Question

The fallen fragment sign is characteristic of:

• A. Simple bone cyst
• B. Osteosarcoma
• C. Adamantinoma
• D. Aneurysmal bone cyst

Hint:

A chip sinks through clear fluid - a single-cavity unicameral cyst.

Answer 1

The Correct Option is A

Step 1: The fallen fragment sign describes a piece of fractured cortical bone that has dropped to the dependent (lowest) part of a fluid-filled cystic bone lesion and is seen lying at the bottom of the cavity on imaging.
Step 2: This sign is considered pathognomonic for a simple (unicameral) bone cyst that has undergone a pathological fracture. Because the cyst contains only fluid, the loose cortical fragment is free to sink to the lowest point, which is what produces the sign. Hence the answer is simple bone cyst.
Step 3: Why the others are wrong. Osteosarcoma is a solid malignant matrix-forming tumour, so a fragment cannot fall through it. An aneurysmal bone cyst contains multiple blood-filled septated locules rather than a single fluid space. Adamantinoma is a solid lytic tumour of the tibia and does not show this sign. Rarely the sign has been reported in eosinophilic granuloma, but the classic association remains the simple bone cyst.