A Baker’s cyst is a pocket of joint fluid that bulges out at the BACK of the knee. It isn’t a tumor, and on its own it isn’t dangerous.
In adults it’s usually a SIGN of something else going on inside the knee, most often arthritis or a torn meniscus. The irritated knee makes extra fluid, and some of it gets pushed into a sac behind the joint.
That’s why treatment is usually aimed at the knee, not the lump. When the knee problem settles, the cyst often shrinks. Some go away on their own, though that can take anywhere from a few months to a few years.
One warning up front: if your calf suddenly becomes swollen, painful, red or warm, get it checked the SAME DAY. A burst cyst and a blood clot can look almost identical.
What a Baker’s cyst is
The knee joint is lined with a membrane that makes synovial fluid, the joint’s natural oil. Behind the knee sits a small sac, called a bursa, that connects to the joint. When the knee is irritated it makes more fluid, and some is squeezed into this sac.
The opening often works like a one-way valve. Fluid gets in but can’t easily flow back, so the sac fills and bulges. Doctors also call it a popliteal cyst, after the popliteal space at the back of the knee.
Why it’s usually a sign of something else
In adults, a Baker’s cyst usually has a cause inside the knee. It’s most common between the ages of 35 and 70.
- Osteoarthritis: worn joint cartilage irritates the lining, which makes more fluid. See knee arthritis
- A torn meniscus: a tear in the knee’s shock-absorbing cartilage can irritate the joint and help push fluid into the sac. See meniscus tear
- Inflammatory arthritis: rheumatoid arthritis and gout both make the knee lining swell and produce fluid
- A previous knee injury: damage from an old sprain or tear can keep the joint irritated
- In children: there’s often no clear cause, and the cyst usually goes away on its own
Symptoms
- A soft lump behind the knee: easiest to see when you stand with the knee straight, and it often softens when you bend it
- Tightness or fullness: especially when you bend the knee fully or squat
- Aching: often worse after walking or a long time on your feet
- Stiffness: the knee may not bend as far as it used to
Many cysts cause no symptoms at all and turn up on a scan done for something else.
Treat the knee, and the cyst usually follows.
Treatment
Not every Baker’s cyst needs treatment. If it isn’t bothering you, keeping an eye on it is a reasonable choice.
- Settle the knee: ice, raising the leg, simple pain relief and easing off the activities that flare it
- Physio: strengthening the thigh and hip muscles and keeping the knee moving. The aim is to calm the arthritis or meniscus problem underneath
- Steroid injection: an injection into the knee joint can reduce inflammation and fluid. Sometimes the cyst itself is injected
- Draining the cyst: a needle, often guided by ultrasound, removes the fluid. It can come back if the knee problem isn’t treated
- Surgery: rarely needed. When it is, it usually deals with the problem inside the knee, such as a meniscus tear, rather than cutting the cyst out
When a swollen calf is a burst cyst or a clot
A Baker’s cyst can leak or burst. The fluid tracks down into the calf and irritates the tissues around it. This causes:
- Sudden calf pain: often sharp, sometimes after a squat or a long walk
- Calf swelling, redness and warmth: the calf can look and feel inflamed
- Bruising around the ankle: sometimes appears a few days later, often around the inner ankle, as the fluid drains down with gravity
These are the same signs as a DVT (deep vein thrombosis), a blood clot in a deep vein of the leg. A clot matters because part of it can break off and travel to the lungs.
You can’t tell the two apart by looking or feeling. An ultrasound scan can, and it’s quick. The treatments are very different, which is why you need that scan the same day rather than a guess. The NHS guide to Baker’s cyst gives the same advice.
A confirmed burst cyst usually settles with rest, ice, raising the leg and pain relief, though it can take several weeks.
Call emergency services if
- Your calf is painful or swollen and you’re also short of breath or have chest pain. That can mean a clot has reached the lungs. Call 911 in the US, 999 in the UK or 112 in Europe
- Your calf becomes very tight, tense and extremely painful, and the pain keeps getting worse
Get checked the same day if
- Your calf suddenly becomes swollen, painful, warm or red, or you notice new bruising around the ankle
- Your calf swells after recent surgery, a long journey or a spell of being laid up
- Your knee becomes hot, red and swollen and you feel feverish
See your doctor or physio if
- You find a new lump behind your knee that hasn’t been checked
- The lump is growing, painful or stopping you from bending the knee
- Your knee locks, catches or gives way, which can point to a meniscus tear
The lump sits behind your knee. The cause usually sits inside it.