What causes knee swelling?

A sudden swollen knee after a twist, fall, or change of direction can reflect damage to a ligament such as the ACL, or irritation of cartilage and other tissues. Swelling that develops gradually may occur with knee osteoarthritis or repeated strain. Bursitis affects a small fluid sac around the knee, while a Baker's cyst is fluid that collects behind the joint. Gout can trigger an intensely painful, hot, swollen knee when crystals irritate the joint. Septic arthritis is an infection within the joint and needs urgent medical assessment.

The location, speed of onset, warmth, redness, fever, ability to bear weight, and history of injury all matter. A clinician may examine the knee and, when appropriate, remove a sample of joint fluid for testing; joint aspiration can also relieve pressure in selected cases.

Reduce activities that increase pain, avoid kneeling and impact exercise, and use a cold pack wrapped in cloth for short periods. Elevating the leg may reduce the feeling of pressure. Do not try to drain the swelling yourself. Arrange an assessment if it persists, returns, limits movement, or follows a significant injury. A clinician can decide whether imaging, blood tests, or joint-fluid testing is useful.

Seek urgent care for a swollen knee that is hot or red with fever, severe pain, rapid onset, or inability to move or bear weight. Prompt assessment is also appropriate after a major injury, a deformity, or a knee that locks. These features can signal infection or substantial internal damage and should not be judged by swelling alone.

Ask what the swelling pattern suggests, whether the knee needs fluid testing or imaging, which activities are safe, and what changes should prompt urgent review. Mention any recent injury, fever, skin infection, new medicine, or previous gout episode.