Knee Swelling (Swollen Knee)

Inside vs outside the knee

Joint effusion is fluid inside the joint capsule. The whole knee looks puffy, the dimples beside the kneecap disappear, and the knee feels tight when bending. The kneecap may feel like it floats.

Swelling outside the joint sits in a bursa (a fluid sac) or the soft tissues. It is usually localized, such as a soft egg-shaped lump on the front of the kneecap (prepatellar bursitis) or a lump at the back of the knee (Baker's cyst).

Knee swelling after an injury

How fast the knee swells after injury is one of the best clues:

• ACL tear: a twisting or landing injury with a pop, immediate swelling and a sense of the knee giving way. Common in soccer, basketball and skiing.
• Meniscus tear: twisting with the foot planted, swelling over the next day, pain along the joint line, catching or locking. In older adults, it can happen with minimal force.
• Fracture: a fall or direct blow, with severe pain and difficulty bearing weight.
• Kneecap dislocation: the kneecap slips out to the outer side, usually in teenagers. It often pops back but leaves a swollen knee.

See knee ligament and meniscus tears for details on these injuries.

Swollen knee without injury

Osteoarthritis

Osteoarthritis is the most common cause in people over 50. The knee aches with activity, stiffens after rest for a few minutes, and swells intermittently. It may creak (crepitus) and gradually change shape.

Gout and pseudogout

Gout is caused by uric acid crystals. It brings sudden, intense pain, redness, heat and swelling, often overnight, and can mimic infection. Pseudogout (calcium pyrophosphate crystals) is common in older adults and frequently affects the knee.

Inflammatory arthritis

Rheumatoid arthritis, psoriatic arthritis and reactive arthritis cause warm, swollen knees, often with other joints involved, morning stiffness and fatigue. Lyme disease can cause a large, relatively painless knee swelling weeks to months after a tick bite.

Septic arthritis

Septic arthritis is a bacterial infection of the joint. The knee is red, hot, very painful and swollen, and any movement hurts. Fever is common but not always present. It can destroy cartilage within days and needs urgent drainage and antibiotics. Higher risk with diabetes, rheumatoid arthritis, joint replacements, injecting drug use and recent joint injections.

Bursitis

Prepatellar bursitis ("housemaid's knee") causes a soft swelling on the front of the kneecap from kneeling. If it becomes red and hot, it may be infected. Pes anserine bursitis causes pain on the inner knee below the joint.

Baker's cyst

A Baker's cyst is a fluid-filled swelling at the back of the knee, usually from arthritis or a meniscus tear. If it bursts, fluid tracks into the calf and causes calf pain and swelling that can look like a blood clot.

Bleeding into the joint

Hemarthrosis without significant injury can happen in hemophilia, with blood thinners, or with certain growths in the joint lining.

How doctors diagnose knee swelling

A doctor will ask about injury, timing, fever, other joint symptoms, gout or arthritis history and medicines. The examination checks for effusion, warmth, range of motion, ligament stability, joint line tenderness and the calf.

Tests may include:

• X-ray: fracture, arthritis, alignment, loose bodies
• Joint aspiration (arthrocentesis): draining fluid with a needle. Clear fluid suggests osteoarthritis, cloudy fluid suggests inflammation or infection, and blood suggests a ligament tear or fracture. Fluid is tested for crystals, cell count and bacteria. Removing it also relieves pressure.
• MRI: ligament and meniscus tears and cartilage damage
• Ultrasound: Baker's cysts, bursitis, and to rule out a blood clot in the calf
• Blood tests: inflammatory markers, uric acid, rheumatoid factor and Lyme serology

How to reduce knee swelling

First aid and home care

• Rest from activities that cause pain, but keep the knee gently moving.
• Ice wrapped in a cloth for 15 to 20 minutes every 2 to 3 hours in the first 48 to 72 hours.
• Compression with an elastic bandage or knee sleeve, not too tight.
• Elevation above the level of the heart when sitting or lying.
• Pain relief: acetaminophen (paracetamol) or ibuprofen, following the label dose. Avoid NSAIDs if you have kidney disease, stomach ulcers or take blood thinners. Topical NSAID gels are a good option for arthritis.
• Crutches if you cannot walk normally.

Medical treatment

• Physical therapy: strengthening the quadriceps and hip muscles is central to recovery from most knee problems.
• Draining the fluid and steroid injections for arthritis or bursitis (once infection is ruled out).
• Gout treatment: anti-inflammatory medicines, colchicine or steroids for attacks, and uric acid-lowering medicine to prevent them.
• Antibiotics and joint washout for septic arthritis.
• Surgery: ACL reconstruction, meniscus repair, fracture fixation or knee replacement for advanced arthritis.

Preventing knee swelling

• Keep a healthy weight. Each pound of body weight adds several pounds of force across the knee when walking.
• Build leg strength and balance, especially for sports with cutting and jumping.
• Use knee pads for kneeling work.
• Increase training gradually.
• Limit alcohol and high-purine foods if you have gout.

Frequently asked questions

What causes a swollen knee without injury?

The most common causes are osteoarthritis, gout, pseudogout, inflammatory arthritis such as rheumatoid arthritis, bursitis and a Baker's cyst. Infection (septic arthritis) is less common but serious. Lyme disease can also cause knee swelling. A doctor may drain fluid to find the cause.

What does water on the knee mean?

Water on the knee is a common name for a knee effusion: excess fluid inside the joint. It is a sign of a problem rather than a diagnosis, caused by injury, arthritis, gout or infection. The fluid can be drained and tested to identify the cause.

How long does knee swelling last?

After a minor sprain, swelling usually improves within 1 to 2 weeks. Swelling from an ACL or meniscus tear can last several weeks. Arthritis-related swelling comes and goes with flares. Swelling that lasts more than 2 weeks or keeps returning should be checked.

How do I reduce swelling in my knee fast?

Rest, apply ice wrapped in a cloth for 15 to 20 minutes several times a day, use a compression sleeve, and keep the leg raised above heart level. Over-the-counter anti-inflammatory medicine can help. If the knee is hot, red or you have a fever, see a doctor instead.

When is a swollen knee an emergency?

Get urgent care if the knee is hot, red and very painful with fever, if it swells rapidly after an injury, if you cannot bear weight, if the knee is deformed, or if the leg below is cold, pale or numb. Calf swelling with pain may signal a blood clot.