Mechanisms of anular failure resulting from excessive intradiscal pressure: a microstructural-micromechanical investigation.

STUDY DESIGN: Microstructural/micromechanical investigation of pathways of anular wall disruption. OBJECTIVE: To investigate the fundamental mechanisms by which both intralamellar and interlamellar relationships are disrupted by nuclear pressurization. SUMMARY OF BACKGROUND DATA: Understanding how anular failure might occur following increased nuclear pressurization requires an experimental approach that avoids artifactual injury to the anulus but reveals structural disruption resulting directly from the pressurization event. METHODS: Bovine motion segments were subjected to internal pressurization using a novel "through vertebra" gel injection method. Intralamellar and interlamellar sections were deliberately chosen so as to expose systematic patterns of structural disruption resulting from the pressurization event. This microdisruption was investigated using a novel method that combined microtensile manipulation and simultaneous differential contrast imaging of the fully hydrated unstained sections. RESULTS: The inner anulus was most severely disrupted, the middle regions developed a series of regular clefts along axes of weakness within the in-plane arrays, with only mild array disruption occurring in the outer regions. CONCLUSIONS: A mechanism is proposed whereby an anular failure pathway, driven by hydrostatic nuclear pressure, could track through the complex anular structures via a set of disruptive events causing weakening of both the in-plane and interlamellar junction interconnections.

The structural basis of interlamellar cohesion in the intervertebral disc wall.

The purpose of this study was to investigate the structural mechanisms that create cohesion between the concentric lamellae comprising the disc annulus. Sections, 50-60 microm thick, were obtained using a carefully chosen cutting plane that incorporated the fibrous component in alternating lamellae as in-plane and cross-sectioned arrays. These sections were then subjected to microtensile stretching both across (radial) and along (tangential) the in-plane fibre direction, in their fully hydrated state. Structural responses were studied by simultaneously viewing the sections using high-resolution Nomarski interference contrast light microscopy. Additional bulk samples of annulus were fixed while held in a constant, radially stretched state in order to investigate the potential for interlamellar separation to occur in a state more representative of the intact disc wall. The study has provided a detailed picture of the structural architecture creating disc wall cohesion, revealing a complex hierarchy of interconnecting relationships within the disc wall, not previously described. Importantly, because our experimental approach offers a high-resolution view of the response of the interlamellar junction to deformation in its fully hydrated condition, it is a potentially useful method for investigating subtle changes in junction cohesion resulting from both early degeneration and whole-disc trauma.

Intralamellar relationships within the collagenous architecture of the annulus fibrosus imaged in its fully hydrated state.

The anisotropic, inhomogeneous, multiply collagenous architecture of the annulus reflects the complex pattern of mainly tensile stresses developed in this region of the disc during normal function. Structural and mechanical responses of fully hydrated in-plane sections taken from within single lamellae of the outer annulus of healthy bovine caudal discs have been investigated using a micromechanical technique in combination with simultaneous high-resolution differential interference contrast optical imaging. Responses both along and across (i.e. transverse to) the primary direction of the mono-array of collagen fibres were studied. Stretching along the alignment direction revealed a biomechanical response consistent with the behaviour of an array whose overall strength is governed primarily by the strength of embedding of the fibres in the vertebral endplates, rather than from interfibre cohesion along their length. The mono-aligned array, even when lacerated, is highly resistant to any further tearing across the alignment direction. Although not visible in the relaxed mono-arrays, transverse stretching revealed a highly complex set of interconnecting structures embodying hierarchical relationships not previously revealed. It is suggested that these structures might play an important role in the containment under pressure of the nuclear contents. The dramatic differences in rupture behaviour observed along vs. across the primary fibre direction are consistent with the known clinical consequences arising from varying degrees of annular wall damage, and might also explain various types of disc herniation. The lamellar architecture of the healthy disc revealed by this investigation provides an important reference framework for exploring structural changes associated with disc trauma and degeneration.