Solitary (unicameral) bone cyst. The fallen fragment sign revisited.

The fallen fragment sign is a prominent radiologic feature in a minority of cases of unicameral bone cyst (20% in this series). This sign is always associated with pathologic fracture. Intramedullary fracture fragments may be single or multiple and may or may not be entirely dislodged from overlying periosteum. The finding appears limited to unicameral bone cysts in patients with open physes. When present, the fallen fragment is a pathognomonic finding as it defines the interior of the cyst as nonsolid. This is particularly helpful in cases where absence of the cyst wall secondary to fracture can simulate an intramedullary malignancy with cortical erosion.