Baker's cyst: the lump behind the knee and what it is telling you

It forms in the popliteal fossa, the hollow at the back of the knee. It should not be confused with the antecubital fossa, which is in the crease of the elbow: the body has several fossae, and the one where a Baker's cyst forms is the one at the back of the knee.

Basically, the cyst is a sac that fills with fluid when the knee is inflamed. That fluid is called synovial fluid. Its size can vary from one person to another.

A Baker's cyst is related to what is happening inside the knee. It can appear at the same time as knee osteoarthritis or arthritis, together with meniscus injuries, or also without any of these problems.

In most cases, people do not come in because of the cyst alone, but because they have:

• Knee osteoarthritis (gonarthrosis) or arthritis
• Meniscus problems
• An injury from a blow or a fall
• Knee pain in general

In those cases, the Baker's cyst is one more condition that comes with the main knee problem.

The most typical sign is a small lump you can feel at the back of the knee. When it hurts, the discomfort comes from the inflammation and the pressure on the soft tissues of that area.

On its own it is usually not a serious condition: many people live their whole lives with a Baker's cyst without needing care. When there is pain, it is worth doing something about it.

There is one important exception: if the calf suddenly swells, turns red or warm, or the pain behind the knee gets much worse, seek medical care that same day. Those signs can be due to other problems that need urgent attention.

When a Baker's cyst hurts, these simple measures can help:

• Ice: apply it for 10 minutes every hour on the back of the knee. Ice helps bring down the pain and inflammation in the area. Always put a cloth or a thin towel between the ice and the skin.
• Raise the leg: lifting the affected leg several times a day helps you feel better.
• A pillow under the knee at night: resting the knee on a pillow when you lie down can help you sleep better.

If the pain or swelling increases with these measures, stop them and get your case checked.

One important tip: do not mistake the lump at the back of the knee for a pimple, and do not try to squeeze it.

The lump is a warning sign that the problem is in your knee. That is why it is very important to treat the root of the problem and not just the cyst.

That is why it is worth checking what is happening in the knee: whether there is osteoarthritis or arthritis, a meniscus injury or a history of a blow or a fall, which are the problems that often come with the cyst. When there is inflammation, management relies on physiotherapy and conservative treatment.