Summary

A Baker's cyst (popliteal cyst) is a fluid-filled synovial cyst that forms in the popliteal fossa, typically as a distension of the gastrocnemius-semimembranosus bursa. It is almost always secondary to intra-articular knee pathology (osteoarthritis, meniscal tears, inflammatory arthritis) that drives excess synovial fluid production. A one-way valve mechanism allows fluid to escape from the knee joint into the bursa but not return. Patients present with posterior knee swelling that is firm on extension and soft on flexion (Foucher's sign). The main clinical concern is rupture, which causes sudden calf pain and swelling mimicking DVT ("pseudothrombophlebitis syndrome"). Treatment is directed at the underlying knee pathology.

Key Facts

• Location: Popliteal fossa (posterior knee)
• Bursa: Gastrocnemius-semimembranosus bursa
• Cause: Secondary to knee joint pathology (OA, meniscal tears, RA)
• Mechanism: One-way valve - fluid enters but can't return
• Clinical Sign: Foucher's sign (firm on extension, soft on flexion)
• Complication: Rupture mimics DVT → Must rule out DVT
• Treatment: Treat underlying knee pathology

Clinical Pearls

"Pseudothrombophlebitis Syndrome": A ruptured Baker's cyst causes sudden calf pain, swelling, and bruising - clinically IDENTICAL to DVT. Always rule out DVT first.

"Foucher's Sign": A Baker's cyst is firm when the knee is extended (tense) and soft when flexed (relaxed). This helps differentiate from solid masses.

"Treat the Knee, Not the Cyst": Aspirating a Baker's cyst gives temporary relief but high recurrence. The underlying knee pathology must be addressed.

"Crescent Sign": Bruising around the ankle (from tracking of extravasated fluid) is a classic sign of ruptured Baker's cyst.

Inspection

• Swelling in popliteal fossa (medial aspect)
• Best seen with patient standing
• May be large and visible

Palpation

• Foucher's Sign: Firm on extension, soft on flexion
• Smooth, well-defined mass
• Non-tender (unless ruptured)
• Transillumination may be positive

Special Tests

Knee Examination

• Effusion (cross-fluctuation, patellar tap)
• Signs of OA (crepitus, reduced ROM)
• Meniscal tests (McMurray)

Natural History

• Often stable or slowly progressive
• Rupture common without warning
• Recurrence common if underlying knee pathology persists

With Treatment

• Addressing underlying knee pathology often resolves or reduces cyst
• Knee replacement for OA: Often resolves cyst
• Aspiration alone: 50%+ recurrence

Factors Affecting Prognosis

Key Guidelines

1. NICE CKS: Knee Pain - Assessment
2. American Academy of Orthopaedic Surgeons

Key Evidence

Association with Knee Pathology

• 94% of Baker's cysts are associated with intra-articular pathology
• Treating underlying cause is most effective management

MRI Studies

• Baker's cysts found in 19-47% of patients with knee OA on MRI
• Many asymptomatic

What is a Baker's Cyst?

A Baker's cyst is a fluid-filled swelling at the back of the knee. It's also called a popliteal cyst. It's not really a separate problem - it usually happens because of something going on inside the knee, like arthritis or a cartilage tear.

What Causes It?

The knee produces extra fluid when it's damaged or inflamed. This fluid can push out through a weak spot at the back of the knee, creating a balloon-like swelling.

What Does It Feel Like?

• A soft lump behind the knee
• Worse when you stand up straight
• May cause aching or stiffness
• Some cysts cause no symptoms at all

Can It Burst?

Yes. If the cyst ruptures, the fluid leaks down into your calf. This causes:

• Sudden calf pain
• Swelling in the calf
• Bruising around the ankle

This can look very similar to a blood clot (DVT), so it's important to see a doctor urgently to rule this out.

How is it Treated?

• Treating the underlying knee problem (e.g., physiotherapy, weight loss, sometimes surgery)
• Draining the cyst with a needle (but it often comes back)
• Surgery (rarely needed)

Do I Need to Worry?

Baker's cysts are generally harmless. The main concern is if it ruptures (see above) or if the swelling is actually something else. If you notice a lump behind your knee, it's worth getting checked by your doctor.