A 12-year-old boy presents with shoulder pain after minor trauma. Radiograph shows a well-defined central lytic lesion in the proximal humeral metaphysis with a cortical fracture through it. Within the lesion, a small bony fragment is seen lying dependently at the base. What is the most likely diagnosis?
- A Aneurysmal bone cyst
- B Unicameral (simple) bone cyst ✓
- C Non-ossifying fibroma
- D Enchondroma
Correct answer: B. Unicameral (simple) bone cyst
Explanation
The fallen fragment sign, a cortical fragment settling to the dependent portion of a lucent lesion after pathological fracture, is considered pathognomonic of a unicameral bone cyst. It occurs because the cyst is filled with fluid of lower density than the fragment, allowing the fragment to sink. In solid tumors such as non-ossifying fibroma or enchondroma, the matrix density prevents the fragment from falling to the base. Aneurysmal bone cysts are eccentric, expansile, and show fluid-fluid levels on MRI rather than a fallen fragment.
Reference: Grainger and Allison's Diagnostic Radiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.