What are the possible causes and recommended work‑up for small palpable nodules extending from the posterior knee toward the hip?

Popliteal (Baker's) Cyst with Proximal Extension

The most likely diagnosis is a popliteal (Baker's) cyst extending proximally into the thigh, which requires ultrasound confirmation and evaluation for underlying knee joint pathology. 1

Immediate Diagnostic Approach

Obtain high-frequency ultrasound (≥10 MHz) of the posterior knee and thigh as the first-line imaging modality. 2, 3 This will:

• Confirm the characteristic comma-shaped fluid collection between the medial gastrocnemius head and semimembranosus tendon 1
• Define the precise extent of proximal extension into the thigh musculature 1
• Identify any cyst rupture, which can mimic deep vein thrombosis clinically 1
• Detect associated knee joint effusion or synovial proliferation 2

Critical Differential Diagnosis to Exclude

Before assuming a benign Baker's cyst, you must rule out vascular pathology, particularly popliteal artery aneurysm, which can present as a pulsatile popliteal mass and be mistaken for a cyst. 4 Ultrasound with color Doppler capability will immediately distinguish vascular from cystic lesions. 5

Other considerations include:

• Deep vein thrombosis (especially if there is calf swelling or pain) 1, 6
• Soft tissue tumor (rare but requires exclusion) 4
• Proliferative synovial disease extending posteriorly 1

Evaluation for Underlying Knee Pathology

A Baker's cyst is almost always secondary to intra-articular knee pathology—it is not a primary disease. 5, 7 The ultrasound should assess for:

• Joint effusion (highly correlated with cyst presence, p < 0.001) 5
• Synovial proliferation or thickening 2
• Evidence of inflammatory arthritis 5

If the etiology remains unclear or if there are concerning features (fever, systemic symptoms, acute onset), perform ultrasound- or fluoroscopy-guided joint aspiration for synovial fluid analysis to rule out:

• Septic arthritis (urgent—requires fever assessment, inability to bear weight) 2
• Crystal arthropathy (gout or pseudogout) 2
• Inflammatory versus non-inflammatory causes 2

Advanced Imaging Indications

Consider MRI without contrast if:

• Symptoms are not explained by ultrasound findings 2
• You need to evaluate for underlying structural pathology (meniscal tears, cartilage lesions, loose bodies) 2
• There is concern for atypical pathology or tumor 3

Power Doppler ultrasound can demonstrate active synovial inflammation if inflammatory arthritis is suspected. 2

Common Pitfalls to Avoid

• Never assume the lumps are benign without imaging confirmation—popliteal artery aneurysm can be catastrophic if missed 4
• Clinical examination alone misses 57% of Baker's cysts (only 43.3% were clinically detected in one series despite being present on ultrasound) 5
• Ultrasound cannot rule out loose bodies if not visualized—absence of findings does not exclude their presence 1, 2
• Do not rely solely on clinical assessment to distinguish cyst rupture from DVT—both require imaging confirmation 1, 6

Treatment Algorithm Based on Findings

For confirmed Baker's cyst:

• Address the underlying intra-articular pathology (this is the primary treatment) 7, 8
• Conservative management for asymptomatic or mildly symptomatic cysts 7
• Corticosteroid injection for inflammatory/osteoarthritic effusions (provides 1-2 weeks of synovitis reduction in two-thirds of patients, though 70% develop recurrent symptoms) 2
• Arthroscopic treatment of the valve-like posterior transverse synovial infold and intra-articular lesions for persistent symptomatic cysts 7, 8

For septic arthritis (if identified):

• Urgent orthopedic consultation for surgical drainage 2
• Empiric antibiotics pending culture results 2
• Serial aspirations as needed 2

For crystal arthropathy:

• Standard medical management based on crystal analysis 2

Monitoring

Serial ultrasound assessments track treatment response more accurately than clinical examination alone and should be used for follow-up. 2