Swelling or a Lump Behind the Knee: What It Usually Is
A lump behind the knee is usually a Baker’s cyst: fluid from inside the knee collecting in the back of it. In my practice, other causes like a swollen lymph node, a lipoma or a tumor are rare. The cyst usually isn’t the real problem. It’s a sign the knee is making extra fluid, most often from arthritis or a meniscus tear, so I treat what’s happening inside the knee rather than drain the cyst.
Key takeaways
- A lump behind the knee is usually a Baker’s cyst. If there’s fluid in the knee, I assume there’s a Baker’s cyst too.
- The cyst is usually a sign of a problem inside the knee, most often arthritis or a meniscus tear.
- I don’t usually drain Baker’s cysts. I treat the root problem and calm the knee down.
- A swollen lymph node, lipoma, meniscal cyst, aneurysm or tumor behind the knee is rare. Those are very far down the list.
- A burst cyst can be very painful, with pain and swelling down into the calf. Calf swelling should be checked to make sure it isn’t a blood clot.
A soft lump or swelling behind the knee is usually a Baker’s cyst, also called a popliteal cyst. Most people who find one are worried it’s something serious. It almost never is. But it usually tells us something is going on inside the knee.
“I felt a soft bump behind my knee and got worried.”
“It feels like a water balloon back there.”
“It’s bigger after a long day on my feet.”
“It’s tight when I bend my knee all the way.”
“Something popped and now my calf hurts.”
What it usually is: a Baker’s cyst
When something irritates the knee, such as arthritis, a meniscus tear or an injury, the knee makes extra fluid. Some of that fluid collects in a pocket at the back of the knee. That pocket is the Baker’s cyst.
Fluid in the knee means a Baker’s cyst
Baker’s cysts are very common. If there’s fluid in the knee, I just assume there’s a Baker’s cyst too. Sometimes you can feel it, and sometimes you can’t.
That’s why the cyst usually isn’t the real problem. It’s a sign that the knee is swelling, and the question is why. See what causes knee swelling.
When it isn’t a Baker’s cyst
Other things can cause a lump behind the knee:
- A meniscal cyst
- A lipoma (a soft, fatty lump)
- A blood clot
- An aneurysm (a bulging artery)
- A tumor
Still, get it checked if the lump is hard, keeps growing, or comes with calf swelling.
Lymph nodes behind the knee
Yes, there are small lymph nodes behind the knee, called popliteal lymph nodes. They drain the lower leg and foot, so when one swells, it’s usually reacting to something below it, such as a cut, a sore or an infection in the foot or leg.
In my practice, a swollen lymph node behind the knee is very uncommon. The two usually feel different:
- A Baker’s cyst is soft and feels like fluid, and it goes along with your knee symptoms.
- A swollen lymph node is a firmer, smaller lump, often with an infection or wound lower in the leg.
Who to see
A lump that goes along with knee pain, swelling or stiffness: see an orthopedic surgeon. A firm lump with a foot or leg infection, or with fevers, night sweats or weight loss: see your primary care doctor.
When a Baker’s cyst bursts
A Baker’s cyst can burst, or rupture. When it does, it can be very painful. People often feel a pop, then pain and swelling down into the calf.
Calf pain and swelling: get checked the same day
A burst cyst and a blood clot in the calf can look alike. Blood clots are uncommon, but they’re the thing to rule out, so get seen the same day if your calf suddenly swells and hurts.
How I check it
- Physical exam. Feeling the back of the knee and checking for fluid inside the knee.
- X-rays. To look for arthritis and the alignment of the knee.
- Ultrasound. Rarely needed.
- MRI. If the knee doesn’t improve with treatment, to look at the meniscus, cartilage and ligaments.
Treatment
I don’t typically drain a Baker’s cyst. It fills from inside the knee, so draining it doesn’t fix the problem. Instead, I treat the root problem and calm down the inflammation in the knee.
How I usually treat it
- Rest, ice, compression and elevation
- An injection into the knee to calm the inflammation: cortisone, gel or PRP
- An MRI if it isn’t improving
- If arthritis is the cause and it keeps affecting your quality of life after treatment: a knee replacement
More on cortisone, gel, PRP and knee replacement. When the knee is treated, the cyst usually settles down on its own.
When to come in
If a lump behind your knee comes with knee pain, swelling or stiffness, come in. We’ll examine the knee, get X-rays and find out what’s causing the fluid.
Find out what’s behind it
I see patients in Washington, D.C. and Germantown, Maryland. Request an appointment.
Frequently asked questions
What is the lump behind my knee?
Usually a Baker’s cyst. When the knee makes extra fluid, from arthritis, a meniscus tear or an injury, some of it collects in a pocket at the back of the knee. If there’s fluid in the knee, I assume there’s a Baker’s cyst as well.
What else could a lump behind the knee be, if it’s not a Baker’s cyst?
It could be a meniscal cyst, a lipoma (fatty lump), a blood clot, an aneurysm or a tumor, but in my experience those rarely happen. They are very far down the list. A firm lump that keeps growing, or one that comes with calf swelling, should be checked.
Are there lymph nodes behind the knee?
Yes, there are small lymph nodes behind the knee, called popliteal lymph nodes. A swollen one is very uncommon in my practice. Typically, a lump behind the knee is a Baker’s cyst. If you have a firm lump along with a cut, sore or infection in your foot or leg, show it to your primary care doctor.
Should a Baker’s cyst be drained?
I don’t typically drain them. The cyst comes from fluid inside the knee, so draining it doesn’t fix the problem. I treat the root cause and calm down the inflammation in the knee, often with a cortisone, gel or PRP injection.
What happens when a Baker’s cyst bursts?
It can be very painful. People often feel a pop, then pain and swelling down into the calf. A blood clot can look similar, so if your calf is swollen and painful, get checked the same day.
Do I need an ultrasound or MRI for a lump behind my knee?
Usually not at first. An exam and X-rays tell me most of what I need to know. An ultrasound is rarely needed. If the knee doesn’t improve with treatment, an MRI shows what’s happening inside the knee.
Does a Baker’s cyst mean I need a knee replacement?
Not on its own. When the cyst comes from arthritis and the arthritis is affecting your quality of life after treatments haven’t worked, that’s when we talk about a knee replacement. Once the arthritic knee is treated, the cyst usually settles down.
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.
What patients say
“He shared so much about my knee and gave me a plan moving forward — I feel like I know exactly what to do.”
“He took the time to listen, thoroughly explained my options, and answered every question with patience and clarity.”
“An excellent physician in whom I have total faith and trust — caring, attentive, and he explains everything without ever talking down to me.”
Where to go from here
Signs You May Need a Knee Replacement
Most people with knee arthritis don’t need surgery any time soon — and surgery is rarely the first step. But it helps to recognize when arthritis is genuinely limiting your life, because with a knee replacement, you largely decide when it’s time. The decision is driven by your pain, function, and quality of life — not by an X-ray.
Read articleArthritis & Joint PainBone on Bone Arthritis: Do You Need Knee Replacement Surgery?
If you’ve been told you have “bone-on-bone” arthritis, the first question is usually whether you need a knee replacement. The answer may surprise you: not necessarily. A severe X-ray is only part of the picture — what matters far more is how the arthritis is affecting your life. We treat patients, not X-rays.
Read articleWondering what’s causing your hip or knee pain?
Schedule an evaluation with Dr. Harb to understand your diagnosis and build a plan — from nonsurgical care to replacement, when the time is right.
Not ready to book? Read how to choose a hip or knee surgeon