A cyst is a bag-like growth that contains fluid, air, or other substances. A cyst can grow almost anywhere on the body.

A Baker’s cyst — also called a popliteal cyst — is a fluid-filled cyst on the back of the knee. It can bulge out, causing a feeling of tightness that becomes painful when you extend or flex your knee.

A Baker’s cyst is often caused by a knee joint problem such as arthritis, gout, or a knee injury that causes your knee to produce too much fluid.

A Baker’s cyst can rupture (burst). If it does, the fluid from the cyst leaks down into your calf, which will become red and swollen.

If your Baker’s cyst ruptures, you will most likely experience a sharp pain in your calf and you might feel a sensation similar to water running down the back of your leg. You might also see what appears to be a bruise on your inner ankle.

Your doctor will most likely recommend that you keep your calf elevated. They might also suggest applying ice to the back of the knee. Some doctors may also prescribe painkillers, although over-the-counter painkillers are usually sufficient.

In most cases, it takes a few weeks until the fluid is reabsorbed into your body.

If you have a Baker’s cyst that doesn’t clear up on its own, you should check with your doctor. After a physical examination, your doctor might decide to drain (aspirate) the cyst. Yours doctor will also want to address the underlying condition that caused the cyst.

It’s important not to self-diagnose. What appears to be a Baker’s cyst may in fact be something more serious, such as a:

If you have bulge behind your knee, it could be a Baker’s cyst. If the pain and swelling behind your knee results in a swollen, red calf, you could have a ruptured Baker’s cyst.

In either case, it’s a good idea to see your doctor for a proper diagnosis. If it is a Baker’s cyst, you doctor will recommend appropriate treatment. There’s a chance that what you think is a Baker’s cyst is a more serious condition.