We investigated the distribution of compressive 'stress' within cadaver intervertebral discs, using a pressure transducer mounted in a 1.3 mm diameter needle. The needle was pulled along the midsagittal diameter of a lumbar disc with the face of the transducer either vertical or horizontal while the disc was subjected to a constant compressive force. The resulting 'stress profiles' were analysed in order to characterise the distribution of vertical and horizontal compressive stress within each disc. A total of 87 discs from subjects aged between 16 and 87 years was examined. Our results showed that age-related degenerative changes reduced the diameter of the central hydrostatic region of each disc (the 'functional nucleus') by approximately 50%, and the pressure within this region fell by 30%. The width of the functional annulus increased by 80% and the height of compressive 'stress peaks' within it by 160%. The effects of age and degeneration were greater at L4/L5 than at L2/L3, and the posterior annulus was affected more than the anterior. Age and degeneration were themselves closely related, but the stage of degeneration had the greater effect on stress distributions. We suggest that structural changes within the annulus and endplate lead to a transfer of load from the nucleus to the posterior annulus. High 'stress' concentrations within the annulus may cause pain, and lead to further disruption.

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We investigated the distribution of compressive 'stress' within cadaver intervertebral discs, using a pressure transducer mounted in a 1.3 mm diameter needle. The needle was pulled along the midsagittal diameter of a lumbar disc with the face of the transducer either vertical or horizontal while the disc was subjected to a constant compressive force. The resulting 'stress profiles' were analysed in order to characterise the distribution of vertical and horizontal compressive stress within each disc. A total of 87 discs from subjects aged between 16 and 87 years was examined.Our results showed that age-related degenerative changes reduced the diameter of the central hydrostatic region of each disc (the 'functional nucleus') by approximately 50%, and the pressure within this region fell by 30%. The width of the functional annulus increased by 80% and the height of compressive 'stress peaks' within it by 160%. The effects of age and degeneration were greater at L4/L5 than at L2/L3, and the posterior annulus was affected more than the anterior. Age and degeneration were themselves closely related, but the stage of degeneration had the greater effect on stress distributions.We suggest that structural changes within the annulus and endplate lead to a transfer of load from the nucleus to the posterior annulus. High 'stress' concentrations within the annulus may cause pain, and lead to further disruption.

The intervertebral disk is routinely subjected to compressive loads that alter with posture and muscle activity and can produce pressures > 2 MPa in human lumbar disks in vivo (A. Nachemson and G. Elfstrom. Scand. J. Rehabil. Med. 2, Suppl. 1:1-40, 1979; A. Nachemson and J. M. Morris. J. Bone Jt. Surg. Am. Vol. 46A: 1077-1092, 1964). We measured the effect of load on hydrostatic pressures in bovine caudal disks. With increase in applied load, pressure increased linearly in the nucleus and inner annulus. The resting pressure measured after slaughter (0.19 +/- 0.05 MPa) and the pressure at failure (34 MPa, estimated from the vertebrae/disk segment failure load of 7,430 +/- 590 N) define the limits that can occur in vivo. Because hydrostatic pressure influences matrix synthesis in articular cartilage, we have examined the effects of pressures in the range 1-10 MPa applied for 20 s or 2 h on proteoglycan synthesis in bovine caudal and human lumbar intervertebral disks in vitro. In the nucleus pulposus and inner annulus of bovine disks, application of hydrostatic pressure in the range of 1-7.5 MPa for only 20 s stimulated matrix synthesis over the following 2 h at atmospheric pressure. The maximum stimulation in the bovine disks was seen in the inner annulus after application of 2.5 MPa, where proteoglycan synthesis rates doubled. Exposure to 2.5 MPa also stimulated synthesis in the nucleus pulposus of human disks taken at surgery, whereas 7.5 MPa inhibited synthesis in five out of six specimens. With 2-h continuous exposure to the same levels of pressure, no stimulation was seen in the nucleus of bovine disks, and significant stimulation was only observed at 5.0 MPa in the inner annulus. Exposure to 10 MPa for either 20 s or 2 h inhibited proteoglycan synthesis in these regions of the disks. In contrast, in the outer annulus, where loading does not lead to a rise in hydrostatic pressure in vivo, there was no significant response to hydrostatic pressure over the range of 1-10 MPa in bovine or human disks.

Stress profilometry is an effective investigation of the mechanics of intervertebral discs in vivo. Discogenic pain is caused by changes in the pattern of loading of the posterolateral anulus or nucleus pulposus.

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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.