Baker's cyst Dr Rob Hicks A Baker's cyst is an accumulation of fluid behind the knee and is named after the British surgeon who first described the condition, William Morrant Baker. It's also known as a popliteal cyst.

In this article

What causes it? Who's affected?

What are the symptoms? What's the treatment?

Like all joints, the knee needs lubrication to function properly. This lubricant is a jelly-like substance called synovial fluid.

When the knee's damaged, more synovial fluid is produced. Under pressure, this fluid stretches the lining of the joint, called the joint capsule, out into the back of the knee. This causes a bulge, known as a Baker's cyst.

Herniation of the joint capsule is responsible for most Baker's cysts in adults.

The most common cause of damage that triggers the process is arthritis, usually osteoarthritis. Other types of knee injury, such as tears to the cartilage, may also be responsible.

It's estimated that up to one in three people with an existing knee problem is likely to develop a Baker's cyst.

They can occur at any age, but are particularly common in children aged between four and seven and adults aged between 35 and 70.

If the cyst is small it may remain unnoticed and many Baker's cysts don't cause any symptoms at all.

People become aware of the cyst when they notice a bulge behind one knee that isn't behind the other. In most cases when touched or prodded the bulge doesn't feel sore, but rather like a water-filled balloon.

When a Baker's cyst does cause symptoms, these usually include pain or tightness behind the knee, particularly when the joint is bent or straightened.

One possible complication is rupture of the cyst, leading to fluid leaking down inside the leg. This can cause pain, bruising, redness and swelling of the calf, mimicking the symptoms of a deep vein thrombosis.

Often no specific treatment is necessary - in time the cyst may resolve and symptoms improve of their own accord.

However, it's important to treat the underlying cause of the problem, such as arthritis, to prevent further development of the cyst. Doing this often helps to relieve any swelling and discomfort caused by the cyst.

If the cyst is large or causing pain, drawing off the fluid with a needle can help, but the cyst will often recur following this procedure.

A cortisone injection can help by reducing the amount of fluid produced and relieving painful inflammation.

Surgery to remove the cyst and to repair any causative damage, for example a torn cartilage, can be performed if other measures have not solved the problem.

This article was last medically reviewed by Dr Rob Hicks in November 2007