1) A line ① is drawn along the top of the vertebral body of the lower spinal segment.
2) Then at the top-back most portion of the lower vertebral body, draw line ② at 90 degrees to line ①, till it projects well into the body of the vertebra above.
3) Then draw another line ③ parallel to the line just drawn ② this time at the posterior most lower portion of the upper vertebral body.
4) The distance between the upright lines ② and ③ is measured. Any distance of 2mm or greater is a retrolisthesis. This measurement represents the degree of translation (slippage) of the upper of the two segments. See Photo.

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