"Retrolisthesis (backwards slippage of one vertebral body on another) has historically been regarded as an incidental finding, one which doesn’t cause any symptoms, and is considered to be of little or no clinical significance. Few studies have been done to date and little is known about this condition. The literature has found a possible association between retrolisthesis and increased back pain and impaired back function. 1-4
Retrolisthesis may occur more commonly than initially believed. Series have shown that retrolisthesis may be present in up to 30% of extension radiographs of patients complaining of chronic low back pain.5 Retrolisthesis has been found to be associated with disc degeneration, decrease in lumbar lordosis, and decrease in vertebral endplate angle."

The above is taken from a study on the NIH website.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2278018/

It may be that this, along with all your other degenerative changes are causing the increase in pain. Retrolisthesis rarely occurs by itself so is not thought to be a pain generator as much as having a degenerative situation with the discs, endplates, etc. At this point, it is likely you are having some nerve compression that is causing your renewed leg pain. The nerve might be getting "squished" by the movement caused by retrolisthesis.

Hope this helps a bit.
Gwennie

I had Retro in my lumbar and the fusion was the best thing. Mine was grade 3 almost completely dislocated. Any movement caused the disc to press into my spinal column.

Mine did not show up on MRI either but the flexion/extension views. My NS said I was probably born this way and it worsened with age.

Julie