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1. Injury or Trauma
The most common reason for swelling in just one knee is trauma or overuse. You may not even remember the specific moment of injury—especially if it was a repetitive motion injury. Some possible issues include:
Ligament sprain or tear (ACL, MCL)
Meniscus tear
Patellar dislocation
Fracture or bone bruise
In these cases, swelling often appears within hours and may be accompanied by pain, stiffness, or a feeling of instability in the joint.
2. Bursitis
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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5. Osteoarthritis (Localized)
Though osteoarthritis typically affects both knees over time, it can start in just one knee, especially if that knee has experienced past injuries or bears more weight. Swelling may be mild to moderate and worsen with activity.
