What causes a banana fracture on radiography?
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Answer:
A banana fracture is a transverse insufficiency fracture arising on the convex, tension side of a long bone that has been bowed and softened by Paget disease. It runs perpendicular to the long axis, begins at the outer cortex and extends inwards for a variable distance, and may be complete or incomplete. The characteristic sites are the lateral cortex of a bowed femur and the anterior cortex of a bowed tibia, the surfaces under maximal tensile stress. Multiple parallel fissures along the same convexity are common, giving the appearance of transverse clefts along a curved bone. They are painful and progressive, may go on to complete fracture with minimal trauma, and are the reason bowing deformity in Paget disease is monitored radiographically. They are not a sign of malignant change, though sarcomatous transformation, most often in the femur or pelvis, is a separate and much more serious complication and should be considered if there is cortical destruction, a soft-tissue mass or disproportionate pain.
Why is it called so?
The transverse clefts along the convex margin of a curved bone resemble the segmented skin of a peeled banana, and the bowed bone itself is banana-shaped.
Pathophysiology
In Paget disease, disordered osteoclastic resorption and rapid, poorly organised osteoblastic repair produce bone that is enlarged and dense but mechanically weak, with coarse disorganised trabeculae and woven rather than lamellar cortex. Weight bearing on this abnormal bone leads to progressive bowing. The convex surface of a bowed bone carries tensile stress, and pagetic bone tolerates tension poorly, so repetitive loading initiates microfractures that coalesce into a transverse fissure propagating inwards from the convex cortex. Because the process is one of accumulated insufficiency rather than acute trauma, the fissures are often multiple and appear in sequence along the same convexity.
Alternative names: Fissure fracture of Paget disease, chalk stick fracture (when complete and transverse)
Other associated named signs: Blade of grass sign, cotton wool appearance of the skull, picture frame vertebra, and osteoporosis circumscripta
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