Noticing that just one of your knees is swollen—while the other remains completely normal—can be concerning. While swelling in both knees often suggests a systemic or chronic condition like rheumatoid arthritis, swelling isolated to one knee typically points to a localized issue. Knowing the possible causes of one-sided knee swelling can help you determine whether it's a minor issue you can manage at home or a more serious condition that requires medical evaluation.

1. Injury or Trauma

One of the most common reasons for swelling in a single knee is injury or overuse. The knee is a complex joint that bears a lot of pressure and is vulnerable to both acute injuries and wear-and-tear damage. Sometimes, the injury is obvious—like a fall or sports-related impact—but other times, it results from repetitive strain and develops gradually.

Common causes include:

• Ligament sprain or tear (such as ACL or MCL injuries)

• Meniscus tear

• Patellar dislocation

• Bone bruise or fracture

Swelling from trauma often develops quickly, sometimes within hours, and may be accompanied by sharp pain, limited range of motion, stiffness, or a sensation of the knee "giving out."

2. Bursitis

The knee contains several small, fluid-filled sacs called bursae that cushion the joint and reduce friction. When one of these bursae becomes inflamed—often due to repeated kneeling, direct impact, or prolonged pressure—it can lead to prepatellar bursitis, commonly known as “housemaid’s knee.”

This typically causes visible swelling at the front of the knee and may feel tender or warm to the touch. While not usually dangerous, it can be uncomfortable and limit movement.

3. Infection (Septic Arthritis)

A joint infection is a serious and potentially dangerous cause of one-sided knee swelling. Known as septic arthritis, this condition occurs when bacteria enter the joint space, leading to rapid inflammation and fluid buildup.

Symptoms include:

• Sudden swelling and intense pain

• Redness and warmth around the knee

• Fever, chills, or general malaise

This condition requires immediate medical treatment—often hospitalization, antibiotics, and joint drainage—to prevent permanent damage.

4. Gout or Pseudogout

If the knee swells up quickly and becomes extremely painful—especially during the night—the cause might be gout or pseudogout. These are forms of arthritis triggered by the buildup of microscopic crystals in the joint:

• Gout is caused by uric acid crystals

• Pseudogout involves calcium pyrophosphate crystals

Both typically affect only one joint at a time and can come on suddenly. The affected joint may also be red, warm, and difficult to move. These flare-ups can resolve on their own but often require medication to manage symptoms and prevent recurrences.

5. Localized Osteoarthritis

Although osteoarthritis often affects both knees over time, it can begin in just one—particularly if that knee has sustained an injury in the past or bears more of your body weight due to posture or biomechanics.

In this case, swelling tends to be:

• Mild to moderate

• Worse after physical activity or long periods of standing

• Accompanied by stiffness, especially in the morning or after rest

Over time, wear and tear of the joint cartilage leads to inflammation and fluid buildup.

6. Baker’s Cyst

A Baker’s cyst is a fluid-filled sac that forms at the back of the knee, often due to underlying joint problems like arthritis or a cartilage tear. It can make the knee feel swollen, especially when bending or extending the leg fully.

Although not always painful, the cyst may burst, causing sharp pain and swelling in the calf—sometimes mistaken for a blood clot. Ultrasound or MRI can confirm the diagnosis.

7. Blood Clot or Circulatory Issue

While uncommon, a deep vein thrombosis (DVT)—a blood clot in the leg—can cause swelling around the knee or lower leg. This is more likely if the swelling is accompanied by:

• Pain in the calf

• Warmth or redness in the leg

• Shortness of breath (in severe cases)

A DVT is a medical emergency and needs immediate intervention to prevent serious complications like a pulmonary embolism.

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When to See a Doctor

You should seek medical attention if:

• The swelling appeared suddenly and is severe

• The knee feels hot, red, or painful to touch

• You have a fever or chills

• You can’t walk or bear weight on the knee

• Swelling persists for more than a few days without improvement

Delaying treatment can lead to worsening of the condition or permanent joint damage, especially in cases of infection, gout, or significant injury.

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Final Thoughts

While having one swollen knee isn't always cause for alarm, it's not something to ignore either. The cause might be as simple as a minor sprain or as serious as an infection or blood clot. Monitoring the symptoms and getting timely medical advice are key to avoiding long-term complications. Taking care of your knees today ensures they’ll keep supporting you well into the future.