An 11-year-old boy presents with mild shoulder pain after minor trauma. Radiograph of the proximal humerus shows a well-defined central lytic lesion in the metaphysis with a thin sclerotic rim. Within the lesion, a small bony fragment lies dependent in the inferior portion. What is the most likely diagnosis?
- A Aneurysmal bone cyst
- B Simple (unicameral) bone cyst ✓
- C Non-ossifying fibroma
- D Enchondroma
Correct answer: B. Simple (unicameral) bone cyst
Explanation
The fallen fragment sign is pathognomonic of a simple bone cyst. The cyst wall is thin and lacks a solid matrix, so a pathological fracture allows a cortical fragment to sink to the dependent part of the fluid-filled cavity. In solid tumors such as non-ossifying fibroma the fragment cannot move, so it stays at the fracture site. Aneurysmal bone cyst is eccentric, expansile and multiloculated rather than central with a free fragment.
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.