We spoke with J. Dean Cole, MD, an orthopedic surgeon who specializes in limb trauma care, to answer some questions about the causes of a swollen knee and how it’s diagnosed.

See What Causes a Swollen Knee (Water on the Knee)?

Knee bursitis that becomes infected, known as septic bursitis, requires prompt medical attention. Read more about Septic Bursitis

If a person has a swollen knee, when should he or she see a doctor? Is knee swelling ever dangerous?

If a patient has a severe swollen knee without a history of trauma, he or she should be examined by a doctor. If there are signs of infection—such as fever, chills, and malaise—a patient should receive urgent care.

See How to Care for a Swollen Knee

There are a lot of potential causes for knee swelling. When you’re diagnosing a patient for knee swelling, what are some of the things you look for?

The diagnosis considerations for a swollen knee should include infection, trauma, and—in the cases of gout and pseudogout—the joint tissue’s response to unwelcome microscopic crystals.

Inflammatory and viral conditions are other causes of knee swelling. For example, several types of arthritis, including rheumatoid arthritis and osteoarthritis, can also cause swelling and inflammation in the knees. To aid in the diagnosis, testing of the knee fluid (known as arthrocentesis) is important.

When do you decide to use medical imaging, such as an ultrasound or MRI, to help diagnose knee swelling that occurs absent any trauma? When is medical imaging not necessary?

Medical imaging is helpful to rule out knee swelling causes such as trauma. Change to the delicate synovial tissue that surrounds the joint is also something that can be evaluated by an MRI. If the patient’s exam and x-rays do not help identify diagnosis, then an MRI is useful.


When do you recommend a patient undergo arthrocentesis to extract fluid from the knee?

Knee arthrocentesis is a very helpful diagnostic procedure when we don’t know what is causing the knee swelling, because it allows us to extract and analyze the joint fluid. I believe this is an important diagnostic procedure for the initial evaluation if I did not know the patients diagnosis.

See Diagnosis through Synovial Fluid Analysis

Knee arthrocentesis can also be used when a patient has a traumatic or arthritic condition that has caused effusion (fluid build-up) in order to relieve pressure from the knee, which can help improve the patient’s pain and ability to stay mobile.

See The Joint Aspiration Procedure

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More Causes of a Swollen Knee (Water on the Knee)