Low back pain

Editor—I disagree with only one bit of Speed's review of low back pain¹: except in inflammatory disease or infection, I am not certain that the sacroiliac joint is responsible for much of what is called sacroiliac pain. The rigidity of the joint is such that mechanical disruption is almost impossible: the pelvis will fracture first. Cures of pain reported by injection of the joint are, I think, not because the joint has been injected but because the injection has gone into part of the gluteal muscle origin.

Many years ago a colleague tried to assess the accuracy of joint injection. Injecting corpses, with Indian ink, using classical localisation and x ray screening, he dissected the region and found no evidence of ink in the joint on any of eight attempts. The ink was all in the muscle, or in some cases had tracked into the vertebral venous plexuses. (I imagine such an experiment would nowadays be impossible to arrange.)

Although the finding of a “stress reaction” on radiography (sclerosis without erosion on the ilial side only, without erosion) might represent some sort of mechanical instability that could cause pain, I suspect that most cases of “sacroiliac strain” are really injuries of the gluteal origin—and injections work because that, and not the joint, is what has been injected. Unilateral pain spreading across the iliac crest, worse when the patient bends to the opposite side, might also, in theory, arise from the iliolumbar ligament rather than the disc.