If you’ve noticed that only one of your knees is swollen, red, or tender—while the other feels completely normal—it’s natural to feel concerned. While swelling in both knees often points to systemic issues like arthritis or gout, swelling in just one knee usually indicates something localized, specific, and potentially treatable.
Understanding the possible causes, symptoms to watch for, and treatment options can help you manage the condition effectively or seek timely medical care.
What Does Unilateral Knee Swelling Mean?
Unilateral knee swelling refers to inflammation and fluid accumulation in only one knee joint. This swelling may be accompanied by pain, warmth, stiffness, redness, and limited movement. The causes vary widely—from minor injuries to serious infections—so careful evaluation is important.
Swelling occurs when excess fluid builds up inside or around the knee joint, often due to injury, inflammation, or infection.
Common Causes of One-Sided Knee Swelling
1. Injury or Trauma
One of the most common reasons for swelling in a single knee is injury. This can include:
Ligament injuries such as ACL (anterior cruciate ligament) or MCL (medial collateral ligament) tears, often caused by twisting motions or sudden impact.
Meniscal tears that occur from sudden twisting or degenerative changes with age.
Fractures or bone bruises around the knee from falls or accidents.
Contusions or bruising causing localized swelling and tenderness.
Symptoms include immediate or delayed swelling, bruising, pain with movement, and sometimes instability or difficulty bearing weight.
2. Bursitis
Bursae are small fluid-filled sacs that cushion the knee joint. Inflammation of these sacs—called bursitis—can cause noticeable swelling.
Prepatellar bursitis is swelling over the kneecap, often from prolonged kneeling (“housemaid’s knee”).
Pes anserine bursitis occurs on the inner side of the knee.
Bursitis causes localized swelling, warmth, and tenderness, often worsened by pressure or movement.
3. Infection (Septic Arthritis)
Septic arthritis is an infection inside the knee joint and requires immediate medical attention.
Symptoms include rapid swelling, intense pain, redness, warmth, fever, and difficulty moving the knee.
It can occur from bacteria entering through a wound, bloodstream, or after surgery.
Left untreated, septic arthritis can cause joint destruction and systemic illness.
4. Crystal-Induced Arthritis (Gout and Pseudogout)
Gout is caused by the buildup of uric acid crystals, while pseudogout involves calcium pyrophosphate crystals.
Both conditions usually affect one joint initially.
Symptoms include sudden, severe pain, swelling, redness, and heat.
Attacks may last days to weeks and often recur.
5. Osteoarthritis (Localized Degenerative Joint Disease)
While osteoarthritis often affects multiple joints, one knee may show more significant wear and tear, leading to swelling due to inflammation and joint fluid buildup.
Symptoms include chronic pain, stiffness, swelling after activity, and decreased range of motion.
6. Baker’s Cyst Rupture
A Baker’s cyst is a fluid-filled swelling behind the knee caused by excess joint fluid.
It can enlarge and sometimes rupture, causing pain, swelling, and bruising in the back of the knee and calf.
It’s often associated with underlying knee problems like arthritis or meniscal tears.
7. Other Causes
Tendonitis or tendinopathy causing inflammation around the knee tendons.
Deep vein thrombosis (DVT) can cause calf and knee swelling but usually includes other symptoms like pain, warmth, and redness in the leg.
Tumors or cysts (rare) may cause localized swelling.
Symptoms to Monitor
If your swollen knee is accompanied by any of the following, you should seek prompt medical care:
Severe pain or inability to put weight on the leg
Rapid swelling that develops over hours
Redness, warmth, and fever (signs of infection)
Deformity or instability in the knee
Persistent swelling lasting more than a week without improvement
History of trauma or recent surgery
How Is Unilateral Knee Swelling Diagnosed?
Doctors will typically begin with:
Medical history and physical exam: Assessing injury history, pain characteristics, and swelling location.
Imaging: X-rays to check for fractures or arthritis; MRI for soft tissue injuries (ligaments, meniscus); ultrasound for cysts or fluid pockets.
Joint aspiration: Removing and analyzing joint fluid to check for infection, crystals, or inflammation.
Blood tests: To identify infection or markers of systemic diseases.
Treatment Options
At-Home Care for Mild Cases
Rest: Avoid activities that aggravate symptoms.
Ice: Apply ice packs for 15-20 minutes several times daily.
Compression: Use a wrap or compression sleeve to reduce swelling.
Elevation: Keep the leg raised above heart level to help fluid drain.
Over-the-counter medications: NSAIDs (ibuprofen, naproxen) can reduce pain and inflammation.
Medical Treatments
Physical therapy: Strengthening and mobility exercises for recovery and preventing future injury.
Medications: Antibiotics for infections; corticosteroids or colchicine for gout; pain management medications.
Aspiration and injections: Removing excess fluid or injecting anti-inflammatory medications.
Surgery: In cases of severe ligament tears, fractures, or persistent cysts.
When to See a Doctor
If swelling persists, worsens, or is accompanied by severe symptoms, a healthcare provider should evaluate you to prevent complications. Early diagnosis and treatment are key to avoiding long-term joint damage.
Final Thoughts
One knee swelling while the other remains normal is often a sign of a localized problem that ranges from minor injuries to more serious conditions like infection or gout. Monitoring your symptoms, seeking prompt care when needed, and following treatment recommendations will help you get back on your feet and protect your joint health.
