Three Small Lumps in a Row Behind the Knee
The most likely diagnosis is a Baker's cyst with loculated fluid compartments, which presents as swelling and tenderness behind the knee in the popliteal fossa. 1
Most Probable Diagnosis: Baker's Cyst
Baker's cysts are fluid accumulations in the bursa of the gastrocnemius or semimembranosus muscles that frequently communicate with the knee joint space. 1 The "three lumps in a row" appearance likely represents:
- Loculated or multiloculated fluid collections within the cyst structure 1
- The characteristic comma-shaped appearance when viewed with ultrasound between the medial head of gastrocnemius and semimembranosus tendon 1
- Extension of the cyst into surrounding tissues, as Baker's cysts can extend far into thigh and calf muscles 1
Key Clinical Features to Assess
- Visible and palpable swelling in the popliteal fossa, especially with the knee extended 1
- Fluctuant mass that may increase in size with knee extension 1
- Tenderness in the popliteal fossa is common 1
- Pain may worsen with knee movement or exercise, and unlike vascular claudication, symptoms can be present at rest 1
- Possible limitation in range of motion of the knee joint 1
Immediate Diagnostic Approach
Ultrasound is the preferred initial diagnostic tool to confirm the presence of a Baker's cyst, with comparable accuracy to MRI for this specific purpose. 1, 2 This approach is supported by the 2025 UK guidelines emphasizing ultrasound as the most effective initial triage tool. 2
Imaging Algorithm:
Plain radiographs of the knee first (AP, lateral, sunrise/Merchant, and tunnel views) to evaluate for underlying joint pathology 1
Ultrasound examination to:
MRI without IV contrast if:
Critical Differential Diagnoses to Exclude
Popliteal Artery Aneurysm (Most Important)
Clinicians should not assume all popliteal masses are benign Baker's cysts and must obtain imaging to exclude popliteal artery aneurysm, especially in patients with a history of other arterial aneurysms. 1 This is a life-threatening condition that can mimic a Baker's cyst.
Ruptured Baker's Cyst vs. Deep Vein Thrombosis
- A ruptured Baker's cyst can clinically mimic deep vein thrombosis with calf pain and swelling 1
- Rupture causes sudden calf pain, swelling, and dissemination of fluid into the calf 1
- Ultrasound can differentiate between these conditions 1
Soft Tissue Sarcoma (Less Likely but Must Consider)
While the most common soft tissue mass of the limbs is benign lipoma 3, 2, certain features warrant urgent evaluation:
- Consider urgent direct access ultrasound (within 2 weeks) for any unexplained lump that is increasing in size 3
- Deep-seated masses that are larger tend to be more concerning for atypical lipomatous tumors 3
- However, superficial masses are overwhelmingly most commonly lipomas (94.1% sensitivity and 99.7% specificity on ultrasound) 2
Management Based on Diagnosis
If Baker's Cyst Confirmed:
- Ultrasound-guided aspiration may provide temporary relief for symptomatic cysts 1
- Corticosteroid injection into the knee joint may be beneficial when there is associated synovitis 1
- Treat underlying knee pathology (the cyst is usually secondary to intra-articular pathology) 1
If Uncertain or Concerning Features:
- Proceed to MRI for definitive characterization 1
- Consider suspected cancer pathway referral (within 2 weeks) if ultrasound findings are suggestive of soft tissue sarcoma or if clinical concern persists 3
Common Pitfalls to Avoid
- Assuming all popliteal masses are benign without imaging 1
- Missing a popliteal artery aneurysm, which requires vascular imaging 1
- Failing to distinguish between ruptured Baker's cyst and DVT, both requiring different urgent management 1
- Not evaluating for underlying knee joint pathology that may be causing the cyst 1