Low back pain on passive straight leg raising: the anterior theca as a source of pain

A prospective clinical and radiologic investigation of two groups of patients presenting with either acute back pain only or acute leg pain only, yet similar restriction in straight leg raising (SLR).

To highlight a group of patients presenting with acute low back pain only, yet a restricted SLR normally associated with leg pain (sciatica) caused by a posterolateral disc prolapse. To determine the anatomic source of the pain in the low back pain only group.

A restricted SLR is commonly associated with leg pain (sciatica) due to compression of a nerve root by an intervertebral disc prolapse. Previous studies investigating pain patterns on SLR have suggested that central disc prolapses tend to induce back pain whereas the more lateral prolapses induce leg pain. Such research work has involved patients presenting with typical sciatic pain investigated by myelography and undergoing decompressive surgery. There are no studies specifically investigating patients presenting with low back pain only and reduced SLR.

Two groups of patients, one with acute low back pain only and one with acute leg pain only, yet showing similar restriction in SLR underwent MR imaging. The scans were reviewed "blind" by an experienced spinal radiologist and imaging features recorded according to a detailed protocol. The MR findings were then compared.

Significant disc prolapses were seen equally in both groups. The disc prolapses in the back pain only group were more likely to be central, smaller, to compress the theca only, and to be at a higher lumbar level as compared with the leg pain only group.

Acute low back pain associated with significant restriction in SLR is likely to be caused by a disc prolapse compressing the anterior theca.