A swollen knee, often called “water on the knee” or knee effusion, occurs when excess fluid accumulates within or around the knee joint. This swelling is a symptom of an underlying issue, not a standalone diagnosis. The fluid can range from mild puffiness to significant swelling that restricts movement and causes discomfort.
Injuries to the Knee
Injuries are a common cause of knee swelling, resulting from direct trauma or repetitive stress. When a knee is injured, the body’s natural response involves inflammation and fluid buildup. These injuries can affect ligaments, cartilage, and bones within the knee.
Ligament tears frequently lead to knee swelling. The anterior cruciate ligament (ACL) and medial collateral ligament (MCL) are susceptible to injury during sports involving sudden stops, changes in direction, or direct impact. An ACL tear often results in immediate, significant swelling and a feeling of instability. Sprains and tears of the collateral ligaments also cause pain and swelling.
Meniscal tears, involving the C-shaped cartilage that cushions the knee joint, also contribute to fluid accumulation. These tears can occur from forceful twisting or rotating the knee, especially while bearing weight. Swelling from a meniscal tear may develop gradually over a few days, leading to stiffness and limited bending.
Fractures around the knee, such as a broken kneecap (patella) or the upper shin bone (tibial plateau), commonly cause severe pain and swelling. These bone injuries often result from high-impact trauma like falls or car accidents. Overuse injuries, stemming from repetitive stress, can also lead to irritation and fluid buildup.
Inflammatory Joint Conditions
Various inflammatory conditions can cause chronic or recurring knee swelling. Arthritis, a broad term for joint inflammation, frequently affects the knee. Osteoarthritis, the most common type, involves the gradual breakdown of cartilage that cushions the joint, leading to bone-on-bone friction, inflammation, and fluid accumulation.
Rheumatoid arthritis (RA) is an autoimmune disease where the body’s immune system mistakenly attacks the synovial membrane, the lining of the knee joint. This causes inflammation and swelling, typically affecting both knees symmetrically. The inflamed synovial membrane produces excess fluid, contributing to swelling, stiffness, and pain that can worsen after periods of rest.
Gout is an inflammatory condition characterized by the buildup of uric acid crystals in the joints, including the knee. These sharp crystals trigger sudden, intense episodes of pain, redness, and swelling. Pseudogout, similar to gout, involves the accumulation of calcium pyrophosphate crystals in the joint, also causing acute inflammation and swelling. Psoriatic arthritis, a form of arthritis affecting some people with psoriasis, can also cause knee inflammation and swelling.
Conditions affecting the soft tissues around the knee, such as bursitis and tendonitis, can also lead to localized swelling. Bursitis involves the inflammation of bursae, small fluid-filled sacs that cushion bones, tendons, and muscles. Overuse or injury can irritate these bursae, causing them to swell and become painful. Tendonitis refers to the inflammation of tendons, which connect muscles to bones, and can result from overuse or repetitive movements, leading to swelling and pain.
Infections and Other Underlying Issues
Infections are a less common but serious cause of knee swelling, requiring prompt medical attention. Septic arthritis occurs when a bacterial infection directly invades the joint, leading to severe pain, warmth, redness, rapid swelling, and often fever. This condition can quickly damage joint cartilage if left untreated. Cellulitis, a bacterial infection of the skin and underlying tissues around the knee, can also cause significant redness, warmth, and swelling.
A Baker’s cyst, also known as a popliteal cyst, is a fluid-filled sac that forms behind the knee. It typically arises as a complication of other knee problems that cause excess fluid production, such as arthritis or a meniscal tear. While often painless, a large Baker’s cyst can cause tightness, pain, and swelling behind the knee or extend into the calf. In rare instances, tumors can also contribute to knee swelling, though this is an infrequent cause compared to injuries or inflammatory conditions.
Recognizing When to Seek Medical Care
While some knee swelling may resolve with rest and home care, certain symptoms indicate the need for prompt medical evaluation. Seek immediate medical attention if you experience sudden, severe swelling, especially after an injury. Intense pain that prevents you from bearing weight on the knee or moving it normally warrants urgent care. A knee that appears deformed or makes a “popping” sound at the time of injury also requires immediate assessment.
Concerning signs include redness or warmth around the joint, which can signal an infection. If swelling is accompanied by fever, chills, or other systemic symptoms, professional medical evaluation is necessary. Persistent swelling that does not improve with home remedies within a few days should also prompt a visit to a healthcare provider. Early diagnosis and treatment can help prevent complications and address the underlying cause.