Classic radiographic signs of spondylolysis and spondylolisthesis

(A) Lateral lumbar spine. Note the pars defects (arrow) and anterior displacement of the L5 vertebra.
(B) Oblique lumbar spine. Observe the clearly visible lucent collar (arrow).
(C) Stepladder sign. On an oblique film, the alignment of the facet joints changes abruptly at the level of slippage.
(D) Axial CT L5. Note that the pars defects are readily identified (arrows). Observe the asymmetry of the defects, which is usually present in spondylolysis and may explain why some pars defects are hard to demonstrate on symmetrically positioned 45° oblique lumbar radiographs.