The Lump Behind the Knee

What a Baker's Cyst Actually Is

A Baker's cyst (also called a popliteal cyst) is a fluid-filled swelling that forms behind the knee when synovial fluid, the natural lubricating fluid inside the knee joint, gets pushed backward through a small, one-way weak spot in the joint capsule and pools in a bursa behind the knee. It is not a tumor or a growth of new tissue; it is existing joint fluid collecting somewhere it does not normally accumulate in that volume.

Because the fluid is coming from the knee joint itself, a Baker's cyst is almost always a sign that something inside the knee is producing more fluid than usual, rather than an isolated problem confined to the back of the knee. Treating the cyst without addressing what is driving the extra fluid production tends to lead to it coming back.

What Causes the Extra Fluid

Knee osteoarthritis is the most common underlying cause in adults, since the arthritic joint produces more synovial fluid as it tries to cushion itself, and that excess volume finds its way into a Baker's cyst more easily than in a healthy knee 1Ref 1American Academy of Orthopaedic Surgeons (OrthoInfo) (2024).Arthritis of the Knee.Patient-facing overview of knee arthritis, used to support that knee osteoarthritis is a common cause of excess joint fluid production.. Osteoarthritis is the most common form of arthritis overall, and the knee is one of the joints it affects most often 2Ref 2National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) (2023).Osteoarthritis.Osteoarthritis is the most common form of arthritis and commonly affects the knee; supports general framing of osteoarthritis prevalence and joint involvement..

A torn meniscus is the other frequent driver, particularly in people who develop a cyst more suddenly or at a younger age, since a tear can cause the joint to swell with reactive fluid in the same way arthritis does 3Ref 3American Academy of Orthopaedic Surgeons (OrthoInfo) (2024).Meniscus Tears.Patient-facing overview of meniscus tears as a common knee injury, used to support meniscus tear as a cause of reactive joint fluid and cyst formation.. Rheumatoid arthritis and other inflammatory joint conditions can also produce enough synovial fluid to cause a cyst, and in children, a Baker's cyst sometimes appears without any clear underlying joint problem at all and can resolve on its own.

What It Feels Like

A Baker's cyst typically feels like a soft, somewhat squishy fullness behind the knee, most noticeable when the knee is fully straightened and less obvious when it's bent, since bending relaxes the tissue around it. Many are painless and are simply noticed as an unusual fullness; others cause tightness, aching, or stiffness behind the knee, particularly after standing or walking for a while.

• Fullness or a lump behind the knee, more prominent with the leg straight
• Tightness or aching, especially after activity, rather than sharp pain
• Size that can fluctuate, larger after a day of activity, smaller after rest
• A knee that also shows signs of its underlying cause, such as arthritis stiffness or a locking, catching sensation from a meniscus tear

The cyst itself is rarely what limits someone's activity; the underlying knee problem generating the fluid usually accounts for more of the day-to-day symptoms. Some people only discover the cyst incidentally, on an MRI ordered for another reason, without ever having noticed a lump at all.

When It Ruptures: Baker's Cyst vs. Blood Clot

A Baker's cyst can occasionally rupture, releasing its fluid into the surrounding calf tissue, and when that happens it causes sudden calf pain, swelling, redness, and sometimes bruising that tracks down toward the ankle. This is the scenario that causes the most confusion and the most concern, because a ruptured Baker's cyst can look and feel very similar to a deep vein thrombosis, a blood clot in the calf, which is a genuinely urgent problem that a cyst rupture is not.

There is no way to tell the two apart with confidence just by how the calf looks or feels; both cause a swollen, tender, sometimes discolored calf, and both can come on suddenly. Anyone with new calf swelling, especially one-sided swelling with pain, warmth, or redness, needs prompt medical evaluation, typically with an ultrasound, to sort out which one is actually happening, because treating a real blood clot as a ruptured cyst carries real risk.

How It's Diagnosed

A clinician can often recognize a Baker's cyst on physical exam alone, based on its location, softness, and how it changes with knee position, but ultrasound is the typical next step to confirm the diagnosis and rule out other causes of swelling behind the knee, including a blood clot if there's any suspicion of one. Ultrasound can also usually identify whether the cyst has ruptured.

Because a Baker's cyst is a downstream sign of another knee problem, evaluation often includes assessing the knee itself, sometimes with an MRI, to identify whether arthritis, a meniscus tear, or another cause is driving the fluid production, particularly when the cyst is large, painful, or recurrent.

What Treatment Looks Like

Many Baker's cysts, especially smaller ones that aren't causing significant symptoms, don't need any specific treatment and can simply be monitored. When treatment is needed, it usually targets the underlying cause rather than the cyst directly: physical therapy and anti-inflammatory measures for arthritis, or addressing a meniscus tear if one is present and symptomatic.

For a cyst that is large, uncomfortable, or limiting knee motion, a clinician may drain the fluid with a needle (aspiration), sometimes combined with a corticosteroid injection into the joint to reduce ongoing fluid production. Surgical removal of the cyst itself is uncommon and generally reserved for cysts that keep recurring despite treating the underlying cause, since removing the cyst without addressing the source problem behind it tends not to hold.

Rest, ice, and elevation of the leg can ease discomfort in the days after a cyst first becomes noticeable or after it ruptures, much the way those measures help with other soft tissue swelling. A compression wrap around the calf is sometimes used for comfort during that period, though it does nothing to change the underlying fluid production driving the cyst itself.