What are the treatment options for a movable mass on the knee?

Treatment Options for a Movable Mass on the Knee

A movable mass on the knee requires immediate duplex ultrasound evaluation to determine if it is vascular (aneurysm) or non-vascular in origin, as this fundamentally determines management—vascular masses ≥2 cm or symptomatic require surgical intervention, while non-vascular masses need tissue diagnosis to guide treatment. 1

Initial Diagnostic Approach

Imaging Algorithm

• Duplex ultrasound is the first-line imaging modality to differentiate between vascular and non-vascular masses 1
• If the mass is vascular (popliteal aneurysm):
  • Measure diameter precisely 1
  • Assess for mural thrombus 1
  • Evaluate tibioperoneal outflow with arteriography if surgical intervention is considered 1
• If the mass is non-vascular:
  • Plain radiographs in two planes should be obtained to evaluate for osseous changes 1
  • MRI without IV contrast is the most useful tool to evaluate soft tissue masses, their extent, and relation to vessels and nerves 1
  • Consider ultrasound for superficial soft tissue masses to characterize their nature 1

Treatment Based on Etiology

Vascular Masses (Popliteal Aneurysm)

Surgical treatment is indicated for:

• Any symptomatic aneurysm (pain, limb-threatening ischemia, distal embolization) regardless of size 1
• Asymptomatic aneurysms ≥2 cm in diameter 1
• Presence of mural thrombus even if diameter appears smaller on arteriogram 1

Surgical approach:

• Bypass grafting is the standard treatment 1
• Saphenous vein grafts provide superior long-term patency and limb salvage rates compared to synthetic grafts 1
• PTFE grafts are approximately twice as likely as polyester grafts to remain patent 1
• Consider catheter-directed thrombolysis preoperatively if recent thromboembolic events have compromised runoff 1

Observation may be appropriate for:

• Asymptomatic aneurysms <2 cm in low-risk patients with limited life expectancy 1
• Annual surveillance with duplex ultrasound is required 1

Non-Vascular Masses

Benign soft tissue masses:

• Conservative management with observation if asymptomatic 2
• Surgical excision if symptomatic, enlarging, or causing mechanical symptoms 2

Suspected malignancy (osteosarcoma or soft tissue sarcoma):

• Patients with findings suggestive of bone or soft tissue malignancy must be referred to a specialized center BEFORE biopsy, as inappropriate biopsy techniques can compromise limb salvage or cure 1
• Open biopsy should be performed by the treating surgeon at the reference center 1
• Confirmation by a pathologist with expertise in bone/soft tissue tumors is essential 1
• Staging includes chest CT, bone scan, and MRI of the entire involved bone and neighboring joints 1

Inflammatory masses or pseudotumors:

• MRI is beneficial for workup of periarticular inflammatory pseudotumors 1
• Treatment depends on underlying etiology (infection, inflammatory arthropathy, etc.) 3

Common Pitfalls to Avoid

• Never perform biopsy of a suspected malignant mass before referral to a specialized center, as this can compromise definitive treatment 1
• Do not rely on arteriography alone to measure popliteal aneurysm size, as mural thrombus makes the diameter appear smaller than the true size on duplex or CT 1
• Avoid dismissing low-velocity trauma in obese patients, as seemingly trivial injuries can cause significant knee pathology including dislocations 4
• Do not overlook vascular injury in patients with knee trauma or masses, particularly in high-energy mechanisms or obese patients 4

Special Considerations

For post-surgical or post-traumatic masses:

• Consider pseudoaneurysm from previous vascular reconstruction or arterial access 1
• Duplex ultrasound and/or contrast-enhanced CT can confirm diagnosis 1
• Surgical repair is typically required for pseudoaneurysms 1

For masses associated with systemic disease:

• Consider crystal deposition, inflammatory arthropathy, or infection in appropriate clinical contexts 3
• Arthrocentesis may be diagnostic and therapeutic 3