What Is a Baker's Cyst, and What Is Causing It?

Key takeaways

• A Baker's cyst is a symptom, not a standalone condition. Treating the underlying cause is the priority.
• Common triggers include osteoarthritis, meniscal tears and inflammatory joint conditions.
• A painful, swollen calf needs urgent medical review to rule out a blood clot (DVT).
• Standing whole-body imaging can reveal knee alignment problems that lying-down scans may miss.
• EOS imaging uses about 90% less radiation than a standard X-ray, around ten times less.

What does a Baker's cyst feel like?

A Baker's cyst usually feels like a smooth, soft lump at the back of the knee, in the area doctors call the popliteal fossa. Some people notice it only when they straighten the leg fully. Others feel a constant tightness or pressure that gets worse with activity.

Many small Baker's cysts cause no symptoms at all and are found by chance during a scan for another reason. If yours is painful or growing, it is worth investigating what is driving it.

What causes a Baker's cyst to form?

A Baker's cyst forms when the knee produces more fluid than it can reabsorb. That excess fluid is pushed through a natural weak point in the joint capsule at the back of the knee, creating a pouch. The real question is why the knee is making too much fluid in the first place.

Because a Baker's cyst is secondary to another problem, draining it or waiting for it to shrink on its own does not address the root cause. If the underlying condition remains, the cyst often comes back. Other swellings around the knee, such as infrapatellar bursitis, have different causes and require separate assessment.

What happens if a Baker's cyst bursts?

A burst Baker's cyst leaks fluid into the calf muscle. This causes sudden, sharp pain behind the knee and into the calf, along with redness, warmth and swelling. The symptoms often settle on their own over a few weeks as the body reabsorbs the fluid.

How does knee alignment affect a Baker's cyst?

Knee alignment plays a larger role than many people realise. When your leg is not well aligned, the load through the knee joint is uneven. One side of the joint bears more weight than the other, which accelerates wear on the cartilage and irritates the joint lining. Over time, this produces excess fluid, and that fluid has to go somewhere.

The body works as a connected chain from the feet to the head. A tilted pelvis or a rotated hip can change the angle at your knee. Flat feet or an ankle that rolls inward can do the same from below. The knee sits in the middle and absorbs the consequences. That is why the source of a knee problem is not always in the knee itself.

Standard imaging usually looks at one area at a time. An X-ray of just the knee will show whether you have arthritis there, but it will not reveal how your overall alignment is loading the joint. Worse, most scans are done lying down. When you lie on a table, gravity is removed, muscles relax, and the subtle alignment problems that cause uneven loading simply disappear from the image.

How can an EOS standing scan help with a Baker's cyst?

An EOS scan captures a full-length, weight-bearing image of your skeleton while you stand naturally. It shows the alignment of your legs, knees, hips and spine in one shot, exactly as your body experiences it during daily life. This gives your clinician the context that single-area, lying-down imaging cannot provide.

For a Baker's cyst, that context matters. If misalignment is accelerating joint wear on one side, a targeted plan to address the loading pattern is far more useful than simply draining the cyst. The EOS system uses about 90% less radiation than a standard X-ray, around ten times less, so it is a low-dose option for detailed skeletal assessment.

At ScanAlign standing EOS imaging, you stand in the scanner for around 20 seconds. There is no enclosed tunnel, no loud noise, and no preparation needed. Your images are reviewed by a specialist and a detailed report is produced for you and your referring clinician.

What helps manage a Baker's cyst?

Because a Baker's cyst is a symptom rather than a standalone condition, management focuses on the underlying cause. Your doctor, physiotherapist or surgeon will decide what is appropriate based on your imaging and clinical picture. Options they may consider include:

• Addressing the joint condition driving excess fluid, such as arthritis or a cartilage problem.
• Physiotherapy to strengthen the muscles around the knee and improve movement.
• Aspiration (draining the cyst with a needle) for temporary relief, though it may refill.
• If alignment is contributing, targeted rehabilitation or orthotics to improve load distribution.

ScanAlign is a diagnostic imaging clinic and does not provide treatment. What we can do is give you and your clinician a clear, full-length picture of how your skeleton is aligned under load, so any treatment plan is based on accurate information. If you would like to explore this, you can book a standing knee scan to get started.

Frequently asked questions about Baker's cysts