Baker's cyst
Exam Tips
- In adults, treat a Baker’s cyst as secondary until proven otherwise: actively look for OA, meniscal injury, or inflammatory arthritis.
- Foucher’s sign is high-yield: cyst becomes more tense in extension and softer/less obvious in flexion.
- Acute calf pain/swelling with a known cyst is pseudo-thrombophlebitis until DVT is excluded urgently.
- Ultrasound is first-line and is recommended in all children to confirm diagnosis and exclude serious mimics.
- Primary care does not usually perform direct cyst aspiration; focus on analgesia, underlying pathology, and red-flag triage.
- Any neurovascular compromise, severe escalating pain, systemic illness, or inability to weight bear needs urgent same-day secondary care assessment.
Definition
A Baker’s cyst (popliteal cyst) is a fluid-filled distension of the gastrocnemius-semimembranosus bursa in the posteromedial knee, rather than a true epithelial-lined cyst. In adults it is usually secondary to intra-articular pathology (for example osteoarthritis, inflammatory arthritis, or meniscal injury), whereas in children it is more often primary and idiopathic.
Pathophysiology
Synovial fluid tracks from the knee joint into the gastrocnemius-semimembranosus bursa through a communicating slit-like opening that can function as a one-way valve, so fluid enters more easily than it exits. Anything that increases intra-articular effusion (degenerative, inflammatory, traumatic, or infective joint disease) can enlarge the cyst. Distension causes posterior knee tightness and reduced flexion/extension; rupture or dissection of fluid into calf tissue can trigger an acute inflammatory picture (pseudo-thrombophlebitis) that clinically mimics DVT. See Figure: posteromedial popliteal fossa anatomy and gastrocnemius-semimembranosus bursa in standard MSK anatomy texts.
Risk Factors
- Knee osteoarthritis
- Inflammatory arthropathy (for example rheumatoid arthritis, seronegative disease)
- Meniscal tear
- Anterior cruciate ligament injury or other internal derangement
- Gout
- Juvenile idiopathic arthritis (in children with secondary cysts)
- Less common associations: pigmented villonodular synovitis, connective tissue disease, septic arthritis, Charcot arthropathy, tuberculosis
Clinical Features
Symptoms
- Posterior knee swelling or palpable lump (may be incidental, especially in children)
- Posterior knee ache, pressure, or tightness
- Pain worsened by activity/walking
- Stiffness or reduced knee range of movement (larger cysts)
- Acute calf pain/swelling/warmth if rupture or dissection occurs
Signs
- Round/fluctuant bulge in medial popliteal fossa, more apparent standing
- Tender popliteal mass
- Foucher’s sign: mass is tense in extension and softens/disappears in flexion
- Restricted knee movement in larger cysts
- If complicated: calf erythema, warmth, tenderness (consider DVT urgently)
Investigations
Management
Lifestyle Modifications
- Reassure if asymptomatic (especially primary cysts in children), as many resolve spontaneously
- Activity modification during painful flares; avoid deep flexion that provokes symptoms
- Physiotherapy to optimize knee mechanics and function
- Treat and optimize the underlying knee disorder (for example OA, RA, gout)
- Safety-net urgently for sudden calf swelling/pain, neurovascular symptoms, fever, inability to weight bear, or rapidly changing mass
Pharmacological Treatment
Simple analgesic
- Paracetamol 1 g orally every 4-6 hours when required (maximum 4 g in 24 hours)
First-line for pain if no contraindication; reduce maximum daily dose in low body weight, severe liver disease, or chronic alcohol misuse.
NSAID
- Ibuprofen 200-400 mg orally three times daily with food (usual prescription max 2.4 g/day; lower limits used for self-care products)
Use the lowest effective dose for shortest duration. Avoid/caution in CKD, peptic ulcer disease, heart failure, uncontrolled hypertension, anticoagulant use, aspirin-sensitive asthma, and in late pregnancy; consider gastroprotection (for example omeprazole) if GI risk is high.
Intra-cyst/interventional corticosteroid (secondary care)
- Ultrasound-guided aspiration/fenestration with corticosteroid injection (for example triamcinolone acetonide, local protocol dose)
Not recommended as direct aspiration in primary care. Exclude infection before steroid injection; counsel on recurrence and post-procedure flare/infection risk.
Surgical / Interventional
- Primary care: no routine aspiration or excision
- Secondary care: ultrasound-guided aspiration with cyst wall fenestration and steroid injection in selected symptomatic cases
- Arthroscopic treatment of intra-articular pathology with communication enlargement/decompression when persistent and function-limiting
- Open cyst excision rarely, usually only for large persistent symptomatic cysts after treating underlying pathology; recurrence can be high
Complications
- Chronic pain and reduced mobility
- Cyst rupture (often clinically silent) or dissection into calf
- Pseudo-thrombophlebitis syndrome mimicking DVT
- Haemorrhage into cyst/calf (higher risk with anticoagulation)
- Compression effects: true DVT (venous compression), arterial compromise/ischaemia, nerve entrapment
- Compartment syndrome after rupture/dissection (rare but limb-threatening)
- Infection (spontaneous or post-injection)
Prognosis
Children without underlying joint disease usually improve spontaneously over time. Adults commonly improve or stabilize, particularly when the underlying joint pathology is treated; persistent cysts are more likely with severe OA or active inflammatory disease. Operative approaches can relieve symptoms in selected patients but recurrence remains a recognized issue.
Sources & References
🏥BMJ Best Practice(1)
âś…NICE Guidelines(1)
- Baker's cyst[overview]