What is a Knee Cyst?

Jacquelyn Gilchrist

A knee cyst is a small growth filled with joint fluid that extends to the back of the knee. Also called a Baker's or popliteal cyst, it normally forms in response to arthritis or a cartilage injury. Though most are not very serious, they do sometimes cause pain, and must be diagnosed by a healthcare professional. Both non-surgical and surgical treatments can be used to address cysts, depending on their severity.

Elastic bandage provides support for knee with Baker's or popliteal cyst.
Elastic bandage provides support for knee with Baker's or popliteal cyst.


When fluid from the knee projects to its back section, called the popliteal area, a cyst results. The fluid is known as synovial fluid, a substance that lubricates parts of the joints to reduce friction and wear and tear. Inflammation of the knee, which often is associated with arthritis in older patients, can lead to too much synovial fluid. A knee injury, which may cause a tear in the cartilage, can also lead to a knee cyst, as can a tear in a ligament.

Knee cysts caused by arthritis can be identified with an x-ray.
Knee cysts caused by arthritis can be identified with an x-ray.

Symptoms and Diagnosis

The symptoms of a popliteal growth can vary. Some people may don't have any symptoms, while others have stiffness, swelling and knee pain that may get worse when the leg is extended or during activity. Many people have a noticeable, tender bulge that feels like a water balloon on the back of the knee. Anyone who experiences quick bruising and swelling accompanied by pain in the knee should seek professional care immediately, because these symptoms are often associated with blood clots.

Stiffness, swelling, and pain may be a symptom of a knee cyst.
Stiffness, swelling, and pain may be a symptom of a knee cyst.

When diagnosing a knee cyst, healthcare professionals first conduct a physical exam. They check the back of the knee for growths, and evaluate the person's range of motion with the affected leg. To see if the cartilage in the knee is torn, a healthcare professional can use a procedure known as transillumination to pass light through the cyst to look for fluid. Magnetic Resonance Imaging (MRI) scans can also help determine if the growth might be a tumor, especially if the person has a fever. X-rays might be required to determine if the cyst is associated with arthritis.

Physical therapy is a non-surgical option to treat a knee cyst.
Physical therapy is a non-surgical option to treat a knee cyst.

Non-Surgical Treatments

Cysts that cause pain and limited mobility are usually treated non-surgically first. Healthcare professionals usually inject a corticosteroid into the knee to relieve pain. After this, a person with a knee cyst needs to rest as much as possible, apply ice to the area and wrap the knee in bandages. Physical therapy can help improve range of motion. Large growths can be aspirated, or drained with a needle. In some cases, a knee cyst will burst on its own, causing bruising, swelling, and pain. Though this is unpleasant, it usually doesn't require any treatment aside from painkillers, as the body will absorb the fluids in a matter of weeks.

Some knee cysts require surgical treatments.
Some knee cysts require surgical treatments.

If the growth is not painful or interfering with activities, it may not require treatment at all. These growths often dissipate over time on their own, although this can take months or years. If a healthcare professional decides to hold off on treatment, he or she may advise a person to wear an elastic knee bandage to support the knee.


An individual who has torn cartilage may undergo surgery to fix or remove this damaged area of the knee. When surgically treating a knee cyst, the health care professional aims to eradicate the swollen tissue that contributes to the formation of the growth. If no other treatments work, then he or she can remove the fluid-filled sac as a final resort. Most healthcare professionals try to avoid this, however, as there is a risk of damaging the surrounding tissues, nerves, and blood vessels in the knee.


A person who is susceptible to developing knee cysts should avoid performing exercises that are hard on the knees or legs. Diets that are low in sodium further reduce inflammation, which can cause cysts. In addition, an individual might need to lose weight to prevent putting too much strain on the knee joint.

An ice pack might help with the swelling caused by a knee cyst.
An ice pack might help with the swelling caused by a knee cyst.

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Discussion Comments


I had a standing job for 12 hours a day. The stress made both of my knees to go out, and I woke up unable to walk for two months. At the same time I had cysts and rashes growing on my hands and wrists. Now I am researching these cysts as the cause of my knee injury. I know I had dental decay in 2 molars, and had recently taken two MRI's with dye before these rashes appeared.


I've felt knee pain all my life. I'm 19 now almost 20. I was involved in taekwondo, marching band and bowling in high school.

I've noticed that there is a hard marble like lump on the inner side of my right knee. It stiffens and is painful to move sometimes. I had people say it's a torn cartilage and/or a cyst. I used to wear knee braces (max ones) but they don't work anymore.

I can't afford to go get the procedures done to see what it is, but does anyone know something that will help this?


@naturesgurl3 -- It sounds like you you might have a Baker's cyst, especially since there's no pain, but in my experience Baker's cysts tend to show up on the side or front of the knee rather than the back.

Are you sure that your cyst isn't a sebaceous cyst (one in the skin)? That would seem slightly more likely for a cyst behind the knee, especially since the back of the knees is usually pretty fleshy on most people.

Again, I'm not saying you don't have a Baker's cyst of the knee, but I'm just saying you might want to get a doctor to check it out before you start hitting up the knee cyst treatment aisle at your local drugstore -- they can give you a little better idea of what could be going on, and then you'll be able to treat it in the most effective way.


If you have a cyst behind the knee joint, what is the treatment? I randomly noticed a lump on the back of my knee the other week, and I thought that I had just banged my leg or something, but it didn't go down.

So now I'm worried that it could be a cyst. I read that a lot of knee pain is from cysts, but I'm lacking that symptom. There's no pain, just the lump. Does that sound like it could be a bakers lump, or a cyst in my knee?

I'm not too worried about it since it's not getting any bigger, and it's not painful, but still, I'd like to keep an eye on it. Is this the best course, or should I see my orthopedist?


I've recently been experiencing a lot of pain in my knee, so I'm starting to wonder if it could be caused by a cyst.

I am a hard-core runner (I do 6 miles a day, minimum), so I'm kind of used to knee pain, but this is different than just muscle soreness or a joint issue.

Do you think that I could possibly have a cyst in the knee? There's no lump, and no other symptoms besides pain. I had a friend who once had a baker cyst behind the knee take a look at it, but he said he wasn't sure. What do you think could be going on?

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