The fallen fragment sign refers to the presence of a bone fracture fragment resting dependently in a cystic bone lesion. This finding is said to be pathognomonic for a simple (unicameral) bone cyst following a pathological fracture.
The finding exists because a simple bone cyst is fluid filled, and therefore a bony fragment may descend through the fluid uninhibited. In contrast, the common differential lesions for a pathologic fracture through a lucent bone lesion (such as a lytic tumour, fibrous dysplasia, aneurysmal bone cyst or enchondroma) have a solid interior matrix, and therefore would not permit such downward fragment migration.
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