Abstract
Introduction: Compound mechanical loadings have been used to re-create clinically relevant annular disruptions in vitro. However, the role that individual loading parameters play in disrupting the lumbar disc’s annulus remains unclear. Using the recently described technique of nuclear inflation, the role that elevated nuclear pressures play in disrupting the lumbar intervertebral disc’s annulus fibrosus was investigated.
Methods: The nuclei of 12 ovine lumbar motion segments, posterior elements removed, were gradually pressurized by injecting a viscous radio-opaque gel via an injection screw fitted axially through their inferior vertebrae. Pressurization was conducted until catastrophic failure of the disc occurred. Investigation of the resulting annular disruption was carried out in tandem using micro-computed tomography and differential interference contrast microscopy.
Results: 3 of the 12 motion segments tested were excluded from the results due to improper placement of the injection screw, resulting in pressurization of the inferior vertebra rather than the nucleus. Mean failure pressure of the remaining 9 motion segments was 14.1 ± 3.9 MPa. Peak rates of pressurization ranged from 0.1–0.4MPa/s. Gel extrusion from the posterior annulus occurred in 7 discs and was the most common mode of failure. Unlike other aspects of the annular wall, the posterior region was unable to distribute hydrostatic pressures circumferentially. In each extrusion case, sever disruption to the posterior annulus occurred. While 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.
Discussion: 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. The limited ability of the injected gel to cross the posterior-posterolateral boundaries, effectively concentrating hydrostatic stress within the posterior annulus, indicates that the laminate architecture along these radial lines is of mechanical significance. Within the outer posterior annulus, the prominence of inter-lamellar rather than intralamellar disruption indicates weak interlamellar cohesion. This suggests that nuclear material migrating down a radial fissure may easily track circumferentially within an interlamellar space upon reaching the inner lamellae of the outer annulus. This may explain why the majority of herniations are limited to protrusions contained within the outer annular wall.. The tendency for annular fibres to rupture in the mid-axial plane when loaded hydrostatically suggests that for a radial fissure or herniation to occur at the annular-endplate junction, a compounding bending or torsional load is required.
Correspondence should be addressed to Dr Owen Williamson, Editorial Secretary, Spine Society of Australia, 25 Erin Street, Richmond, Victoria 3121, Australia.
