A 12-year-old boy has a proximal humeral radiograph showing a central lytic metaphyseal lesion with a thin sclerotic rim. Within the lesion, a small bony fragment lies in the dependent portion. The most likely diagnosis is:

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

Explanation

The fallen fragment sign is considered pathognomonic of a unicameral bone cyst. After a pathological fracture, a piece of cortical bone detaches and, because the cyst is filled with fluid rather than solid tissue, it sinks by gravity to the dependent part of the cavity. In solid tumors such as non-ossifying fibroma or chondromyxoid fibroma, a detached fragment cannot migrate and remains at the fracture site.

Reference: Grainger and Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.