ISSLS prize winner: microstructure and mechanical disruption of the lumbar disc annulus: part II: how the annulus fails under hydrostatic pressure.

Study design Mechanically induced annular disruption of lumbar intervertebral discs followed by microstructural investigation. Objective To investigate the role that elevated nuclear pressures play in disrupting the lumbar intervertebral disc's annulus fibrosus. Summary of background data Compound mechanical loadings have been used to recreate clinically relevant annular disruptions in vitro. However, the role that individual loading parameters play in disrupting the lumbar disc's annulus remains unclear. Methods The nuclei of ovine lumbar intervertebral discs were gradually pressurized by injecting a viscous radio-opaque gel via their inferior vertebrae. Pressurization was conducted until catastrophic failure of the disc occurred. Investigation of the resulting annular disruption was carried out using microcomputed tomography and differential interference contrast microscopy. Results Gel extrusion from the posterior annulus was the most common mode of disc failure. Unlike other aspects of the annular wall, the posterior region was unable to distribute hydrostatic pressures circumferentially. In each extrusion case, severe disruption of the posterior annulus occurred. Although intralamellar disruption occurred in the mid annulus, interlamellar disruption occurred in the outer posterior annulus. Radial ruptures between lamellae always occurred in the mid-axial plane. Conclusion With respect to the annular wall, the posterior region is most susceptible to failure in the presence of high nuclear pressure, even when loaded in the neutral position. Weak interlamellar cohesion of the outer posterior lamellae may explain why the majority of herniations remain contained as protrusions within the outer annular wall.