Napoleon Hat Sign

A term for marked antero-inferior displacement of the anterior edge of lumbar vertebra, L5 on S1, which appears as a symmetrical navicular density on a frontal plain film of the lower vertebrae in congenital spondylolisthesis, which has been fancifully likened to an inverted napoleonic hat. On the AP radiograph, the anterior border of the tranverse process is continuous with the anterior border of the body of the same vertebra. In spondylolisthesis, this continuous anterior border is projected against the shadow of the sacrum, and has the hat-like appearance
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