Introduction: Peripheral rim tears in the annulus fibrosus are a common finding in autopsy specimens, and animal experiments suggest that they lead eventually to degenerative changes throughout the disc. We test the hypothesis that injury to the outer annulus decompresses the nucleus, thereby providing a progressive stimulus for disc degeneration.

Methods: Seven human cadaveric lumbar “motion segments” aged 49–70 yrs were compressed at 2 kN while the distribution of compressive stress was measured in each disc by pulling a 1.3 mm-diameter pressure transducer along its mid-sagittal diameter. Measurements were repeated after “rim tears” were simulated by 10 mm-deep scalpel cuts into the anterior annulus, as follows. 1st cut: horizontal, 15 mm right lateral; 2nd cut: vertical, 15 mm left lateral; 3rd cut: horizontal, midline (through the transducer needle track). Stress measurements were repeated a final time following compressive overload sufficient to fracture the endplate.

Results: “Rim tears” had negligible effect on compressive stress distributions more than 15mm from the scalpel cut, and nucleus pressure fell by only 1.0% (STD 1.3%, NS). However, compressive stresses in the outer annulus adjacent to the cut were greatly reduced, and a steep stress gradient appeared in the middle annulus. The effective decrease in the A-P diameter of the disc was 7.1% (STD 1.7%, P< 0.01). Endplate fracture reduced nucleus pressure by 36.1% (STD 16.7%, P< 0.001).

Discussion: Stress gradients generated in the middle annulus could cause the “rim tear” to progress inwards until it reached the nucleus, at which point it might decompress it. However, the present results suggest that injuries to the outer annulus are unlikely to have any direct effect on the pressure in, or metabolism of, the nucleus pulposus. This is in contrast to injuries to the vertebral endplate, which do affect the nucleus directly.