What causes knee swelling, and when should I see a doctor?

Knee swelling occurs when excess fluid builds up inside or around the knee joint. It can happen after an injury, repeated overuse, arthritis, or another medical condition. Common causes include ligament injuries such as an ACL tear, meniscus damage, osteoarthritis, fractures, bursitis, gout, inflammatory arthritis, and joint infection.

Mild swelling after increased activity may sometimes improve with rest, avoiding aggravating activities, applying ice for short periods, and elevation. However, repeated swelling should not be ignored because treatment depends on the underlying cause. A doctor may use an examination, X-rays, ultrasound, MRI, or occasionally test fluid from the joint to understand why swelling is occurring.

You should see an orthopaedic doctor if swelling is severe, keeps returning, does not improve, or makes it difficult to bend, straighten, or put weight on the knee. Urgent medical attention is particularly important if the knee suddenly becomes very swollen, red, hot, or extremely painful, especially if there is fever. These symptoms can sometimes indicate an infection or another condition that requires prompt treatment.