Posterior fixation of intervertebral discs is used to treat, and occasionally diagnose, discogenic pain since it is thought that it will reduce the internal loading of the discs in vitro. We measured the internal loading of ten intervertebral discs using stress profilometry under simulated physiological loads and then after posterior fixation. Partial discectomies were performed to simulate advanced disc degeneration and the sequence repeated.

Posterior fixation had very little effect on the magnitude of the loads acting on the disc and none when disc degeneration was simulated. It did, however, reduce bulging of the anterior annulus under combined bending and compression (p < 0.03). Recent experiments in vivo have shown that discogenic pain is associated with abnormal bulging of the annulus which suggests that the clinical benefit of fixation may be due to this.

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.