INTRA-LAMELLAR RELATIONSHIPS IN THE COLLAGENOUS ARCHITECTURE OF THE ANNULUS FIBROSIS: A ‘LIVE’ TISSUE STUDY

Abstract

Introduction Compressive loads applied to the disc are translated into an internal hydrostatic pressure in the nucleus which is resisted by the annulus. 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. While many previous investigators have analysed the tensile behaviour of the annulus there still remains much to be learned about the fundamental structural relationships within the disc wall and upon which normal function depends. There is also much to be learned about how alterations in these relationships might lead to disc malfunction. Both intra and inter-lamellar structural relationships will be fundamental to the maintenance of annular wall strength. The aim of this study was to use high resolution ‘live’ imaging to explore the fundamental structural relationships governing the elasticity, intrinsic strength and rupture behaviour of intra-lamellar sections.

Methods In-plane intra-lamellar sections of nominal thickness 70–90μm were cut from the outer lamellae of bovine discs using a sledging microtome. Using a micro-mechanical technique in combination with simultaneous high resolution differential interference contrast optical microscopy (DIC) structural responses both along and transverse to the primary direction of the mono-array of collagen fibres were studied.

Results and Discussion Stretching along the primary alignment direction revealed a biomechanical response consistent with the behaviour of an array of strong fibres whose strength is governed primarily by the strength of embedding in the vertebral endplates rather than from inter-fibre 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 a series of 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 versus across the primary fibre direction are consistent with 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 ‘live’ tissue investigation provides an important reference framework for exploring structural changes associated with disc trauma and degeneration.