This article describes 12 common causes of knee swelling. Below are causes 7 through 12; causes 1 through 6 are listed on the previous page of this article.

7. Baker's Cyst
Swelling at the back of the knee can indicate a Baker's cyst. The cyst may have no other symptoms or may be accompanied by pain and stiffness.

Patients should treat the swollen knee at home using the R.I.C.E. formula (rest, ice, compression, and elevation) and also consult with their doctor, who can eliminate other possible diagnoses.

See How to Care for a Swollen Knee for an explanation of the R.I.C.E. method

8. Juvenile Rheumatoid Arthritis
Aching, swollen joints may cause a child to limp or seem clumsy and could be signs of juvenile rheumatoid arthritis.

Children with juvenile rheumatoid arthritis sometimes also have a fever or rash. Caregivers should contact a doctor if a child's symptoms persist for a week or more.

See Juvenile Idiopathic Arthritis and Other Rheumatologic Diseases in Children


9. Osgood-Schlatter Disease
Most common in active tweens and teens, Osgood-Schlatter disease is an inflammation of the patellar tendon in the knee. After diagnosis, this condition can usually be treated at home and will resolve as the child grows.

10. Septic Arthritis
Bacteria or other microorganisms can penetrate the delicate lining that surrounds the knee joint, infecting the joint and potentially causing it to fill with pus. Sudden knee swelling, intense knee pain, and fever are signs of septic arthritis.

See How Infection Can Lead to Severe Knee Pain

Patients should seek medical attention immediately if they suspect symptoms are caused by septic arthritis.

11. Reactive Arthritis (Including Reiter's Syndrome)
Certain types of bacterial infections (e.g. chlamydia and gastrointestinal infections) can spur an inflammatory immune response in the body that may cause pain and swelling in joints.

See Infections that Can Cause Reactive Arthritis

Sometimes reactive arthritis symptoms resolve on their own or with over-the-counter anti-inflammatory medications; however, serious or chronic cases require medical attention.

See How Doctors Treat Reactive Arthritis

12. Tumor
While relatively uncommon, a benign or malignant tumor can cause a swollen knee. A tumor may be accompanied by pain that is more noticeable at night, night sweats, fever, and weight loss.

In This Article:

When to Contact a Doctor

If the cause of the swollen knee is uncertain, a doctor may want to draw fluid from the joint, a process called arthrocentesis or joint aspiration, or order imaging studies, such as X-rays or an MRI.

See The Joint Aspiration Procedure


A patient should contact a doctor immediately if he or she suspects septic bursitis or septic arthritis. Left untreated the infection can spread to other parts of the body and become a grave medical condition.

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