A laboratory model of lumbar disc protrusion. Fissure and fragment.

Discs of 20 human lumbar motion segments from donors between 20 and 52 years of age were subjected to a procedure that effected a radial fissure of the anulus, sparing a peripheral layer of approximately 1 mm in thickness. In addition, fragmented tissue pieces that resembled those retrieved at surgery for prolapse were created in the center of the disc. The disc contour was measured under pure axial load as well as in flexion and extension. In the intact specimen, the disc contour shifts in ventral direction in flexion and in dorsal direction in extension. In the 'fissure and fragment' discs a broad-based protrusion develops dorsolaterally at the location of the fissure. The magnitude of the protrusion is independent of flexion or extension angles in the range of +/- 5 degrees. The 'fissure and fragment' discs exhibit disc prolapse at loads between 0.9 and 6.1 kN and flexion angles below 10 degrees, i.e., under loading conditions well in the physiologic range. The findings of this experiment support the hypothesis that disc prolapse--aside from the hyperflexion trauma described in the literature--has to be preceded by generation of radial fissures and tissue fragmentation within the disc. Thus, prolapse appears to be a late event during the course of a long-term degenerative process.