Reprinted from THE BACK LETTER Vol. 7
Number 7. 1992
Thus far, researchers have mostly identified what doesn't cause back pain
Some of the world's most renowned specialists will freely admit that the cause of back pain is still a mystery. "Unfortunately, for the majority of patients, physicians do not know what specific structures in and around the motion segment are the source of pain," says Alf Nachemson, MD, the Swedish pioneer. (See Clinical Orthopedics and Related Research, June 1992.)
Over the years, experts have confidently blamed the disc, the sacroiliac joint, the facet joint, the spinal ligaments, spinal muscles, and other structures as the main source of back pain. Even the coccyx was once blamed. Scientific tests have since deflated most of these claims. Confusion about the definition of back pain frequently muddies discussions about its causes. Anatomist Nikolai Bogduk, MB, defines back pain as pain arising from the tissues of the axial skeleton or body wall, "perceived in an area not more than a hand's breadth either side of the lumbosacral vertebral column." (See the Medical Journal of Australia, 3 Feb. 1992.) Sciatica is a separate entity, involving symptoms along the distribution of a lumbar nerve root in the lower extremities.
Many practitioners blame back pain on "degenerative disc disease," and point to the many degenerative changes observed on imaging scans. Yet there is little direct evidence that the disc is a dominant source of pain in the back.
"Degenerative disc disease is a nonvalidated diagnosis," stresses Nachemson, who observes that disc degeneration is as common in individuals without back pain as it is in back pain sufferers.
There is no evidence of free, naked nerve endings inside the vertebral disc, says Nachemson. However, studies do show the outer annulus of the disc, as well as the adjacent peridiscal ligaments, to be amply laced with nerves, and it is possible that these nerves are involved in discogenic pain. They could become sensitized when disc tissue is injured.
Can disc herniation cause back pain? According to Bogduk, nerve-root compression (i.e., from a bulging or herniated disc) is a good explanation for sciatica but is unlikely to be a common source of back pain. "There is no evidence that nerve root irritation causes back pain the absence of radicular lower limb," he says.
Internal disc disruption is another explanation for back in. This condition is marked by internal changes in the disc, with degradation of the nuclear matrix and internal radial fissures. Pain is reputedly produced by chemical or mechanical irritation of nerves in the outer annulus. This is a controversial hypothesis, based largely on clinical observation rather than basic scientific studies.
How about the oft-repeated diagnosis, "sprained ligament"? According to Bogduk, anatomical studies cast doubt on this diagnosis.
"The supraspinous ligament does not exist below L3, and the iliolumbar ligament is muscular until the third decade, so these are unlikely sources of ligament pain," he argues.
The ligamentum flavum isn't known to be affected by painful conditions, and the longitudinal ligaments are so completely associated with the discs that they cannot be separated as a source of pain.
Muscle strain can cause short-term back pain, but there is little evidence that it can cause chronic pain. Muscle spasm and trigger points, other common scapegoats, find little support as causes in scientific studies.
Everyone agrees that the facet joints are richly supplied with nerves and could be a source of back pain. Bogduk notes that in randomly selected groups, the facet joints appear to be the source of pain in 8%, 16%, and 22% of chronic back pain sufferers, according to various studies. "No one can say that the facets cannot be the source of pain," says Alf Nachemson, "but diagnostic capabilities for testing the presence of a facet syndrome have proven invalid."
In the 1920s, sacroiliac dysfunction was a common diagnosis and fusion of this joint was the most common form of back surgery. Since then the diagnosis has gone in and out of fashion.
Nachemson doubts that the sacroiliac joint is a common source of back pain, pointing out that sacroiliac dysfunction hasn't been scientifically validated as a diagnosis. New techniques of selectively anesthetizing the sacroiliac joints should finally make it possible to determine the actual prevalence of sacroiliac problems, says Bogduk.
Lumbar instability is a popular explanation for back pain these days, and is offered as a rationale for thousands of lumbar fusions every year. There is no universally accepted definition for instability, no agreement about how to test for it, and no conclusive proof that it is a dominant cause of back pain.
Other possible causes for back pain - lumbar insufficiency, hypomobility lesions, subluxations, segmental insufficiency - haven't been described in hard pathoanatomical terms that can be evaluated in scientific studies, says Bogduk. He is disdainful of theoreticians who are reluctant to test their pet explanations. "It seems easier to win arguments by saying the same thing often enough and loudly enough than to conduct appropriate studies, the guidelines for which are well established," Bogduk comments.
Fortunately it's not necessary to know the exact cause of back pain to treat back syndromes effectively. Merely recognizing a distinctive pattern of symptoms can lead to effective treatment. But when researchers do pinpoint the exact causes, new and more effective therapies for back pain should emerge.