The morphology of acute disc herniation: A clinically relevant model defining the role of flexion

Study Design. Hydrostatically induced disruption of flexed lumbar intervertebral discs followed by microstructural investigation. Objective. To investigate how flexion affects the anulus' ability to resist rupture during hydrostatic loading, and determine how the characteristics of the resulting disc failures compare with those observed clinically. Summary of Background Data. While compression of neutrally positioned motion segments consistently causes vertebral failure, compression of flexed segments can induce herniation. Why flexion has this effect remains unclear. A vast range of herniation characteristics have been documented clinically; whether flexion-related herniations are likely to possess a subset of these is unknown. Methods. Forty-two ovine lumbar motion segments, dissected from the same 3 levels of 14 spines, were each flexed 7° or 10° from the neutral position. While maintained at one of these angles, the nucleus of each segment was gradually injected with a viscous radio-opaque gel via an injection screw placed longitudinally within the inferior vertebra, until failure occurred. Each segment was then inspected using microcomputed tomography and oblique illumination microscopy in tandem. Results. Eighteen segments suffered disc failure; 14 of these were caused by direct radial rupture of the anular wall. All radial ruptures were located in the central posterior anulus. Nine radial ruptures contained nuclear material, which had breached the posterior longitudinal ligament in 1 disc, and reached it in 5 others forming transligamentous and subligamentous nuclear extrusions, respectively. The most common radial rupture route, occurring in 10 discs, involved a systematic anulus-endplate-anulus failure pattern. Conclusion. Flexion places the anulus at risk by facilitating nuclear flow, limiting circumferential disruption while promoting radial rupture, and rendering the endplate/vertebra junction vulnerable to failure. Flexion may play a developmental role in those herniations possessing a central posterior radial rupture that incorporates a short span of endplate disruption along the apex of the vertebral rim.