Baker’s cysts are also called popliteal cysts. A Baker’s cyst is a fluid-filled sac in the popliteal fossa, causing a lump.
The popliteal fossa is the diamond-shaped hollow area formed by the:
- Semimembranosus and semitendinosus tendons (superior and medial)
- Biceps femoris tendon (superior and lateral)
- Medial head of the gastrocnemius (inferior and medial)
- Lateral head of the gastrocnemius (inferior and lateral)
Pathophysiology
In adults, Baker’s cysts are usually secondary to degenerative changes in the knee joint. They can be associated with:
- Meniscal tears (an important underlying cause)
- Osteoarthritis
- Knee injuries
- Inflammatory arthritis (e.g., rheumatoid arthritis)
Synovial fluid is squeezed out of the knee joint and collects in the popliteal fossa. A connection between the synovial fluid in the joint and the Baker’s cyst can remain, allowing the cyst to continue enlarging as more fluid collects there.
Baker’s cysts are contained within the soft tissues. They do not have their own epithelial lining.
Presentation
Patients may present with symptoms localised to the popliteal fossa:
- Pain or discomfort
- Fullness
- Pressure
- A palpable lump or swelling
- Restricted range of motion in the knee (with larger cysts)
On examination, the lump will be most apparent when the patient stands with their knees fully extended. The lump will get smaller or disappear when the knee is flexed to 45 degrees (Foucher’s sign).
Oedema may occur if the cyst compresses the venous drainage of the leg.
Ruptured Baker’s Cyst
Baker’s cysts can rupture if the pressure is large enough.
Ruptured Baker’s cyst causes inflammation in the surrounding tissues and calf muscle, presenting with:
- Pain
- Swelling
- Erythema
A critical differential diagnosis of a ruptured Baker’s cyst is a deep vein thrombosis (DVT).
A ruptured Baker’s cyst can rarely cause compartment syndrome.
Differential Diagnoses
The key differential diagnoses of a lump in the popliteal fossa are:
- Deep vein thrombosis
- Abscess
- Popliteal artery aneurysm
- Ganglion cyst
- Lipoma
- Varicose veins
- Tumour
Investigations
Ultrasound is usually the first-line investigation to confirm the diagnosis. It is also used to rule out a DVT.
MRI can evaluate the cyst further if required, for example, before surgery. They can also demonstrate underlying knee pathology, such as meniscal tears.
Management
No treatment is required for asymptomatic Baker’s cysts.
Non-surgical management for symptomatic Baker’s cysts include:
- Modified activity to avoid exacerbating symptoms
- Analgesia (e.g., NSAIDs)
- Physiotherapy
- Ultrasound-guided aspiration
- Steroid injections
Surgical management typically involves arthroscopic procedures to treat underlying knee pathology contributing to the cyst, such as degenerative changes or meniscal tears. Resection of the cyst is difficult, and the cyst is likely to recur, particularly when another knee pathology is present.
Last updated August 2021
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