A 12-year-old boy presents with mild shoulder pain after minor trauma. Radiograph shows a central, well-defined lytic lesion in the proximal humeral metaphysis with cortical thinning, and a small bony density lying dependently within the lucent cavity. The most likely diagnosis is:

• A Non-ossifying fibroma
• B Aneurysmal bone cyst
• C Enchondroma
• D Unicameral (simple) bone cyst with a fallen fragment sign ✓

Explanation

The fallen fragment sign, a piece of cortical bone gravitating to the dependent part of a lucent lesion after a pathological fracture, is considered pathognomonic of a unicameral bone cyst, because only a fluid-filled cavity allows the fragment to move freely. Enchondromas show chondroid matrix, non-ossifying fibromas are eccentric with a sclerotic rim, and aneurysmal bone cysts are expansile with fluid-fluid levels on MRI.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.