Researchers1 studying data from the Framingham Heart Study recently came across an interesting relationship unrelated to the original heart study.
This study examined the records of 606 patients who had lateral lumbar radiographs taken in 1967-1968 and again in 1992-1993. The authors recorded the presence of aortic calcification in the posterior wall of the aorta, and the presence of lumbar disc degeneration and report of back pain.
The study found that subjects with aortic calcification were more likely to have general disc degeneration. Furthermore, there was a strong relationship between the level of calcification and the degeneration of the corresponding disc; and, patients with more severe aortic calcification were found to be at a significantly higher risk of back pain.
"We conclude that aortic calcification, a marker for advanced atherosclerosis, increases a person's risk for development of disc degeneration and is associated with the occurrence of back pain. Posterior calcification may be a sign of atheromatous obliteration of a feeding artery or arteries of the lumbar spine, which leads to impaired nutrition of the lumbar spine. The intervertebral disc, with its precarious nutrient supply, may be one of the first structures to suffer from insufficient nutrient supply, thus linking aortic calcification with radiographic disc degeneration. The relationship between aortic calcification and back pain is either through pain-sensitive structures, such as nerve roots, bones, and muscles that suffer from ischemic pain; through spinal dysfunction, caused by ischemic atrophy and degeneration of structures; or through a combination of both."
Nerve Growth in Degenerated Discs
In a separate, but related study, other clues to the enigma of back pain have been published.2 In this study, researchers examined 46 samples of intervertebral discs collected from 38 patients who underwent spinal fusion surgery for chronic low back pain (longer than 12 months). These were compared to 34 samples taken from healthy lumbar spines.
The authors found that nerve growth in the control samples was limited to the outer one third of the anulus fibrosus. In the damaged discs, though, nerve growth had extended into the inner one third of the anulus fibrosus in 46% of the samples, and had infiltrated the nucleus pulposus in 22% of the samples. Most of the time, blood vessels were found along with the nerve growth, but in 14 back pain samples (30%) the nerve cells were isolated in the discal matrix.
"So what is the biological purpose of nerve ingrowth into intervertebral discs? By analogy with other connective tissues, nerve ingrowth into damaged intervertebral discs could mediate various tissue events, notably healing. Initially, an immobilising nociceptive component to nerve ingrowth might be beneficial, but because the healing process is thought to be poor in this tissue, unproductive nerve ingrowth and pain may result."
- Kauppila LI, McAlindon T, Evans S, et al. Disc degeneration/back pain and calcification of the abdominal aorta: a 25-year follow-up study in Framingham. Spine 1997;22(14):1642-1649.
- Freemont AJ, Peacock TE, Goupille P, et al. Nerve ingrowth into diseased intervertebral disc in chronic back pain. The Lancet 1997;350:178-181.