Findings: Lytic lesion involving proximal humeral metaphysis with fracture and "fallen fragment" sign.

Diagnosis: Unicameral bone cyst with fallen fracture fragment.

Discussion: Unicameral (simple) bone cyst is a common lesion of unknown cause. They generally present in the 1st and 2nd decades of life. The simple bone cyst is asymptomatic unless complicated by a fracture. They are most commonly located within tubular bones (usually metaphyseal), but are also found in the calcaneus and ilium. Radiologically they appear as a centrally located region of lucency with cortical thinning and occasionally expansion. A pathologic fracture is common. Such a fracture is often associated with a fracture fragment which settles dependently within the cystic space. This has been called the "fallen fragment" sign. Although bone cysts may undergo regression, they are often treated surgically.

References:
Resnick, Donald, MD. Bone and Joint Imaging 2nd edition.
WB Saunders Inc., Philadelphia, PA, 1996. Pages 1048-1050.

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