Frontal and lateral radiographs of the lumbar spine revealing anterolisthesis of the L5 vertebral body with the typical inverted Napoleon hat sign. This sign is reserved for spondylolisthesis of the L5 vertebral body. With a case of spondylolisthesis, the AP radiograph will reveal the anterior border of the transverse process in continuity with the anterior border of the body of the same vertebrae. This continuous anterior border of the L5 vertebrae will be projected against the shadow of the sacrum, and appear as an upside-down (or inverted) Napoleon hat.
Reference
Garland LH, Thomas SF: Spondylolisthesis. Am J Roentgenol1946, 55: 275-91.
