🦵 Why Is Only One Knee Swollen? What It Might Mean for Your Health

ByJames William

If just one of your knees suddenly becomes swollen while the other stays normal, it can be worrying. Swelling in both knees often points to a general health issue like arthritis. But swelling in only one knee usually means there’s a specific problem with that knee. Knowing what could be causing this can help you figure out if it’s something small or if you need to see a doctor right away.

1. 🚑 Injury or Trauma: The Most Common Reason

The most common cause of swelling in one knee is an injury—even if you don’t remember getting hurt. Injuries can build up slowly, especially from sports or doing the same movement over and over.

Some common injuries that cause swelling include:

  • Sprains or tears in the ACL or MCL

  • Meniscus tears

  • A kneecap that moved out of place

  • Bone bruises or fractures

Swelling from an injury usually happens within a few hours. You might also feel pain, stiffness, or like your knee could give out.

2. 💦 Bursitis: Also Called “Housemaid’s Knee”

Your knees have tiny sacs filled with fluid called bursae. These protect your joints. But when one gets irritated—often from kneeling, getting bumped, or using your knee too much—it can swell up. This is called bursitis, especially prepatellar bursitis, which happens at the front of the knee.

The swelling often looks like a soft, puffy lump and is usually only in one knee.

3. 🦠 Infection: A Serious Emergency

If your knee becomes swollen, red, hot, and very painful, and you also have a fever, it could be an infection in the joint (called septic arthritis). This is a medical emergency and needs quick treatment to avoid serious damage.

Watch for these signs:

  • Sudden swelling

  • Strong pain

  • Warm and red skin

  • Fever or chills

Doctors may need to remove fluid from the knee and give you antibiotics right away.

4. 🧂 Gout or Pseudogout: Swelling From Crystals

If the swelling happens suddenly and is very painful, it might be caused by gout or pseudogout.

  • Gout is caused by a buildup of uric acid crystals.

  • Pseudogout comes from calcium crystals.

Both conditions usually affect only one joint at a time, often the knee or big toe. The pain can come and go, and symptoms can feel like an infection.

5. 🦴 Osteoarthritis (Affects One Knee First)

Osteoarthritis usually affects both knees, but it can start in just one—especially if that knee has been injured or used more.

Signs include:

  • Light to medium swelling
  • Pain that gets worse with movement
  • Stiffness, especially in the morning or after sitting

This kind of swelling builds up slowly and is usually less serious than swelling from an injury or infection.

6. 💧 Baker’s Cyst: Swelling at the Back of the Knee

A Baker’s cyst happens when fluid builds up behind your knee. It’s often caused by arthritis or damage to the cartilage in the joint.

You might notice:

  • A tight or full feeling behind your knee
  • Pain when bending or straightening your leg
  • Swelling that gets worse after being active

If the cyst bursts, it can cause swelling in your lower leg that may look like a blood clot.

7. 🩸 Blood Clot or Circulation Problems

In rare cases, a blood clot in a deep vein (DVT) can cause swelling near your knee and lower leg. Signs to watch for:

  • Swelling in the calf
  • Warm or red skin
  • Pain, especially when walking

If you think you have a blood clot, go to the emergency room right away.

🚨 When You Should Call a Doctor

One swollen knee doesn’t always mean it’s serious—but you shouldn’t ignore it. See a doctor as soon as possible if:

  • The swelling is sudden or really bad
  • The knee feels hot, red, or extremely painful
  • You have a fever or chills
  • You can’t bend or walk on the knee
  • The swelling lasts more than a few days

🧠 Final Thoughts: Don’t Wait to Get Help

A swollen knee could be your body’s way of warning you something’s wrong. It might be a simple strain—or it could be something more serious like an infection or a long-term joint problem. The only way to know for sure is to get the right diagnosis.

Pay attention to how it started, whether it hurts, and how long it lasts. These small clues can help your doctor figure out the real cause and help you get back to moving without pain.

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