serious issues, swelling develops gradually and responds well to the R.I.C.E. protocol:
- Rest
- Ice
- Compression
- Elevation
Applying ice for 10 to 20 minutes several times a day and keeping the leg elevated helps minimize inflammation and reduce discomfort.
Immediate Medical Emergencies
Certain causes of knee swelling demand immediate medical attention due to the risk of rapid joint destruction or systemic illness.
Septic Arthritis
Septic arthritis, an infection within the joint, is a time-sensitive emergency that quickly erodes cartilage and bone. This infection requires rapid diagnosis and treatment, typically involving intravenous antibiotics and draining the infected fluid from the joint.
Acute Hemarthrosis
Acute hemarthrosis, or significant bleeding into the joint space, is an urgent condition often caused by severe trauma, such as an anterior cruciate ligament (ACL) tear or a fracture. The rapid buildup of blood increases pressure inside the joint, causing intense pain and potentially damaging the joint cartilage. Prompt drainage is necessary to relieve pressure and prevent the long-term risk of post-traumatic osteoarthritis.
Deep Vein Thrombosis (DVT)
Swelling that extends down the leg, particularly in the calf or behind the knee, can signal deep vein thrombosis (DVT). A DVT is dangerous because the clot can break loose and travel to the lungs, causing a potentially fatal pulmonary embolism.
Chronic Conditions Linked to Persistent Swelling
Persistent or recurrent knee swelling indicates ongoing joint damage that can lead to permanent disability over time.
Degenerative and Autoimmune Conditions
Osteoarthritis (OA), the most common form of arthritis, is a degenerative condition where joint cartilage wears down, causing inflammation and fluid buildup. This chronic effusion worsens as bone surfaces rub together, eventually leading to stiffness and reduced range of motion. Inflammatory autoimmune diseases like Rheumatoid Arthritis (RA) or metabolic conditions such as gout also cause persistent or flaring swelling. RA causes the immune system to attack the joint lining, leading to painful swelling and potential deformity. Gout results from uric acid crystal buildup, triggering sudden, painful inflammatory attacks.
Mechanical Instability
Meniscus tears or chronic ligament instability may cause intermittent, low-level effusion. While these injuries do not pose an immediate systemic threat, the long-term inflammation and mechanical instability contribute to accelerated joint degeneration. Managing these chronic causes focuses on controlling inflammation and preserving the remaining joint structure to maintain mobility.
Critical Warning Signs for Urgent Care
A swollen knee accompanied by certain symptoms requires immediate medical evaluation. Sudden, significant swelling, especially if it occurs rapidly following an injury, may signal a major ligament tear or an intra-articular fracture. The inability to bear weight on the affected leg, or the knee buckling or giving way, indicates a serious structural injury.
Swelling paired with signs of systemic illness or localized infection warrants an emergency visit. This includes fever or chills, suggesting a widespread infection. Local signs like severe redness, warmth, or tenderness can point toward septic arthritis. If leg swelling is accompanied by shortness of breath or chest pain, seek immediate emergency care due to the risk of pulmonary embolism from a DVT.