A swollen knee, or knee effusion, indicates an accumulation of excess fluid within or around the knee joint. This symptom often signals the body’s response to an injury, overuse, or an underlying medical condition.
The Biology of Knee Swelling
Knee swelling results from fluid buildup inside or surrounding the joint capsule. This fluid can include synovial fluid, blood, or pus, depending on the underlying cause. Inflammation plays a central role, as the body’s immune system responds to damage or irritation by increasing blood flow and vessel permeability, allowing fluid and immune cells to leak into the surrounding tissues.
The synovial membrane, a thin tissue lining the joint, produces synovial fluid. When inflamed (synovitis), it overproduces fluid that collects within the joint, causing noticeable swelling. This fluid accumulation also increases pressure, contributing to discomfort and limited movement.
Common Reasons for a Swollen Knee
Knee swelling stems from injuries or underlying medical conditions.
Injuries
Traumatic causes involve a direct impact or sudden twisting of the knee. Tears to ligaments (e.g., anterior cruciate ligament or ACL, medial collateral ligament or MCL) or cartilage structures (e.g., meniscus) can lead to swelling, often with bleeding and inflammation. A torn meniscus may cause swelling that develops slowly over hours or days, accompanied by pain and stiffness. Fractures or kneecap dislocations also result in significant swelling. Overuse injuries, such as tendinitis or bursitis, involve inflammation of tendons or fluid-filled sacs (bursae) around the knee, leading to localized swelling.
Inflammatory Conditions
Nontraumatic causes commonly involve inflammatory conditions. Osteoarthritis, a degenerative condition, occurs when knee cartilage wears away, causing bones to rub against each other. This leads to irritation, inflammation, and excess fluid production. Rheumatoid arthritis, an autoimmune disease, involves the immune system attacking the synovial membrane, causing inflammation, pain, and swelling, often symmetrically in both knees. Other inflammatory conditions like gout and pseudogout are characterized by crystal buildup in the joint, triggering an inflammatory response and fluid accumulation.
Infections
Infections, such as septic arthritis, are an urgent cause of knee swelling. This condition involves bacteria or other microorganisms infecting the joint, leading to rapid and severe inflammation. Septic arthritis often presents with fever, warmth, and redness, necessitating prompt medical attention due to potential joint damage.
When to Consult a Healthcare Professional
While minor knee swelling might resolve with home care, certain symptoms indicate a need for medical evaluation.
Consult a healthcare professional if you experience:
Severe pain or an inability to bear weight on the affected leg.
Sudden swelling, especially after hearing a “popping” sound, which may suggest an acute ligament or meniscal injury.
Fever, increased warmth, or pronounced redness around the knee, as these can point to a joint infection.
Swelling that does not improve with rest and elevation, or that worsens over time.
Mechanical symptoms, such as the knee “locking” or catching during movement, or a feeling of instability where the knee “gives way,” indicating structural damage like a torn meniscus or ligament.
Numbness or tingling in the leg or foot accompanying knee swelling.
Navigating Diagnosis and Treatment
When seeking medical help for a swollen knee, a healthcare professional typically begins with a detailed review of medical history and a physical examination. This initial assessment helps narrow down potential causes. Imaging tests are often employed for further investigation. X-rays can identify bone fractures or signs of arthritis. An ultrasound can visualize issues with tendons or ligaments and detect fluid accumulation. Magnetic resonance imaging (MRI) offers detailed images of soft tissues, including ligaments, tendons, cartilage, and menisci, making it valuable for diagnosing tears or other internal joint problems.
Joint aspiration, also known as arthrocentesis, is a procedure where a needle withdraws fluid from the knee joint. Analyzing this fluid can help diagnose infections (checking for bacteria), identify crystal-induced conditions (like gout), or determine if bleeding from an injury is present. This procedure can also relieve pressure and discomfort caused by excessive fluid buildup.
Treatment approaches for a swollen knee are tailored to its underlying cause. Initial management often includes the RICE method:
Rest: Avoids further injury.
Ice: Reduces pain and swelling by constricting blood vessels.
Compression: Helps control fluid buildup with an elastic bandage.
Elevation: Uses gravity to reduce swelling by raising the leg above heart level.
Over-the-counter pain relievers, such as ibuprofen, can help manage pain and inflammation.
Physical therapy is frequently recommended to improve knee function, strength, and range of motion. Therapists use various techniques, including therapeutic exercises and hands-on manipulation, to reduce swelling and promote healing. In some cases, prescription medications, such as anti-inflammatories or disease-modifying antirheumatic drugs (DMARDs) for conditions like rheumatoid arthritis, may be prescribed. Surgical intervention becomes an option when conservative treatments are insufficient or for certain structural damages. Procedures like arthroscopy, a minimally invasive surgery, can repair torn cartilage or ligaments or remove damaged tissue. The specific treatment plan is always determined by the diagnosed cause of the knee swelling.