A 12-year-old boy sustains minor trauma to the right arm. X-ray shows a fracture through a well-defined central lytic lesion in the proximal humeral metaphysis. Within the lesion, a bony fragment is seen lying dependently at its lower pole. What is the most likely diagnosis?

• A Aneurysmal bone cyst
• B Unicameral (simple) bone cyst ✓
• C Non-ossifying fibroma
• D Enchondroma

Explanation

The fallen fragment sign, a piece of cortical bone settling to the dependent part of a lytic lesion after a pathological fracture, is considered pathognomonic of a unicameral bone cyst. The cyst is filled with clear serous fluid, so the detached fragment can fall freely and gravitate. In aneurysmal bone cyst and other solid or septated lesions, the fragment cannot migrate to the dependent portion, which excludes option A.

Reference: Maheshwari's Essential Orthopaedics, 6th ed.

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