Knee swelling happens when excess fluid accumulates in or around the joint, and the cause ranges from everyday wear and tear to injuries, gout, and infections. The most common reasons in a primary care setting are osteoarthritis, trauma, and gout. Figuring out which one is driving your swelling depends on how quickly it appeared, whether you injured it, and what other symptoms came along with it.
Osteoarthritis and Everyday Wear
Osteoarthritis is the single most common cause of a swollen knee in adults. Over time, the cartilage cushioning the joint breaks down, and the body responds by producing extra joint fluid. This type of swelling tends to build gradually over weeks or months rather than appearing overnight. It’s often worse after long periods of standing, walking, or climbing stairs, and it may improve with rest. Both knees are frequently affected, though one side can be worse than the other.
The knee contains multiple small fluid-filled sacs called bursae that help tendons and ligaments glide smoothly over bone. When these sacs become irritated from repetitive kneeling, prolonged pressure, or overuse, they swell independently of the joint itself. This is bursitis, and it often produces a localized pocket of swelling rather than a generally puffy knee. Tenderness directly over a tendon or where a tendon attaches to bone can also point to tendonitis, which sometimes accompanies mild swelling around the joint.
Swelling After an Injury
The speed at which your knee swells after an injury is one of the most useful clues about what’s damaged. A knee that balloons up within minutes of a twist, fall, or impact usually means bleeding inside the joint, which points to damage to a ligament (like the ACL) or a fracture. This rapid, tense swelling is called a hemarthrosis.
Swelling that develops 24 to 36 hours after an injury tells a different story. This delayed timeline is more typical of a meniscus tear or a milder ligament sprain. The fluid is usually inflammatory rather than bloody, and the knee may feel stiff and achy rather than acutely painful. If you twisted your knee during a sport or misstep and the swelling crept in the next day, a meniscal injury is a strong possibility.
Gout and Crystal-Related Swelling
Gout attacks in the knee can come on suddenly, often overnight, producing intense pain, warmth, and sometimes dramatic redness. About two-thirds of people with acute gouty arthritis in studies had visible redness over the affected joint. The swelling usually peaks within 12 to 24 hours and can make the knee so tender that even the weight of a bedsheet is uncomfortable. Gout is caused by uric acid crystals forming inside the joint, and it tends to flare after triggers like alcohol, red meat, dehydration, or certain medications.
A related condition called pseudogout involves a different type of crystal (calcium pyrophosphate) and produces similar sudden swelling and pain. The knee is actually the most common joint affected by pseudogout, and it’s more frequent in older adults.
Infection in the Joint
A swollen knee that also feels hot to the touch, looks red, and comes with fever or chills may signal a joint infection, known as septic arthritis. This is the most serious cause of knee swelling and requires urgent treatment. Septic arthritis often causes systemic symptoms like fever and chilling more consistently than gout does, though gout can occasionally produce a low-grade fever too. The key distinguishing feature is that an infected joint typically makes you feel sick overall, not just sore in the knee.
If one knee has a noticeable change in skin color and feels warm compared to the other, seek medical attention promptly. Bacteria can destroy cartilage rapidly if the infection isn’t drained and treated.
How Doctors Figure Out the Cause
A physical exam can detect fluid in the knee using two simple bedside tests. For small amounts of fluid, a doctor will stroke upward along the inner side of the knee to push fluid above the kneecap, then sweep it down the outer side. If fluid visibly shifts back toward the inner knee, the test is positive. For larger collections, pressing down on the kneecap itself will produce a tapping sensation as it bounces against the bone underneath, confirming a significant effusion.
When the cause isn’t obvious, your doctor may draw fluid from the knee with a needle. Normal joint fluid is clear and colorless. Red or blood-tinged fluid suggests bleeding from trauma or inflammation. A yellow-orange tint indicates older bleeding that’s already breaking down. White, cloudy, or light yellow fluid points to inflammation, infection, or crystal deposits. The fluid can be tested for bacteria, uric acid crystals, and cell counts to narrow down a diagnosis quickly.
Managing Swelling at Home
For swelling tied to a minor injury or an osteoarthritis flare, the RICE approach (rest, ice, compression, elevation) is the standard first step. Apply ice with a thin barrier like a towel for 10 to 20 minutes at a time, repeating every hour or two. Keep the leg elevated above heart level when possible, which helps fluid drain away from the joint through gravity. Compression with an elastic bandage can also limit further swelling, though it shouldn’t be wrapped so tightly that it causes numbness or tingling below the knee.
Over-the-counter anti-inflammatory medications like ibuprofen or naproxen can reduce both pain and swelling by dampening the inflammatory response. For osteoarthritis-related swelling that doesn’t respond to oral medications, injections directly into the joint are sometimes used. Research comparing anti-inflammatory injections to steroid injections for knee osteoarthritis found that both produced significant improvements in pain and function at one week, one month, and three months, with benefits lasting up to 24 weeks in some studies. Neither approach clearly outperformed the other for knee osteoarthritis specifically.
Patterns Worth Paying Attention To
A single episode of knee swelling after an obvious cause, like a long hike or a stumble, usually resolves with basic home care within a few days to a couple of weeks. Swelling that keeps coming back, affects both knees, or is accompanied by joint stiffness lasting more than 30 minutes each morning could suggest an inflammatory arthritis like rheumatoid arthritis, which involves the immune system attacking joint tissue.
Swelling that doesn’t improve with rest and ice, that progressively worsens, or that locks the knee so you can’t fully bend or straighten it warrants a closer look. A locked knee with swelling after a twisting injury often means a torn meniscus fragment is caught in the joint. Swelling combined with instability, where the knee feels like it might buckle or give way, raises concern for a ligament tear that may need more targeted treatment.