Fallen fragment sign – the presence of a bone fragment in the dependent portion of a lucent bone lesion. It is said to be pathognomonic of a simple (unicameral) bone cyst and is usually seen after pathologic fracture. Simple bone cysts are fluid filled and therefore fracture fragments can descend through the fluid uninhibited. Other lucent lesions such as fibrous dysplasia, aneurysmal bone cyst and enchondroma all have solid interiors which do not permit fragments to fall.
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