A 9-year-old boy presents with sudden shoulder pain after minor trauma. X-ray shows a pathological fracture through a well-defined central lytic lesion in the metaphysis of the proximal humerus. A bony fragment is seen lying at the dependent (inferior) part of the lesion. What is the most likely diagnosis?

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

Explanation

B central metaphyseal lytic lesion in the proximal humerus of a child that fractures easily is a unicameral bone cyst. The 'fallen fragment' sign, a piece of cortical bone gravitating to the dependent portion of the cavity, is pathognomonic because the cyst is filled with fluid, unlike solid lesions where the fragment stays in place. An aneurysmal bone cyst is typically more eccentric and expansile with internal septations.

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.