What causes fallen fragment sign in bone on plain radiography?
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Answer: A cortical bone fragment displaced into the dependent portion of a unicameral bone cyst (also known as simple bone cyst or solitary bone cyst) following pathologic fracture through the thinned cortex. This sign is pathognomonic for unicameral bone cyst, typically seen in children and adolescents, most commonly in the proximal humerus or femur, presenting with pain after minor trauma.
This radiologic appearance is referred to as the fallen fragment sign on plain radiography.
Why is it called so?
The sign is named for the radiologic appearance of a detached cortical bone fragment that has “fallen” and settled by gravity into the dependent (bottom) portion of the fluid-filled cyst cavity on upright radiographs.
Pathophysiology
Unicameral bone cysts are fluid-filled intramedullary cavities with thinned overlying cortex. Minor trauma causes pathologic fracture through the weakened cortex, detaching a fragment. The intact periosteal sleeve initially may hold it as a hinged flap, but it eventually displaces completely, sinks through the cyst fluid due to gravity, and lies dependently within the lucent lesion, visible as a sclerotic fragment against the cystic background.
Alternative names: None
Other associated named signs: None
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