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Baker's Cyst

(Popliteal cyst)

What is a Baker's cyst?

A Baker's cyst, also known as a popliteal cyst in adults, is a fluid-filled sac that forms at the back of your knee. It can vary in size as the fluid content varies.

What causes a Baker cyst?

A Baker's cyst is normally the result of a knee injury or joint disorder that causes inflammation inside the knee, such as osteoarthritis, rheumatoid arthritis or meniscus tear.

In response to the inflammation, the knee produces an excess of synovial fluid which is able to pass into the popliteal bursa at the back of the knee. When fluid accumulates in the bursa causing it to bulge it is called a Baker's cyst.

What are the symptoms of a Baker's cyst?

Many people that have a Baker's cyst are unaware of it because the majority of cases do not have any symptoms. The first time you may become aware of it is if you feel a bulge at the back of the knee or you experience a restriction in your range of motion when bending to kneel or to do a squat. Sometimes the bulge is so large that you may struggle to fully bend or straighten the knee.

A Baker's cyst can cause pain at the back of the knee and calf region. Symptoms can change depending on the position of the knee. 

What are the potential complications of a Baker's cyst?

Ruptured cyst - On rare occasions, a Baker's cyst can rupture causing pain, warmth and redness in the region of the calf muscles. The appearance and symptoms are similar to a deep vein thrombosis (DVT)/blood clot, or infection (cellulitis) both of which are conditions that require urgent medical attention.

Not every swelling at the back of the knee is a Baker's cyst. There is always the possibility of a soft tissue tumour which your doctor will look to rule out.

If you develop a pain in your calves, with or without warmth, redness and swelling you should make an appointment to see a doctor urgently.

How is a Baker cyst diagnosed?

Diagnosing a Baker's cyst involves getting your medical history and performing a physical examination to look for conditions that can contribute to the formation of the cyst.

A diagnostic ultrasound scan can be carried out on the same day of your appointment to confirm the diagnosis. X-rays of the knee joint or other blood tests may be requested if an underlying condition is suspected.

Magnetic resonance imaging (MRI) are not routinely requested, but more often painless Baker's cysts are incidentally found when MRI scans are done for other reasons, such as when investigating the cause of joint pain.

What are the treatment options?

Most Baker's cysts are not problematic, so if you don't get any symptoms from it you don't need treatment.

Aspiration (fluid-drainage) and injection of a corticosteroid and local anaesthetic can be done if a Baker's cyst is symptomatic. This is ideally done using ultrasound guidance to provide real-time visualisation of the needle placement and avoid injury to nearby blood vessels.

Chances of re-accumulation of synovial fluid following drainage of a Baker's cyst is high, but this may be reduced by addressing the underlying cause of the cyst in the first place. For example, if your cyst is a result of having osteoarthritis of the knee joint this will also need addressing.

Before undergoing drainage of a symptomatic Baker's cyst make sure any underlying cause has been explored and addressed, otherwise you risk having rapid re-accumulation.

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