Knee Swelling: What Causes a Swollen Knee and What to Do

When a patient comes in and says their knee is swollen, my first question is how it happened. Was it an injury, or is this something chronic that has been building for a while? The answer usually points to the cause.

First question: how did it happen?

These are the things I ask about with a swollen knee:

• An injury. A fall, a twist, a sports injury. Did they tear their ACL, sprain a ligament, or injure the meniscus?
• Something chronic. Usually osteoarthritis, or an old injury that never fully healed.
• Gout. Have they had gout flares before?
• Bursitis. Swelling right over the front of the kneecap (prepatellar bursitis) is in a sac outside the joint, not inside it.
• Autoimmune conditions, such as rheumatoid arthritis.
• Activity level. Are they athletic, and what were they doing when it started?

There’s no single answer, so the diagnosis really comes from the history, the exam and X-rays together.

Swelling out of nowhere

A knee can swell up with no injury for many reasons, including the ones above. Most often it’s a chronic condition. Arthritis, and injuries that don’t fully heal, are long, drawn-out processes. Many people are doing fine until they’re not, and then the knee, often bone-on-bone by that point, swells up.

So swelling that seems to come out of nowhere is often a slow problem that has finally shown itself. A gout flare or another inflammatory condition can also come on suddenly.

Swelling after a fall or twist

What I worry about depends on how it happened:

• A twist without contact. That’s when I start to worry about an ACL tear.
• Pain with deep bending. That makes me think about a meniscus tear.
• Younger, active patients. I worry more about ligament and meniscus injuries.

After a real injury, come in and get evaluated. I always recommend it.

What the fluid is, and why your knee makes it

“Water on the knee” is extra fluid inside the joint. The medical term is an effusion. Most of the time I see it with acute injuries, meniscus tears and arthritis.

When the knee is full of fluid, it’s also hard to bend or straighten. That’s one of the biggest reasons an arthritic knee gets stiff. See knee stiffness. A lot of fluid in the knee often means a Baker’s cyst behind the knee too.

How I check a swollen knee

Most people have some difference between their two knees, and a knee that had surgery before may look bigger than the other. When the difference is significant, here’s what I do:

• Look at the sides of the kneecap. I have the patient straighten the knee. Normally the skin on either side of the kneecap dimples in. When there’s fluid in the knee, it fills those dimples and bulges out.
• A fluid shift test. I push the fluid from one side of the knee to the other. It tells me pretty quickly whether there’s water on the knee.
• X-rays. They show the alignment of the knee and whether there’s arthritis. It’s a very quick way to tell whether this is chronic arthritis or something else, such as a problem with the kneecap.
• An MRI if the knee doesn’t improve, to look at the meniscus, ligaments and cartilage.

An ultrasound is rarely needed for a swollen knee.

Should a swollen knee be drained?

Very rarely. A knee can be drained if the swelling is significant and nothing else is helping. The problem is that draining doesn’t fix what’s causing the fluid, so the knee usually fills back up.

Treatment, step by step

More on each option: nonsurgical care, cortisone, gel, and PRP. When a swollen, arthritic knee keeps limiting your life and those treatments haven’t worked, that’s when a knee replacement comes up. See the signs you need a knee replacement.

Red flags: when to be seen right away