What are the differential diagnoses and appropriate work‑up and management for a firm, palpable swelling just below the knee in an adult with no history of knee injury, arthritis, or clotting disorder?

Firm Area of Swelling Below the Knee: Differential Diagnosis and Management

Begin with plain radiographs of the knee to exclude fracture, degenerative changes, or tumor, then proceed to ultrasound examination to differentiate between the most common causes: Baker's cyst, deep vein thrombosis, soft-tissue infection, or popliteal artery aneurysm. 1, 2

Initial Clinical Assessment

Key Historical Features to Elicit

• Onset and progression: Acute onset with calf pain suggests ruptured Baker's cyst or DVT, while gradual onset suggests intact Baker's cyst or soft-tissue mass 2, 3
• Pain characteristics: Behind-the-knee tenderness with swelling that worsens with exercise but persists at rest indicates Baker's cyst, distinct from intermittent claudication 1, 2
• Risk factors for DVT: Recent immobilization, surgery within 12 weeks, active cancer, or indwelling catheters increase DVT probability 1
• History of knee pathology: Osteoarthritis, meniscal tears, or rheumatoid arthritis commonly associate with Baker's cysts 4

Critical Physical Examination Findings

• Palpate all lower extremity pulses (femoral, popliteal, dorsalis pedis, posterior tibial) - a prominent popliteal pulse in the opposite leg may indicate popliteal aneurysm 1
• Assess for popliteal mass: Fluctuant swelling behind the knee that increases with extension suggests Baker's cyst 2, 4
• Evaluate for vascular compromise: Absent pulses, pallor, cool extremity, or delayed capillary refill indicates potential popliteal artery injury requiring emergent intervention 5
• Check for signs of infection: Erythema, warmth, and skin lesions suggest soft-tissue infection or cellulitis 1

Diagnostic Algorithm

Step 1: Plain Radiographs (Initial Study)

Obtain anteroposterior, lateral, and patellofemoral views of the knee to outline anatomic detail, evaluate for fractures, degenerative changes, tumor, or radiodense foreign bodies. 1, 2

Step 2: Risk Stratification for DVT

• Calculate Wells score if DVT is suspected clinically - a score ≥2 indicates likely DVT 1
• Common pitfall: Do not rely on Wells score and D-dimer alone to exclude ruptured Baker's cyst, as these methods cannot distinguish between the two conditions 2
• If Wells score <2 (unlikely DVT) and high-sensitivity D-dimer is negative, DVT is safely excluded and ultrasound is not needed 1

Step 3: Ultrasound Examination (Preferred Next Study)

Order complete duplex ultrasound from inguinal ligament to ankle with compression at 2-cm intervals to simultaneously evaluate for DVT and Baker's cyst. 1, 2

• For Baker's cyst: Look for comma-shaped fluid collection between medial gastrocnemius head and semimembranosus tendon 2, 4
• For DVT: Non-compressible vein indicates acute thrombosis 1
• For soft-tissue infection: Ultrasound detects fluid collections, abscesses, and joint effusions 1
• Advantage: Ultrasound has comparable accuracy to MRI for diagnosing Baker's cyst and can detect cyst rupture 2, 6

Step 4: MRI Without IV Contrast (When Additional Evaluation Needed)

Order MRI when ultrasound is inconclusive, concomitant internal knee pathology is suspected, or to evaluate extent of soft-tissue infection. 1, 2

• MRI accurately depicts effusion extent, synovitis, and Baker's cyst rupture 1, 2
• MRI is superior for detecting inflammation, fasciitis, myositis, and areas of necrosis in soft-tissue infections 1
• Do not order MRI with IV contrast for routine evaluation - non-contrast is sufficient 5

Step 5: CT Angiography (If Vascular Injury Suspected)

Order CTA of lower extremity immediately if palpable popliteal mass with absent pulses or history suggesting aneurysm, as popliteal artery aneurysm requires urgent surgical repair. 1, 5

• Popliteal artery aneurysms ≥2.0 cm diameter require repair to prevent thromboembolic complications and limb loss 1
• Critical pitfall: Do not assume all popliteal masses are benign Baker's cysts - always exclude popliteal artery aneurysm with imaging, especially in patients with other arterial aneurysms 2

Differential Diagnosis by Clinical Pattern

Firm Swelling Behind Knee Without Trauma

1. Baker's cyst (most common): Fluctuant popliteal mass, may have underlying knee arthritis or meniscal pathology 2, 4
2. Popliteal artery aneurysm: Pulsatile mass, may have contralateral popliteal or other peripheral aneurysms 1
3. Soft-tissue mass: Requires MRI for characterization if ultrasound shows solid mass 1

Acute Onset Calf Pain and Swelling

1. Ruptured Baker's cyst: Sudden calf pain with fluid dissection into calf muscles, mimics DVT 2, 7, 3
2. Deep vein thrombosis: Non-compressible vein on ultrasound, positive Wells score 1
3. Venous claudication: History of iliofemoral DVT, edema, signs of venous stasis, symptoms subside slowly with leg elevation 1

Swelling With Erythema and Warmth

1. Soft-tissue infection/cellulitis: Skin changes, possible abscess on ultrasound 1
2. Septic arthritis: Joint effusion requiring aspiration for culture 1
3. Charcot neuro-osteoarthropathy (diabetics): Temperature asymmetry >2°C between limbs, flattened midfoot on imaging 8

Management Based on Diagnosis

Baker's Cyst (Intact)

• Conservative management: Most resolve spontaneously with rest and graduated physiotherapy over 3-4 months 9
• Ultrasound-guided aspiration with corticosteroid injection may provide temporary relief for symptomatic cysts with associated synovitis 2
• Treat underlying knee pathology (osteoarthritis, meniscal tears) 4

Ruptured Baker's Cyst

• Conservative management is first-line: Rest, leg elevation, NSAIDs - most improve within 12 weeks 9, 3
• Avoid arthroscopic surgery for degenerative knee disease associated with Baker's cysts, as evidence shows no benefit over conservative management 2

Deep Vein Thrombosis

• Anticoagulation therapy is indicated for proximal DVT (above-the-knee) to reduce risk of pulmonary embolism 1
• For distal DVT (below-the-knee), either anticoagulate or perform serial ultrasound at 1 week to exclude proximal extension 1

Popliteal Artery Aneurysm

• Surgical repair required for aneurysms ≥2.0 cm diameter to prevent thrombosis and limb loss 1
• If acute thrombosis with absent runoff, catheter-directed thrombolysis or mechanical thrombectomy to restore distal flow 1

Soft-Tissue Infection

• MRI to delineate extent of infection, identify abscesses requiring drainage 1
• Ultrasound-guided or CT-guided percutaneous drainage for abscesses 1
• Joint aspiration for suspected septic arthritis - positive culture is diagnostic 1