Intervertebral disc herniation and internal disc disruption are both thought to be primarily mechanically based pathologies. Although several studies have previously disrupted discs in vitro, none have examined the resulting disruptions microscopically.

The technique of nuclear pressurization was used to mechanically disrupt ovine lumbar motion segments. A hollow injection screw was inserted longitudinally through the inferior vertebra of each motion segment, so that the injection screw’s tip was located in the centre of the nucleus. Through this screw, a radio-opaque gel was gradually injected into each segment’s nucleus until failure occurred, marked by a large drop in nuclear pressure, or focal change to the disc’s periphery. Following mechanical testing, the internal failure characteristics of each motion segment were assessed using micro-CT and microscopy. During nuclear pressurization, motion segments were held in one of four postures:

0° flexion,

Group I (0° flexion; n=12): Discs failed at a mean nuclear pressure of 13.2±2.1MPa. In most cases failure occurred in a diffuse manner via sequential circumferential tears within the posterior annulus. Group II (7° flexion; n=17): Discs failed at a mean nuclear pressure of 11.2±2.5MPa. Compared to the Group I discs, 7° flexion led to the creation of radial tears extending through the central posterior disc wall. Two types of radial tear occurred: mid-axial and annular-endplate. Mid-axial radial tears were confined to the annulus. Annular-endplate radial tears incorporated both annular and endplate failure; endplate failure in these tears always occurred adjacent to the mid-annulus at the cartilaginous/vertebral endplate junction. Group III (10° flexion; n=17): Discs failed at a mean nuclear pressure of 9.8±2.6MPa. Compared to the Group II discs, 3° of additional flexion increased the proportion of annular-endplate radial tears. Group IV (7° flexion + 2° axial rotation; n=25): Discs failed at a mean nuclear pressure of 7.9±2.4MPa. Compared to the Group II discs, the addition of 2° axial rotation significantly decreased the nuclear pressure at which discs failed, and reduced the occurrence of mid-axial radial tears.

Postures that reduced the disc wall’s ability to withstand high nuclear pressures were associated with an increase in the proportion of disc failures that incorporated tears of the cartilaginous endplates, specifically at the cartilaginous/vertebral endplate junction adjacent to the mid-annulus. The robustness of this junction appears to be intimately linked to the robustness of the disc wall.

The detailed anatomy of interconnectivity of intervertebral disc annular fibre layers remains unclear and a structural survey of interlammellar connectivity is required to understand this anatomy and mechanical behavior. The subsequent failure modes of the annulus under hydrostatic loading require definition to understand genesis of annular tears and disc herniation.

Interlamellar Connectivity. We imaged anterior annular sections from ovine lumbar discs. Using differential interference contrast microscopy we were able to reconstruct a three-dimensional image of the interconnecting bridging network between layers. Annular Disruption. The nuclei of ovine lumbar discs were gradually pressurised to failure by injecting a viscous radio-opaque gel via their inferior vertebrae. Investigation of the resulting annular disruption was carried out using micro-computed tomography and DIC microscopy. This allowed analysis of annular failure patterns and herniation, with analysis of the pathway of nuclear movement during prolapse in relation to annular fibre separation within and between fibre layers.

Interlamellar Connectivity. A high level of connectivity between apparently disparate bridging elements was revealed. The extended form of the bridging network is that of occasional substantial radial connections spanning many lamellae with a subsidiary fine branching network. The fibrous bridging network is highly integrated with the lamellar architecture via a collagen-based system of interconnectivity. In particular this bridging network appears to have a major role in anchoring leading edges of incomplete annular lamellae. Annular Disruption and Disc Herniation. Gel extrusion from the posterior annulus was the most common mode of disc failure. Unlike other regions of the annular wall, the posterior region was unable to distribute hydrostatic pressures circumferentially. In each extrusion case, severe disruption to the posterior annulus was observed. While intralamellar disruption occurred in the mid annulus, interlamellar disrupt ion occurred in the outer posterior annulus. Radial ruptures between lamellae always propagated in the mid-axial plane.

The interlamellar architecture of the annulus is far more complex than has previously been recognised and this paper further defines the microanatomy of the disc wall. The hydrostatic pressure failure mode of the posterior annulus mirrors clinic al sites of annular tear and disc prolapsed in the neutral loading position.

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.