Why Is One Knee Swollen but Not the Other?
Swelling in just one knee can be alarming, especially when the other is completely normal. While it’s easy to brush it off as a minor injury or strain, a swollen knee could point to several underlying issues—some mild, others more serious.
Here’s a look at the most common reasons only one knee may be swollen:
1. Injury or Overuse
If you’ve recently twisted, bumped, or overused your knee, it may respond with swelling. Common culprits include:
- Ligament injuries (like ACL or MCL tears)
- Meniscus tear
- Tendonitis or bursitis
Even a small strain can trigger fluid buildup in the affected knee.
2. Arthritis (Localized)
Osteoarthritis often starts in one joint and may cause swelling, stiffness, and pain in a single knee. Inflammatory arthritis (like gout or pseudogout) can also affect one knee, causing sudden and intense swelling.
3. Bursitis
Prepatellar bursitis (also called “housemaid’s knee”) occurs when the small fluid-filled sac (bursa) in front of the kneecap gets irritated. It’s often due to kneeling too much, trauma, or infection.
4. Gout or Pseudogout
Crystals forming inside the joint can cause sudden swelling, redness, and pain. Gout is due to uric acid buildup; pseudogout is caused by calcium crystals. These usually affect just one joint at a time.
5. Infection (Septic Arthritis)
A swollen, red, and extremely painful knee—especially if accompanied by fever—could be a sign of joint infection. This is a medical emergency and requires immediate attention to avoid joint damage.
6. Baker’s Cyst
This is a fluid-filled sac that forms behind the knee. It can lead to swelling in the back of the knee and even extend down the leg. It’s often linked to arthritis or knee injuries.
7. Blood Clot or Vascular Issue
While less common, a blood clot or circulation problem can cause one knee or leg to swell. This is serious and needs urgent medical care—especially if swelling is accompanied by warmth, pain, or skin color changes.
When Should You See a Doctor?
You should seek medical attention if:
- The swelling comes on suddenly or is severe
- You can’t bear weight or move the knee
- There’s redness, warmth, or fever
- The swelling persists for more than a few days
A proper diagnosis often includes physical exams, imaging (like X-rays or MRIs), and sometimes fluid aspiration from the joint.
Bottom Line
One swollen knee isn’t something to ignore. Whether it’s due to a mild strain or a more serious issue like infection or arthritis, understanding the cause is key to getting the right treatment—and preventing further damage.
