What is the fallen leaf sign on radiography of bone?
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Answer:
The fallen leaf sign is a large fragment of fractured cortical bone that has fallen to the dependent portion of a fluid-filled cyst cavity, seen on the radiograph as a curved bony fragment lying at the bottom of a lucent lesion. It indicates a pathological fracture through a simple (unicameral) bone cyst and is regarded as essentially diagnostic of that lesion, because the fragment can only descend freely if the cavity contains fluid rather than solid tissue. The closely related fallen fragment sign describes smaller fragments doing the same thing, and the rising bubble sign is the counterpart seen when gas within the cyst moves to the non-dependent part on decubitus imaging. The typical setting is a proximal humerus or femur in a child or adolescent presenting after minor trauma, since simple bone cysts are usually asymptomatic until they fracture. The differential for a lucent metadiaphyseal lesion includes aneurysmal bone cyst, which is expansile with fluid-fluid levels on MRI and thin internal septa, fibrous dysplasia with its ground-glass matrix, non-ossifying fibroma, eosinophilic granuloma and, in an older patient, a lytic metastasis or myeloma. Management is usually conservative for the fracture, with cyst treatment by aspiration and steroid or bone marrow injection, curettage and grafting, or decompression, since many cysts heal after the fracture.
Why is it called so?
The bone fragment drifts down through the fluid and settles at the bottom of the cyst like a leaf falling to the ground.
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 within growing bone, which is why it occurs in the metaphysis adjacent to the physis and migrates diafyseally with growth. Expansion thins the surrounding cortex until minor trauma fractures it. Because the cavity contains serous or serosanguineous fluid rather than tumour or fibrous tissue, a detached cortical fragment is free to sink under gravity to the dependent margin. A solid lesion would hold the fragment where it lay, so the observation is a direct test of cyst content and is best confirmed with the patient repositioned or with a cross-table view.
Alternative names: Fallen fragment sign; fallen leaf appearance
Other associated named signs: Rising bubble sign and the trap door sign of the simple bone cyst
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