Fallen fragment sign | Radiology Signs

Answer:

The fallen fragment sign is a detached cortical fragment lying in the dependent portion of a fluid-filled cyst cavity, seen as a curved sclerotic fragment at the bottom of a lucent lesion after a pathological fracture. It is regarded as essentially diagnostic of a simple (unicameral) bone cyst, because a fragment can only sink freely if the cavity contains fluid rather than solid tissue; the same observation seen with a single large fragment is called the fallen leaf sign. The typical setting is the proximal humerus or femur of a child or adolescent presenting with pain after minor trauma, since simple bone cysts are usually silent until they fracture. Related observations are the trap door sign, where the fragment stays hinged on an intact periosteal sleeve and has not fully descended, and the rising bubble sign, where gas within the cyst moves to the non-dependent part on decubitus or cross-table imaging. The differential for a lucent metadiaphyseal lesion includes aneurysmal bone cyst, which is expansile with septa and fluid-fluid levels on MRI, fibrous dysplasia with a ground-glass matrix, non-ossifying fibroma, eosinophilic granuloma and, in an older patient, lytic metastasis or myeloma. The fracture is usually managed conservatively, with cyst treatment by aspiration and steroid or bone marrow injection, curettage and grafting, or decompression, since many cysts heal after fracturing.

Why is it called so?

The fragment drifts down through the cyst fluid and settles at the bottom of the lesion, like something falling to the floor of an empty room.

Pathophysiology

A simple bone cyst is a solitary, fluid-filled cavity lined by a thin fibrous membrane, thought to arise from a local disturbance of interstitial fluid drainage in growing bone, which is why it starts in the metaphysis next to the physis and migrates towards the diaphysis with growth. Expansion thins the overlying cortex until minor trauma fractures it and detaches a fragment. The intact periosteal sleeve may hold that fragment as a hinged flap at first, but once free it sinks under gravity through the serous or serosanguineous contents to the dependent margin. A solid lesion would hold the fragment where it lay, so the sign is a direct test of cyst content and is best confirmed by repositioning the patient or with a cross-table view.

Alternative names: Fallen leaf sign; fallen fragment appearance

Other associated named signs: Trap door sign of the periosteally hinged fragment and the rising bubble sign of intracystic gas

References

1. Reynolds J. The “fallen fragment sign” in the diagnosis of unicameral bone cysts. Radiology. 1969;92(5):949-53 passim. PMID: 5771835.