Swelling Behind the Knee: What a Baker’s Cyst Is and What to Do About It
Swelling behind the knee in an adult is almost always a Baker’s cyst — a fluid-filled outpouching from inside the knee joint. The cyst itself is rarely the real problem. In adults, it’s usually a visible signal that something else is going on inside the knee — most often arthritis or a meniscus tear producing extra joint fluid. Treating the underlying cause is the answer, not the cyst.
Key takeaways
- In adults, swelling behind the knee is almost always a Baker’s cyst — a fluid-filled outpouching that comes from inside the knee joint.
- The cyst itself is rarely the problem. It’s a visible signal that the knee is producing extra fluid — most often because of arthritis or a meniscus tear.
- Children sometimes get Baker’s cysts as a primary, isolated condition. In adults, there’s almost always an underlying knee cause.
- Treating the cyst directly (aspirating it or excising it) without addressing the underlying cause has a high rate of recurrence.
- Treating the underlying cause — arthritis care, meniscus management, or replacement when appropriate — is what makes the cyst resolve.
- A ruptured Baker’s cyst can look very similar to a blood clot (DVT) in the calf — that’s a situation worth getting checked promptly.
Swelling behind the knee — a soft, sometimes grape-sized lump in the back of the knee — is almost always a Baker's cyst. It's also called a popliteal cyst. Most patients arrive worried it's something serious. The cyst itself is almost never the real problem — but understanding what's causing it usually is.
What a Baker's cyst is
A Baker's cyst is a fluid-filled outpouching from the back of the knee joint. The inside of the knee communicates with a bursa (a small sac) sitting behind the joint, and when the knee produces more joint fluid than normal, that fluid flows back into the bursa and stretches it — a balloon that gets bigger as more water flows in.
The cyst sits in the area called the popliteal fossa — the soft hollow behind your knee. Hence the medical term “popliteal cyst.” The eponym “Baker's cyst” comes from William Morrant Baker, a 19th-century English surgeon who described it.
Why it forms — and why the cyst usually isn't the real problem
Healthy knees produce a small, balanced amount of joint fluid. When something inside the knee irritates the joint lining — arthritis, a meniscus tear, an inflammatory condition — the joint produces more fluid than usual. That extra fluid has to go somewhere; the cyst behind the knee is often where it ends up.
That's the key clinical point:
The cyst is the symptom, not the disease
In an adult, a Baker's cyst is almost always a downstream effect of something else going on inside the knee. The cyst is the visible sign that the joint is making more fluid than it should — which is why treating the cyst alone (aspirating or excising it) without addressing the underlying knee problem has a high recurrence rate. The knee just refills it.
Children are different — Baker's cysts in children are usually a primary, isolated condition without any underlying joint problem, and they often resolve on their own. The discussion that follows is about adult Baker's cysts, which is the much more common clinical picture.
What's usually causing it in adults
The two most common underlying causes:
- Knee osteoarthritis. By far the most common cause in adults over 50. The arthritic knee produces extra fluid, which fills the cyst.
- Meniscus tears. A torn meniscus irritates the joint lining and produces extra fluid in much the same way.
Less common causes worth knowing about:
- Inflammatory arthritis — rheumatoid arthritis, gout, or other inflammatory joint diseases
- Cartilage injury from a sports impact or twisting injury
- Synovitis from any cause — sometimes after surgery or infection
For a fuller picture of what's producing the joint fluid, see knee swelling — causes and what helps. The Baker's cyst story is one piece of the larger swelling-in-the-knee story.
What it feels like
The pattern is fairly consistent:
- A soft, smooth lump behind the knee — sometimes grape-sized, sometimes larger
- A tight or pulling sensation when bending the knee deeply
- A fluid-filled, balloon-like feel rather than a hard or fixed mass
- Size that fluctuates — often bigger at the end of the day or after a long walk
- Discomfort that worsens with walking, stairs, kneeling, or prolonged standing
- Underlying knee symptoms in the front of the knee as well — stiffness, pain, swelling
When to worry — and the DVT mimic
A Baker's cyst is almost always benign. Two situations warrant prompt attention:
Get checked promptly if
- Sudden calf pain, swelling, or bruising — can be a ruptured Baker's cyst leaking fluid into the calf, which looks remarkably similar to a deep vein thrombosis (DVT, or blood clot). DVT is the more serious diagnosis to rule out, and a quick ultrasound can usually tell them apart.
- Sudden increase in size, redness, warmth, or fever — can suggest infection rather than a routine cyst.
- A hard, fixed, or rapidly growing mass behind the knee — should be evaluated to make sure it's actually a cyst rather than something else.
For most patients, none of these apply — the cyst is soft, slow-changing, and not concerning on its own.
How it's diagnosed
Diagnosis usually involves a clinical exam plus imaging when needed:
- Exam. A soft, ballotable swelling behind the knee, more prominent with the knee extended and softer with the knee bent, is classic for a Baker's cyst.
- Ultrasound. Fast, accurate, and the best first test — confirms the cyst, measures it, and helps rule out DVT or a solid mass.
- MRI. Used when we need to evaluate the underlying knee — meniscus, cartilage, ligaments — to understand what's producing the joint fluid.
- X-rays. Don't show the cyst itself, but show arthritis and joint changes — usually part of the workup for any knee evaluation.
What actually helps
The principle is straightforward:
Treat the cause, and the cyst follows
When the underlying knee problem is addressed, the Baker's cyst usually settles down on its own — because the knee is no longer producing extra fluid to fill it. That's why the long-term treatment of a Baker's cyst in an adult is the treatment of the underlying condition.
The pathway looks like:
- Identify the cause. Clinical exam plus weight-bearing X-rays; MRI when meniscus or cartilage need to be evaluated.
- Treat the underlying knee. Most often arthritis care: activity modification, physical therapy, weight management, and selectively cortisone injections into the knee joint. For meniscus issues, the management depends on the pattern of the tear.
- Aspirate the cyst selectively. When the cyst itself is uncomfortably large, ultrasound-guided aspiration with cortisone can give meaningful temporary relief — but it's a bridge while the underlying cause is addressed, not a stand-alone fix.
- Consider replacement when arthritis is advanced. When the underlying arthritis is significantly affecting your life despite real nonsurgical effort, modern knee replacement addresses the joint fluid production at its source — and the Baker's cyst typically resolves once the arthritic joint is replaced. See signs you may need a knee replacement for what that conversation looks like.
What patients tell me
“I felt a soft bump behind my knee and thought it was a tumor.”
“My doctor said it was a Baker’s cyst. What does that even mean?”
“It comes and goes — bigger after a long day on my feet.”
“It feels like there’s a water balloon behind my knee.”
“My calf started hurting and they were worried about a blood clot.”
Frequently asked questions
What is a Baker’s cyst?
A Baker’s cyst (also called a popliteal cyst) is a fluid-filled sac that forms behind the knee. The cyst itself is an outpouching connected to the inside of the knee joint — joint fluid flows back into the cyst when the knee makes more fluid than usual. The cyst is rarely the real problem in an adult; it’s a visible signal that the knee joint inside is producing too much fluid.
What causes a Baker’s cyst in an adult?
In adults, a Baker’s cyst is almost always caused by something else going on inside the knee joint. The most common causes are knee osteoarthritis and meniscus tears — both produce extra joint fluid, which is what fills the cyst. Less commonly, inflammatory arthritis (rheumatoid arthritis, gout) or a cartilage injury can be the source. In children, a Baker’s cyst is usually a primary, isolated condition without an underlying joint problem.
Is a Baker’s cyst dangerous?
Almost always, no. It’s typically a benign, fluid-filled outpouching. The two scenarios that warrant prompt attention are: (1) a sudden increase in size, redness, warmth, or fever — which can suggest infection; and (2) sudden calf pain, swelling, or bruising — which can be a ruptured cyst leaking fluid into the calf, and which looks very similar to a deep vein thrombosis (DVT, or blood clot). DVT is more serious than a ruptured cyst, so a same-day evaluation is reasonable.
Can a Baker’s cyst go away on its own?
Sometimes — when the underlying knee problem improves (an arthritis flare settles, a meniscus tear becomes less reactive), the cyst can shrink or resolve. More often, a Baker’s cyst in an adult is a recurring or fluctuating finding that reflects ongoing joint changes inside the knee.
Do I need to have the cyst drained or removed?
Treating the cyst directly — aspirating (draining) it, or surgically excising it — has a high rate of recurrence if the underlying cause inside the knee isn’t addressed. The joint just refills the cyst with more fluid. The more durable answer is to identify and treat whatever is producing the extra joint fluid: arthritis care, meniscus management, or, when arthritis is advanced, joint replacement. In selected cases — particularly when the cyst is large and symptomatic — a targeted aspiration with cortisone (often done under ultrasound guidance) can help temporarily while the underlying cause is addressed.
Can a Baker’s cyst rupture?
Yes, occasionally. When it does, the cyst’s fluid leaks down into the calf, producing sudden pain, swelling, and sometimes bruising — a picture that overlaps significantly with a deep vein thrombosis (blood clot). Because DVT is more serious, an ultrasound is usually the right next step to tell them apart. A ruptured Baker’s cyst itself usually settles down with rest, ice, elevation, and time.
I have a Baker’s cyst and knee arthritis — does that mean I need a knee replacement?
Not on its own. The cyst is a signal that the joint is making extra fluid, and arthritis is one of the most common reasons that happens. Whether replacement is the right next step depends on how much your arthritis is affecting your life — your pain, your function, what activities you’ve given up — not on the cyst itself. The Baker’s cyst usually resolves or improves when the arthritis is treated, regardless of whether that treatment is nonsurgical care or, eventually, joint replacement.
References
This article is for general education and is not a substitute for personalized medical advice. Please consult Matthew Harb, M.D. about your specific condition.
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