Date issued: September 2019
For review: September 2021
Ref: A-181/Medical specialities/Medicine/ZL/baker's cyst v4
A Baker's cyst is an accumulation of synovial fluid (joint fluid) that forms behind your knee and is named after the British surgeon who first described the condition, William Morrant Baker.
It's estimated that up to one in five people with an existing knee problem is likely to develop a Baker's cyst.
What causes it?
Like all joints, the knee needs lubrication to function properly. This lubricant is a jelly-like substance called synovial fluid which is made by a tissue that is inside of the knee joint called the synovium.
When the knee is 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. As more fluid accumulates in the cyst, it expands like a balloon.
The most common cause of damage that triggers the process is arthritis, usually osteoarthritis, but also other less common forms of arthritis for example rheumatoid arthritis.
What are the symptoms?
If the cyst is small it may remain unnoticed and therefore not cause any symptoms at all. However if it gets bigger, people become aware of it 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. Sometime though it can become inflamed and painful
One possible complication is rupture of the cyst, leading to fluid leaking down inside the leg. This can cause a painless bruise to appear around the inner ankle, or may mimic the symptoms of a deep vein thrombosis causing swelling, redness and pain in the calf.
Your GP may visually examine the back of your knee and may use a light to see how much synovial fluid is inside the cyst. Comparing the affected knee with the normal one can be helpful. Your range of motion of the knee may be limited by pain and/or stiffness.
What's the treatment?
Often no specific treatment is necessary and in time the cyst may resolve and symptoms improve of their own accord. If inflammation occurs in a Baker's cyst anti-inflammatory medication and/or pain-killers can be used with a support bandage around the knee. If this is not effective and the knee/cyst is still painful, a steroid injection into the knee can be done as well.
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.