fallen fragment sign
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. Although it has occasionally been reported with other cystic lesions, e.g. eosinophilic granuloma .
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 tumor, fibrous dysplasia, aneurysmal bone cyst or enchondroma) have a solid interior matrix and therefore would not permit such downward fragment migration.