General Considerations

• Occurs with marked anterolisthesis of L5 on S1
• Visible on frontal radiograph of lumbar spine
• Implies the presence of bilateral spondylolysis and significant spondylolisthesis
  • Spondylolysis resulting in this degree of spondylolisthesis is more often congenital and/or traumatic in origin and less often degenerative
• "Brim" of hat is formed by the downward rotation of the transverse processes
  • "Dome" of hat is body of L5
• The sign can also be seen with marked exaggeration of the normal lordosis at the lumbosacral junction

 

 

 

Inverted Napoleon Hat Sign.The "hat" is seen on the frontal view at left (blue arrows) and almost always indicates marked spondylolisthesis of L5 on S1. A lateral view on right shows L5 (blue arrow) slipped 100% forward on the sacrum (white arrow).
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