Explanation:
The 'fallen leaf' or 'fallen fragment' sign occurs when a piece of the thin cortical wall breaks off during a pathologic fracture and sinks to the dependent portion of a Unicameral Bone Cyst (UBC). Because a UBC is fluid-filled, gravity allows the fragment to fall. In contrast, an Aneurysmal Bone Cyst (ABC) is divided by fibrous septae and filled with blood/tissue, so fractured fragments typically stay in place. Reference: Campbell's Operative Orthopaedics, 14th Edition.