Sciatica and the sacroiliac joint: Is there a relationship?

Inflammation of the sacroiliac joint (SIJ), sacroiliitis, sometimes leads to sciatica. Patients with sacroiliitis can have pain radiating from the buttock down the posterior thigh and leg, even into the great toe¹. Sacroiliitis has been reported to mimic an L5 or S1 radiculopathy in various arthritides including psoriatic arthritis², reactive sacroiliitis², Crohn’s disease³, and unspecified seronegative spondyloarthropathies^1,3,4.

Although this may come as a surprise, before Mixter and Barr’s publication in 1934 linking sciatica to damage of the lumbar intervertebral disc⁵, clinicians such as Yeoman felt that sciatica originated primarily from the sacroiliac joint⁶.

Features of sacroiliitis can overlap with or mimic the signs and symptoms of lumbar disc herniation or degenerative spinal disease. These symptoms include a radicular pain distribution and positive straight leg raise¹. How is it possible that the sacroiliac joint creates some of the same symptoms as a lumbosacral radiculopathy?

Joseph Fortin and his team has identified substance P in the SIJs of patients with chronic low back pain, and state:

“…in a traumatized and inflamed [sacroiliac] joint, extravasation of synovial fluid containing inflammatory mediators including substance P could traverse any of the three pathways [from the sacroiliac joint to surrounding neural structures] described and irritate one or more of the neural elements that compose the sciatic nerve (L4-S2).”

Another study reviewed 133 patients who had no evidence of nerve root damage on imaging studies, yet had relief of sciatica with therapeutic injection into the sacroiliac joint³. The author concluded:

          …”the possibility remains that some needless back surgery is completed in patients whose sciatica-like pain actually arose in their SI joints”

Anatomical studies show that the lumbosacral plexus is indeed in close contact with the sacroiliac joint. In fact, the fifth lumbar nerve root and lumbosacral trunk cross the sacroiliac joint about 2cm inferior from the pelvic brim and are fixated in this position by connective tissue⁷. Based on studies of contrast injection into the SIJ and resultant leakage, three pathways were identified whereby inflammatory mediators could come into contact with to neurological structures⁸:

1.   Posterior extravasation into the dorsal sacral foramina

2.   Superior recess extravasation at the sacral alar level to the fifth lumbar epiradicular sheath

3.   Ventral extravasation to the lumbosacral plexus

Mechanism by which sacroiliitis causes sciatica. Inflammatory mediators such as substance P leak from the sacroiliac joint and come into contact with surrounding neurological structures. Individual images from wikipedia. Diagram by Robert Trager.

Sacroiliac joint pain accounts for 2% of cases of failed back surgery⁹. Based on the evidence presented, this may actually be because inflammation of the sacroiliac joint can cause sciatica and lead to diagnostic confusion. Fortunately, neurological deficits are not commonly found with sacroiliitis, and thus the presence of these findings suggest an alternate diagnosis¹. In addition, orthopedic tests directed at the sacroiliac joint are often helpful in cases of sacroiliac-related sciatica^1,3.

A pain pattern alone is not diagnostic of radiculopathy, and only with a complete examination, and sometimes treatment of the condition, will the accurate diagnosis be revealed.

1.       Buijs, E., Visser, L. & Groen, G. Sciatica and the sacroiliac joint: a forgotten concept. Br. J. Anaesth. 99, 713–716 (2007).

2.       Wong, M., Vijayanathan, S. & Kirkham, B. Sacroiliitis presenting as sciatica. Rheumatology 44, 1323–1324 (2005).

3.       Margules, K. R. & Gall, E. P. Sciatica-like pain arising in the sacroiliac joint. JCR J. Clin. Rheumatol. 3, 9–15 (1997).

4.       Kulcu, D. G. & Naderi, S. Differential diagnosis of intraspinal and extraspinal non-discogenic sciatica. J. Clin. Neurosci. 15, 1246–1252 (2008).

5.       Mixter, W. J. & Barr, J. S. Rupture of the intervertebral disc with involvement of the spinal canal. N Engl J Med 211, 210–5 (1934).

6.       Yeoman, W. THE RELATION OF ARTHRITIS OF THE SACRO-ILIAC JOINT TO SCIATICA, WITH AN ANALYSIS OF 100 CASES. The Lancet 212, 1119–1123 (1928).

7.       Ebraheim, N., Lu, J., Biyani, A., Huntoon, M. & Yeasting, R. The relationship of lumbosacral plexus to the sacrum and the sacroiliac joint. Am. J. Orthop. Belle Mead NJ 26, 105–110 (1997).

8.       Fortin, J. D., Washington, W. J. & Falco, F. J. E. Three Pathways between the Sacroiliac Joint and Neural Structures. Am. J. Neuroradiol. 20, 1429–1434 (1999).

9.       Schofferman, J. et al. Failed back surgery: etiology and diagnostic evaluation. Spine J. 3, 400–403 (2003).