Popliteal (Baker) cyst

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Popliteal cyst, also called Baker cyst, is a synovial fluid-filled distention of a bursa in the popliteal fossa. It can be idiopathic or associated with underlying conditions, e.g., meniscal tear or osteoarthritis. Popliteal cysts are often discovered incidentally on imaging or physical examination but can also cause posterior knee pain and/or limit knee flexion. Large or ruptured cysts can cause calf swelling and may mimic the signs and symptoms of DVT. The diagnosis is usually clinical, but ultrasound is often used to exclude DVT and confirm the diagnosis. X-ray and/or MRI can be obtained to assess for underlying joint disease. Management of symptomatic cysts typically involves a trial of nonsurgical measures, e.g., pain management with NSAIDs and cyst drainage. Surgical resection may be considered for complications such as neurovascular compression or for persistent symptomatic cysts.

• A synovial fluid-filled distention of a bursa in the popliteal fossa, typically located between the medial head of the gastrocnemius muscle and the semimembranosus muscle ^[2]
• Often communicates with the synovial space of the knee joint

• Inflammation of the synovium stimulates excessive production of synovial fluid. ^[4]
• Risk factors: inflammatory, degenerative, or traumatic intraarticular changes in the knee joint (e.g., rheumatoid arthritis, osteoarthritis, meniscal lesions)
• Idiopathic (primary popliteal cysts) : most common in children

Popliteal cysts are typically asymptomatic and found incidentally on physical examination or imaging. ^[3][4]

• Signs
  • Palpable mass in the medial popliteal fossa
  • Foucher sign: Cyst is firm in full knee extension and soft in knee flexion.
• Possible symptoms
  • Posterior knee pain, tenderness, knee flexion limitation
  • Symptoms of the underlying pathology: e.g., meniscal tear, rheumatoid arthritis
• Ruptured popliteal cyst: : leakage of synovial fluid into the calf muscles
  • Calf swelling and pain, mimicking acute compartment syndrome or DVT
  • Can cause bruising below the medial malleolus (crescent sign) ^[5]

A palpable change in cyst consistency when flexing or extending the knee differentiates popliteal cysts from other masses, which are not affected by knee position. ^[4]

• Clinical evaluation ^[3][4]
  • Popliteal cyst is usually a clinical diagnosis.
  • Perform neurovascular examination to rule out neurovascular compression.
• Imaging: to rule out other causes of posterior knee pain or lower limb swelling (e.g., DVT) ^[3][4]
  • Ultrasound: anechoic lesion
  • Plain x-ray: soft tissue mass
  • MRI: can help identify underlying knee joint pathology

Obtain an ultrasound, as a large or ruptured popliteal cyst may cause calf swelling and can mimic the signs of DVT (pseudothrombophlebitis syndrome). ^[3]

• DVT
• Popliteal artery aneurysm
• Seroma or hematoma
• Meniscal cyst
• Soft tissue tumor

The differential diagnoses listed here are not exhaustive.

• Asymptomatic cysts: Treatment is not required in most cases. ^[3][4]
• Symptomatic cysts ^[3][4]
  • Nonsurgical management: preferred for at least 6 weeks if there are no neurovascular complications ^[4]
    • Rest, elevation, compression
    • Pain management, e.g., with NSAIDs ^[3]
    • Treatment of underlying knee joint pathology
    • If no improvement: Refer to orthopedics for consideration of ultrasound-guided drainage and/or intraarticular glucocorticoid injection.
  • Surgical resection: for persistent symptoms and/or neurovascular compression
• Ruptured popliteal cyst: conservative treatment, e.g., pain relief, rest, elevation

Consider referral to orthopedics for persistent symptomatic popliteal cyst despite conservative measures.

• Popliteal cyst rupture: can cause compartment syndrome in rare cases
• Pseudothrombophlebitis syndrome: caused by a large or ruptured popliteal cyst
  • Calf swelling and/or pain
  • Positive Homans sign
• Cyst infection
• Neurovascular compression: e.g., tibial nerve injury, DVT due to venous compression

We list the most important complications. The selection is not exhaustive.