A swollen knee is never normal and warrants a trip to the doctor. But for the medical sleuths among you, here’s how to assess the situation—and prevent it from happening again.
A swollen knee may also be painful, stiff and keep you from fully extending your leg. To figure out why it’s swollen, identify when it first puffed up.
Within an hour or two of activity: Swelling that occurs soon after an activity is much more serious than swelling that shows up, say, the next day. Example: You twist your knee trail running and it swells up. This is a sign of bleeding within the knee, or hemarthrosis. Basically, something has been torn or broken. About 80 percent of hemarthrosis cases are caused by a torn anterior cruciate ligament (ACL). Sudden-onset effusion is a sign that something serious is going on.
Hours later or the day after activity: Swelling that arrives later is generally caused by excess synovial fluid (the lubricant in joints) in the knee, much like too much oil in a car. Overuse and an underlying medical condition are the most common causes.
Something in there is irritated or rubbing during activity, and the body responds by over-lubricating the knee to compensate. Osteoarthritis is one of the most common causes, but far less common maladies can also be the culprits, such as rheumatoid arthritis, infection, gout, bursitis, cysts, bleeding disorders, tumors and Lyme disease. Advancing age and participation in sports that require sudden changes in speed and direction raise your risk.
See a doctor. Anytime you have a swollen joint, you should see a doctor. This is especially true with sudden-onset effusion
Employ dynamic rest. Even if the swelling comes without pain, avoid loading the knee until the swelling subsides. Trade knee-loading exercises for intense upper-body and core work.
Ice it. Apply ice for 15 minutes 4–6 times a day for the first two days of swelling. Elevating the knee as you ice it can also help reduce the swelling.
Strengthen your legs. Strong legs protect your knees. Be sure your workout regimen includes regular lower-body strength training in addition to any running, and biking that you do. You may not be able to prevent knee effusion caused by health issues, but properly trained legs will help your knees recover in the long run no matter what the issue turns out to be.
When to Call a Doctor
My philosophy is that any time you have joint swelling, you should see a doctor because you need to figure out what the problem is. Try to pinpoint when the effusion began in relation to your athletic activities, especially if your knee has swelled up with no discernible cause such as an overt injury and you have no other symptoms that suggest a related illness. A physician can help shed light on the mystery, whether by physical exam, analysis of fluid drawn from the knee, or review of images such as MRIs or X-rays.
Also, if the knee is swollen but has some extra symptoms like redness or warmth of the skin and/or you have a fever, it could signal an infection. Get to an ER pronto.