A Baker’s cyst, also called a popliteal cyst, arises from the knee joint and causes a lump at the back of the knee. We usually confirm the diagnosis by seeing a Baker’s cyst on ultrasound. Sometimes a Baker’s cyst drainage is needed to relieve pressure at the back of the knee, but is drainage really necessary?

This page covers lumps at the back of the knee. For a lump on the side of the knee, see lump on the side of the knee.

What is a Baker’s cyst?

baker's cyst drainage

A Baker’s cyst is a fluid-filled sac. It forms when excess knee fluid pushes the back of the knee capsule outwards, creating a sac of fluid. It’s also called a ‘popliteal cyst’, because it forms in the popliteal area at the back of the knee.

What causes a Baker’s cyst?

A Baker’s cyst forms from excess fluid in the knee. That excess fluid is usually produced by conditions such as osteoarthritis, rheumatoid arthritis, and a torn meniscus.

It’s essential to be sure the lump at the back of the knee isn’t something else. More serious conditions that can mimic a simple cyst include tumours, blood clots, and swollen arteries (aneurysms). An ultrasound or MRI can tell a simple cyst apart from these more serious causes.

What are the symptoms of a Baker’s cyst?

Often, a Baker’s cyst causes no symptoms — the only sign is a soft, palpable lump at the back of the knee. Sometimes the cyst becomes tight or painful, particularly when straightening the knee, and it can grow or shrink depending on how much joint fluid the knee produces.

Occasionally, the cyst bursts, producing redness, warmth, and calf swelling. A ruptured Baker’s cyst is often confused with a blood clot, so if your calf swells, see your doctor to exclude a clot, usually with an ultrasound or a blood test.

Baker’s cyst vs ganglion (meniscal) cyst: how to tell the difference

A meniscal cyst comes from a meniscal tear and usually forms on the inside or outside of the knee. Occasionally, it develops at the back and mimics a Baker’s cyst. An ultrasound helps tell whether the swelling at the back of the knee is a Baker’s cyst or a meniscal cyst.

Can a Baker’s cyst cause calf pain?

Generally not. But a large cyst can press on the muscles, tendons, and nerves so that pain can spread from the back of the knee into the calf. Even so, severe pain is unusual, and we recommend an ultrasound or MRI to rule out other causes such as a tumour or an aneurysm.

Baker’s cyst and rheumatoid arthritis: is there a connection?

Yes. People with rheumatoid arthritis are more likely to develop larger, more painful Baker’s cysts. In these cases, a Baker’s cyst on ultrasound shows significant complex fluid and active synovitis.

What to do for a Baker’s cyst, including drainage

Treatment depends on the cause, but nearly all cases are managed without surgery:

  • Ice to the swollen cyst and knee
  • Oral anti-inflammatory tablets
  • Compression of the knee with a bandage
  • Drainage of the cyst with a needle and syringe
  • Treating the underlying cause — for example, managing knee arthritis with exercise therapy reduces the joint fluid, and so the cyst.

Surgery should be avoided unless the cyst fails simple treatments or becomes painful and bothersome — and even after removal, the cyst can return.

Baker’s cyst aspiration technique

baker's cyst on ultrasound

Drainage suits cysts that grow or become painful despite simple treatment. Confirming a simple cyst on ultrasound first is vital. Ultrasound also lets us place the needle in the right spot and drain the entire cyst, inserting it into the cyst’s centre to remove most of the fluid. A cortisone injection into the sac sometimes prevents it from returning. If the excess fluid is due to arthritis or a torn meniscus, we often inject a small dose of cortisone into the joint at the same time.

Can a Baker’s cyst get infected?

An infected Baker’s cyst is rare. Even so, consider it if you develop both swelling and pain at the back of the knee, and see a doctor.

What happens when a Baker’s cyst ruptures?

Sometimes the cyst swells and bursts, leaking fluid into the calf and causing swelling. This is often misdiagnosed as a more serious blood clot, so an ultrasound must confirm a ruptured cyst and rule out a DVT.

The good news: a ruptured Baker’s cyst is harmless and needs no special treatment beyond leg elevation and calf compression.

Ruptured Baker’s cyst vs DVT

It can be hard to tell these apart — but while a ruptured cyst is harmless, a DVT can be life-threatening. An ultrasound is a quick, accurate way to distinguish them.

Frequently asked questions about Baker’s cyst on ultrasound

Do you need to drain a Baker’s cyst?

Generally no. A drained Baker’s cyst usually comes back. We advise drainage for cosmetic reasons, or if you develop pain or discomfort at the back of the knee, and we recommend injecting the knee joint at the same time to reduce the recurrence.

Should you inject the joint if a Baker’s cyst is drained?

Yes. A Baker’s cyst is usually caused by excess fluid from the joint, so stopping further fluid production helps prevent a recurrence. Options include cortisone, hyaluronic acid, or PRP — though cortisone is probably the most effective.

Can a Baker’s cyst be removed?

We suggest avoiding surgical removal until you’ve tried other treatments, including drainage and a knee joint injection. Even after surgery, a cyst can return if the underlying joint problem isn’t addressed.

Will a Baker’s cyst go away on its own?

It can, especially if the underlying joint swelling settles. A burst cyst often resolves completely. Treating the cause — usually arthritis or a meniscal tear — is the key to long-term resolution.

How is a Baker’s cyst diagnosed on ultrasound?

Ultrasound shows a fluid-filled sac at the back of the knee in the popliteal space, and can distinguish it from a meniscal cyst, a solid tumour, or a DVT. It also guides drainage if needed.

Final word from Sport Doctor London about Baker’s cyst on ultrasound

See your doctor if you’re unsure about a lump behind the knee. Ultrasound easily confirms a Baker’s cyst and rules out other lumps. At Sport Doctor London, we diagnose a Baker’s cyst on ultrasound, drain it under ultrasound if needed, and inject the knee joint at the same time when there’s excess joint fluid.

To book a one-stop knee assessment with Dr Masci in London, contact the team here or call +44 (0) 203 488 0350.

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