Restriction of straight leg raising (SLR) is usually associated with patients suffering leg pain due to a postero-lateral disc protrusion.

We report a group of twelve patients presenting with acute mechanical low back pain only, and no leg pain, who also demonstrated similar restriction of SLR.

The MRI scans of these patients, when compared with the scans of patients suffering typical sciatic pain, revealed that the disc protrusions in the back pain group were more likely to be smaller, central and at a higher lumbar level than the leg pain group.

Anatomical considerations would suggest that the source of back pain was the anterior theca being compressed by a central disc protrusion.