Solitary (unicameral) bone cyst The fallen fragment sign revisited

The fallen fragment sign is a prominent radiologic feature in a minority of cases of unicam- eral bone cyst (20% in this series). This sign is always associated with pathologic fracture. Intra- medullary fracture fragments may be single or mul- tiple 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 interi- or of the cyst as nonsolid. This is particularly help- ful in cases where absence of the cyst wall second- ary to fracture can simulate an intramedullary ma- lignancy with cortical erosion.