Inverted Napoleon hat sign
Low back pain
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There is a high grade anterior spondylolisthesis of L5 vertebra over S1 with overlapping over the sacrum.
The inverted Napoleon hat sign indicates the presence of bilateral spondylolysis and significant spondylolisthesis. The dome of the hat is formed by the overlying body of L5 vertebra and the brim is formed by downward rotation of the transverse processes.