A Baker’s cyst is a fluid-filled sac that forms in the hollow at the back of your knee, called the popliteal fossa. It develops when excess joint fluid gets pushed into a small pocket of tissue behind the knee, creating a visible or palpable bulge. Most Baker’s cysts aren’t dangerous on their own, but they signal that something else is going on inside the knee joint.
How a Baker’s Cyst Forms
Your knee joint naturally produces a slippery fluid that reduces friction and cushions the joint during movement. When the knee is irritated or damaged, it often responds by making more of this fluid than usual. That excess fluid can flow through a one-way valve-like opening into a bursa (a small fluid sac) behind the knee, where it pools and expands into what we call a Baker’s cyst. The cyst sits between two muscles at the back of the knee, the gastrocnemius and the semimembranosus, nestled against the joint capsule.
The two most common triggers are arthritis and cartilage injuries. Osteoarthritis and rheumatoid arthritis both cause chronic inflammation inside the knee, which drives ongoing fluid production. A torn meniscus, the rubbery cartilage disc that acts as a shock absorber in the knee, can do the same thing. In most adults, a Baker’s cyst is secondary to one of these underlying conditions rather than a standalone problem.
What It Feels Like
A small Baker’s cyst may cause no symptoms at all. Many people discover theirs incidentally during an imaging scan for something else. When the cyst is large enough to notice, you’ll typically feel a tight, pressure-like sensation behind the knee, sometimes described as a “water balloon” feeling. Stiffness and mild pain in the back of the knee are common, and both tend to worsen with increased activity or prolonged standing.
One distinctive feature is how the cyst behaves with movement. When you straighten your leg, the muscles behind the knee compress the cyst against the deeper tissue, making it feel firmer and more noticeable. When you bend the knee, those muscles separate and the cyst relaxes, becoming softer and sometimes harder to detect. This change in firmness between a straight and bent knee is a classic clinical sign doctors use during examination.
Diagnosis and Imaging
A Baker’s cyst can often be identified during a physical exam, but imaging confirms the diagnosis and rules out more concerning possibilities. Ultrasound is the preferred method: it’s fast, inexpensive, and clearly shows the fluid-filled nature of the cyst. MRI provides a more detailed view and is particularly useful for identifying the underlying knee problem, such as a meniscus tear or cartilage damage, that caused the cyst in the first place.
Getting the diagnosis right matters because a Baker’s cyst can look and feel a lot like a blood clot in the leg (deep vein thrombosis). Both cause swelling, tightness, and discomfort behind the knee or in the calf. The two conditions are similar enough that clinical examination alone often can’t distinguish them. An ultrasound can differentiate between a fluid-filled cyst and a clot in minutes, which is why imaging is important when swelling appears suddenly behind the knee.
When a Cyst Ruptures
A Baker’s cyst can rupture, releasing its fluid into the calf muscle. When this happens, you may notice sudden sharp pain behind the knee followed by swelling, redness, and a sensation of warmth spreading down the calf. The symptoms closely mimic those of a blood clot, which is why a ruptured cyst is sometimes called “pseudothrombophlebitis.” If you experience sudden calf swelling and pain, getting an ultrasound to rule out a clot is the right move, since the treatment for each condition is very different.
Treatment Options
Because Baker’s cysts are almost always caused by an underlying knee problem, the most effective long-term approach is treating that root cause. A cyst driven by a meniscus tear, for example, often resolves once the tear is repaired. A cyst linked to arthritis may shrink when the inflammation is brought under control. Without addressing the underlying condition, the cyst tends to come back regardless of what’s done to it directly.
Draining and Injection
For cysts that are large or painful enough to interfere with daily life, a doctor can drain the fluid with a needle (aspiration), often guided by ultrasound for precision. A corticosteroid is typically injected at the same time to reduce inflammation and slow fluid re-accumulation. This approach shrinks the cyst in roughly two-thirds of patients within two to seven days. Complete disappearance, however, is far less common, occurring in only about 7% of cases. And even after successful drainage, the cyst recurs within six months about 19% of the time.
Surgery
Surgical removal of a Baker’s cyst is generally a last resort, reserved for cases where the underlying knee condition can’t be fixed and the cyst causes persistent, bothersome symptoms. The recurrence rate after surgical excision is high: one study found that cysts came back in 63% of knees after removal. This reinforces why treating the joint problem itself is more important than removing the cyst. Modern approaches often combine cyst treatment with arthroscopic repair of the underlying damage, which improves long-term outcomes.
Physical Therapy and Self-Care
A rehabilitation program focused on the knee can help manage symptoms and support recovery, whether you’ve had a procedure or are taking a conservative approach. The typical program includes gentle range-of-motion exercises to maintain knee flexibility, a hamstring stretching routine to relieve tension behind the knee, and quadriceps strengthening exercises to improve joint stability. These should be performed several times a day for the best effect.
Ice applied to the back of the knee for 15 to 20 minutes can reduce swelling after activity. Compression wraps or sleeves provide support and help limit fluid accumulation. Avoiding high-impact activities like running or deep squatting during flare-ups is practical, since pain and stiffness behind the knee reliably worsen with increased activity. Low-impact alternatives like swimming or cycling are generally better tolerated while the cyst is symptomatic.