A Baker’s cyst (also known as a popliteal cyst) forms when tissue at the back of your knee fills with synovial fluid – the lubricating fluid from your knee joint. This makes it swollen and inflamed.
Cysts can vary in size and can, very occasionally, develop behind both knees at the same time.
Baker’s cyst is much more common in adults than children, with more women than men affected.
In some cases there are no symptoms, but an MRI scan for another reason finds the cyst. However, the larger the cyst, the more likely it is that you'll notice symptoms other than the swelling itself.
These symptoms can include:
Occasionally a Baker’s cyst ruptures which can be painful, with swelling and redness in your calf and may become infected.
Your GP will examine your knee and ask questions about the swelling and how you’re feeling.
You'll usually get a diagnosis from your GP without any further tests. However, in some cases you might have an ultrasound scan or MRI scan. This is to rule out other causes, eg deep vein thrombosis (DVT) or an aneurysm (a bulge in a blood vessel), or to check the extent of the cyst.
A child may be referred to a specialist paediatric orthopaedic surgeon if there’s the possibility of an underlying joint disease.
It can also affect people who have gout.
Other possible causes of Baker’s cyst include a sports-related injury such as:
In children, the cyst may relate to injury or irritation of the bursa. Cysts may also occur with conditions such as juvenile idiopathic arthritis.
There are things you can do yourself to treat Baker’s cyst including rest and taking ibuprofen or paracetamol, if advised by your doctor.
In many cases the swelling goes away by itself. Using compression bandages and an ice pack can also help.
A cyst may sometimes be aspirated. This means a fine needle is inserted to remove fluid. In some cases, surgery may be recommended to repair damage around the joint.
If there’s an underlying condition, treating it will usually shrink the cyst.
Other treatments related to Baker’s cyst include: