A Baker’s cyst forms when excess synovial fluid, the natural lubricant inside your knee joint, gets pushed into a small bursa (a fluid-filled sac) behind the knee. In nearly every adult case, something inside the knee is producing too much fluid, and that fluid finds its way into the space behind the joint. The cyst itself is a symptom of an underlying knee problem, not a standalone condition.
How Fluid Moves Behind the Knee
The space where a Baker’s cyst develops sits between two muscle tendons at the back of your knee: the medial head of the gastrocnemius (the inner calf muscle) and the semimembranosus (one of the hamstring muscles). This space, called the gastrocnemius-semimembranosus bursa, normally exists in most people and sometimes connects to the knee joint through a small opening.
When the knee produces excess fluid from injury or disease, pressure builds inside the joint. That fluid gets squeezed through this opening into the bursa behind the knee. The key problem is a one-way valve effect: as you bend and straighten your knee, the muscles around that opening act like a gate that lets fluid flow out of the joint but not back in. Fluid accumulates in the bursa, and the more it fills, the more the cyst grows. This is why a Baker’s cyst often gets larger after periods of activity and may feel tighter or more noticeable by the end of the day.
The Most Common Underlying Causes
Baker’s cysts in adults are almost always secondary, meaning they’re caused by another knee condition that triggers excess fluid production. The most frequent culprits fall into a few categories.
Osteoarthritis
Wear-and-tear arthritis is the single most common cause in adults over 40. When cartilage breaks down, the irritated joint lining responds by producing more synovial fluid than it normally would. This chronic overproduction keeps the cyst filled and often makes it a long-term issue rather than something that resolves on its own. If you have knee osteoarthritis and notice a soft bulge behind your knee, the two are very likely connected.
Meniscal Tears
Tears in the meniscus, the C-shaped cartilage that cushions each side of the knee, are the second major cause. Tears in the posterior horn of the medial meniscus are especially linked to Baker’s cyst formation. In one study, 27 of 30 cases showed an oval opening between the meniscus body and the posterior horn that extended to the joint capsule. This opening creates its own one-way valve: the torn meniscus lets fluid escape from the joint cavity into the bursa but blocks it from returning. The leaked synovial fluid then consolidates into a thick, gel-like material that fills the cyst.
Inflammatory Arthritis
Rheumatoid arthritis, gout, and other inflammatory joint conditions cause the knee’s synovial lining to become chronically inflamed. Inflamed tissue produces large volumes of fluid, sometimes rapidly. People with rheumatoid arthritis may develop Baker’s cysts that are larger and more prone to rupture than those caused by osteoarthritis, because the inflammatory fluid is produced in greater quantities and under higher pressure.
Ligament Injuries and Other Damage
ACL tears, cartilage damage, and other traumatic injuries to the knee can trigger significant fluid production as the joint responds to internal damage. In these cases, the cyst may appear in the weeks following injury and can persist until the underlying damage is addressed.
Baker’s Cysts in Children Are Different
In children, Baker’s cysts are typically primary, meaning they form without any underlying knee damage or disease. The cyst develops from the bursa itself rather than from excess joint fluid being pushed backward. These cysts are generally painless, show up as a soft lump behind the knee, and most resolve on their own over months to years without any treatment. This is a key distinction from adult cysts, which rarely disappear unless the underlying cause is managed.
How Common They Are
Baker’s cysts show up on 5% to 41% of knee MRI scans, a wide range that reflects how often they exist without causing any symptoms. Many people have a small cyst behind the knee and never know it. The cyst only becomes a clinical problem when it grows large enough to cause stiffness, pain, or a visible bulge, or when it ruptures.
What Happens When a Baker’s Cyst Ruptures
A cyst can burst when internal pressure gets too high, usually from sudden knee bending or physical activity. When it ruptures, the thick synovial fluid leaks down into the calf muscle compartment. This causes sudden sharp pain behind the knee, swelling in the calf, redness, and warmth that can look and feel remarkably similar to a deep vein thrombosis (blood clot). This mimicry is common enough that it has its own name: pseudothrombophlebitis. Because the symptoms overlap so closely with a DVT, imaging is typically needed to tell the two apart.
The good news is that a ruptured Baker’s cyst is usually self-limiting. Your body gradually reabsorbs the leaked fluid over days to weeks. Rest, ice, compression, elevation, and over-the-counter anti-inflammatory medication are enough for most people. The calf swelling and pain typically improve steadily, though complete resolution can take several weeks.
Why the Cyst Keeps Coming Back
This is the frustration many people experience: you can drain a Baker’s cyst, but if the knee problem driving excess fluid production is still there, the cyst refills. Draining the cyst treats the symptom, not the source. That’s why treatment focuses on the underlying condition, whether that’s managing arthritis, repairing a meniscal tear, or reducing joint inflammation.
For cysts that persist despite conservative measures, arthroscopic surgery to address the intra-articular problem (repairing the torn meniscus, cleaning up damaged cartilage) is the most widely used surgical approach. By fixing what’s wrong inside the knee, the excess fluid production slows or stops, and the cyst deflates. Open surgical removal of the cyst itself is now rarely performed and reserved for unusual cases where arthroscopic treatment isn’t feasible.
Other Conditions That Can Look Like a Baker’s Cyst
Not every lump behind the knee is a Baker’s cyst. A popliteal artery aneurysm, a bulge in the artery that runs behind the knee, can feel similar on physical exam but requires very different management. Ultrasound can quickly distinguish between the two by checking for blood flow within the mass: a Baker’s cyst has no vascular flow, while an aneurysm does. Ganglion cysts, tumors, and other soft tissue masses can also occur in the same area, which is why imaging is standard when a new mass is found behind the knee.