Why Is One Knee Swollen but Not the Other

Bursae are small fluid-filled sacs that cushion the knee. If one becomes irritated due to pressure, trauma, or kneeling for long periods, it can result in prepatellar bursitis (also known as “housemaid’s knee”). This usually affects only one knee and causes visible swelling at the front of the knee.

3. Infection

A joint infection or septic arthritis is a serious condition that can cause rapid swelling, redness, warmth, and severe pain in just one knee. This usually comes with fever or chills and requires urgent medical treatment. Joint aspiration may be needed to drain infected fluid.

4. Gout or Pseudogout

If the swelling comes on suddenly and is very painful, the cause may be gout—a type of arthritis caused by uric acid crystals. Pseudogout, which involves calcium crystals, can cause similar symptoms. These conditions often affect only one joint at a time and may return periodically.

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