A 35-year-old presents with a well-defined lytic lesion at the distal femoral epiphysis extending to the subchondral bone. MRI shows a lobulated mass with fluid-fluid levels internally, the fluid levels representing blood products of different ages. The most likely diagnosis is:

• A Aneurysmal bone cyst (ABC) ✓
• B Giant cell tumor of bone
• C Clear cell chondrosarcoma
• D Osteoblastoma

Explanation

Aneurysmal bone cyst (ABC) is characterised by blood-filled cavities separated by fibrous septa without an endothelial lining. The pathognomonic MRI finding is fluid-fluid levels (blood products of different densities/ages layering in the cyst cavities) — the single most reliable imaging feature. ABCs can be primary (idiopathic) or secondary (arising within another lesion such as GCT, osteoblastoma, chondroblastoma). The epiphyseal location and subchondral involvement here is atypical for primary ABC but should still be considered. GCT is epiphyseal in location but shows a solid/heterogeneous pattern without fluid-fluid levels. Fluid-fluid levels are the key discriminating finding for ABC.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.