What causes the Napoleon hat sign in the lumbar spine on frontal radiographs?
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Napoleon hat sign is caused by severe spondylolisthesis of the fifth lumbar vertebra (L5) over the sacrum (S1), often associated with bilateral spondylolysis and/or marked lumbar lordosis. This sign indicates significant anterior displacement (anterolisthesis) of L5 relative to S1, which can be congenital, traumatic, or less commonly degenerative in origin. On an anteroposterior (AP) frontal radiograph, the overlapping bony structures create the classic appearance.
Why is it called so?
The name “Napoleon hat sign” derives from the characteristic silhouette formed on frontal spine imaging. On an AP lumbar spine radiograph, the superimposed L5 vertebral body and sacrum resemble a stylized bicorne hat. The L5 vertebral body forms the “dome” of the hat, while the laterally displaced transverse processes simulate the hat’s “brim,” reminiscent of the bicorne headgear famously worn by Napoleon Bonaparte.
Pathophysiology
The pathophysiology involves bilateral defects in the pars interarticularis (spondylolysis), resulting in loss of structural integrity of the L5 vertebral arch. This weakness permits anterior slippage of L5 on S1. The displacement and overlap on frontal imaging produce the Napoleon hat sign. Contributing factors may include congenital malformation of the pars, repetitive trauma, or degenerative changes leading to progressive spondylolisthesis and hyperlordotic alignment of the lumbar spine.
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