Baker’s Cyst

What is a Baker's cyst?

A Baker's cyst (also called a popliteal cyst) is a fluid-filled swelling at the back of the knee. It forms when the synovial fluid that normally lubricates the joint accumulates in a sac that bulges out behind the knee.

In adults, a Baker's cyst is almost always secondary to another problem inside the knee, most commonly a meniscus tear or osteoarthritis. The underlying joint condition produces excess fluid, which finds its way into the cyst.

What are the symptoms?

• a visible or palpable lump at the back of the knee
• tightness or pressure behind the knee, especially with deep flexion
• swelling that fluctuates with activity
• occasionally, calf pain if the cyst ruptures and fluid tracks down the leg
• symptoms of the underlying knee problem (joint-line pain, stiffness)

How is it diagnosed?

• clinical examination, the cyst is usually palpable behind the knee
• ultrasound, confirms the cyst and excludes a deep vein thrombosis (DVT)
• MRI, visualises the cyst and identifies the underlying knee pathology

How is it treated?

Treatment depends on the underlying cause and the severity of symptoms:

• small asymptomatic cysts, usually need no treatment, just observation
• symptomatic cysts, treatment of the underlying joint problem (meniscus surgery, arthritis management)
• large painful cysts, ultrasound-guided aspiration and corticosteroid injection
• recurrent cysts, surgical excision is rarely needed and only considered after other options

In most cases, treating the underlying cause causes the cyst to resolve naturally.

When should I be concerned?

A Baker's cyst should be evaluated when:

• the swelling is large or progressively enlarging
• there is significant pain, stiffness or limitation of activity
• sudden calf pain, swelling or redness develops, this requires urgent assessment to exclude DVT or cyst rupture
• there is associated knee pain or mechanical symptoms suggesting an internal joint problem