Abnormal stress concentrations in lumbar intervertebral discs following damage to the vertebral bodies: a cause of disc failure?

The purpose of this investigation was to test the hypothesis that damage to a lumbar vertebral body can lead to abnormal stress concentrations in the adjacent intervertebral discs. Twenty-three cadaveric lumbar "motion segments", from persons who had died aged between 19 and 87 years, were subjected to substantial compressive loading while in the neutral, lordotic and flexed "postures". During the loading period, a miniature pressure transducer was pulled through the disc along its mid-sagittal diameter and graphs of horizontal and vertical compressive stress against distance were obtained. Measurements were repeated after each motion segment had been compressed up to the point of mechanical failure: at this point the vertebral bodies suffered minor damage to the trabecular arcades, and sometimes to the end-plate, but the structure remained essentially intact and motion segment height was reduced by only 1%-2%. After damage, the stress in the nucleus and anterior annulus fell by about 30%, and high stress peaks appeared in the inner posterior annulus. These changes were more pronounced in lordotic posture and less pronounced in flexion. The youngest discs showed the smallest changes. It is concluded that minor compressive damage to the vertebral body can lead to high stress concentrations in the posterior annulus. Since the vertebral body is the "weak link" of the lumbar spine, this may be a frequent precipitating cause of isolated disc failure in living people.