Overview
what it is and why it mattersA Baker's cyst, also called a popliteal cyst, is a fluid-filled pouch that forms in the soft tissue behind your knee (the popliteal fossa). It develops when excess fluid inside the knee joint is pushed backward through a natural one-way valve between the joint capsule and a small fluid-filled sac that sits between two muscles at the back of the knee. Fluid can flow out into this sac, but the valve makes it difficult for the fluid to flow back in, so the pouch fills and stays filled.
A Baker's cyst is almost always a secondary finding. The cyst is not the disease; it is the knee's overflow system telling you something else is wrong inside the joint. The most common drivers are a meniscus tear, osteoarthritis, or an inflammatory condition like rheumatoid arthritis, anything that makes the knee produce more fluid than usual. Treating the cyst alone without addressing the source almost guarantees it will come back.
Symptoms
what patients describeMany Baker's cysts are discovered by accident: your provider feels a soft lump behind the knee during an exam for something else, or it shows up on an MRI ordered for knee pain. When the cyst is small, you may not notice it at all.
As it enlarges, the most common complaint is a sense of tightness or fullness behind the knee, especially when you bend the knee fully (squatting, kneeling, or pulling your heel toward your buttock). The lump is most visible and easiest to feel when you stand with the knee straight. Some patients describe a vague aching behind the knee after prolonged walking or standing.
If the cyst ruptures, and they do rupture, the fluid spills down into the calf, causing sudden pain, swelling, and sometimes bruising along the inner ankle. A ruptured Baker's cyst can look and feel remarkably like a DVT (a blood clot in a deep vein), which is why your provider may order an ultrasound of the leg veins to rule that out before diagnosing a rupture.
Diagnosis
exam first, imaging secondYour provider can often make the diagnosis by feel alone: a soft, smooth mass in the hollow behind the knee that is most prominent when the knee is fully straight and tends to shrink or become harder to find when the knee is bent. That physical finding, combined with knee symptoms that suggest an underlying problem, is usually enough to confirm the diagnosis.
Ultrasound is the most cost-effective imaging study to confirm the cyst, measure its size, and, importantly, distinguish it from a popliteal artery aneurysm (a bulging blood vessel that can feel similar on exam but requires very different treatment). MRI does double duty: it shows the cyst and, more importantly, reveals the intra-articular problem driving it, whether that is a meniscus tear, cartilage loss, or an inflamed joint lining.
How We Treat It
fix the cause first, then the cystOne idea guides everything here: a Baker's cyst is a symptom, not the disease, so the most effective care goes after whatever is making the knee produce extra fluid. Treat that source and the cyst usually shrinks or disappears on its own. Drain the cyst without addressing the source and it tends to come back, because the knee simply refills the pouch. The steps below move in that order.
1
Treatment of the Underlying Cause
Because the cyst is a symptom rather than the disease, the most effective treatment is fixing what is making the knee produce excess fluid. If a meniscus tear is driving the effusion, repairing or trimming the torn fragment reduces fluid production and the cyst shrinks or disappears on its own. If osteoarthritis or an inflammatory condition is the source, anti-inflammatory medication, activity modification, and sometimes a corticosteroid injection into the knee joint bring the fluid level down, and the cyst follows.
2
Aspiration
When the cyst is large enough to cause discomfort or limit knee bending, your provider can drain it with a needle under ultrasound guidance. Aspiration provides quick relief, the tightness behind the knee improves immediately, but the recurrence rate is high if the underlying joint problem is not addressed, because the knee simply refills the pouch.
Surgical Options
if non-operative care isn't enoughDirect surgical removal of a Baker's cyst is rarely needed. In the uncommon case where the underlying joint problem has been treated (for example, an arthroscopic meniscus repair) and the cyst still persists months later, usually because the one-way valve has become a permanent channel, your surgeon can excise the cyst through a small incision behind the knee, often at the same time as the arthroscopic procedure. Most patients never reach this point.
Providers Who Treat Baker's Cyst
sports-medicine team
Frequently Asked
questions we hear in clinicIs a Baker's cyst dangerous?
The cyst itself is almost always a secondary finding rather than the disease, it is the knee's overflow system signaling that something else is going on inside the joint. The most common drivers are a meniscus tear, osteoarthritis, or an inflammatory condition such as rheumatoid arthritis. The bigger reason to have it looked at is to confirm what it is and to find and treat the underlying joint problem.
Why does my cyst keep coming back after it's drained?
Draining the cyst with a needle gives quick relief and the tightness behind the knee improves immediately, but the recurrence rate is high if the underlying joint problem is not addressed. The knee simply refills the pouch. Treating the source of the extra fluid is what makes the cyst stay down.
What happens if the cyst ruptures?
When a Baker's cyst ruptures, the fluid spills down into the calf and causes sudden pain, swelling, and sometimes bruising along the inner ankle. A ruptured cyst can look and feel a lot like a blood clot in a deep leg vein (DVT), so your provider may order an ultrasound of the leg veins to rule that out before diagnosing a rupture.
Do I need an MRI?
Not always. Your provider can often make the diagnosis by feel, and ultrasound is the most cost-effective scan to confirm the cyst, measure it, and tell it apart from a popliteal artery aneurysm. MRI does double duty: it shows the cyst and, more importantly, reveals the joint problem driving it, such as a meniscus tear, cartilage loss, or an inflamed joint lining.
Will I need surgery to remove it?
Rarely. Direct surgical removal of a Baker's cyst is uncommon. In the unusual case where the underlying joint problem has been treated and the cyst still persists months later, a surgeon can excise it through a small incision behind the knee, often at the same time as an arthroscopic procedure. Most patients never reach this point.
Further Reading
authoritative sourcesExternal patient-education references and related OSI pages for additional background: