The fallen fragment sign in unicameral bone cyst.

The roentgenographic findings in unicameral bone cyst may be difficult to differentiate from a solid tumor of bone. When present, the fallen fragment sign may aid in determining the roentgenographic diagnosis of unicameral bone cyst. In this finding, with a pathologic fracture a fragment of cortical bone may become dislodged and gravitate to the dependent portion of the unicameral bone cyst. Two examples of the fallen fragment sign demonstrate the displacement of a fragment of cortical bone to the dependent portion of the cystic cavity.