The “inverted Napoleon’s hat” sign

Excerpt

Imagine Napoleon Bonaparte’s iconic two-cornered hat (Fig. 1) abandoned and lying upside down as the emperor fled the Battle of Waterloo [1]. The image of an “inverted Napoleon’s hat” has been used as a metaphor for the frontal radiographic appearance of severe spondylolisthesis at the lumbosacral junction, which can be observed on frontal abdominal radiographs (Fig. 2). Spondylolisthesis is the anterior displacement of one vertebral body with respect to the adjacent inferior vertebral body or sacrum. With severe subluxation of the L5 vertebral body, the anterior slip, focal kyphotic curvature, and compensatory hyperlordosis of the lumbar spine result in the projection of the anterior cortex of the L5 vertebral body over the sacrum on the frontal view. The anterior cortex forms the “dome” of the inverted hat and creates a continuous line with the L5 transverse processes which represent the two tapered “corners” (Fig. 2a, b) [2].

…