The Biomechanics of the Inter-Lamellar Matrix and the Lamellae During Progression to Lumbar Disc Herniation: Which is the Weakest Structure?

Abstract

While microstructural observations have improved our understanding of possible pathways of herniation progression, no studies have measured the mechanical failure properties of the inter-lamellar matrix (ILM), nor of the adjacent lamellae during progression to herniation. The aim of this study was to employ multiscale, biomechanical and microstructural techniques to evaluate the effects of progressive induced herniation on the ILM and lamellae in control, pre-herniated and herniated discs (N = 7), using 2 year-old ovine spines. Pre-herniated and herniated (experimental) groups were subjected to macroscopic compression while held in flexion (13°), before micro-mechanical testing. Micro-tensile testing of the ILM and the lamella from anterior and posterolateral regions was performed in radial and circumferential directions to measure failure stress, modulus, and toughness in all three groups. The failure stress of the ILM was significantly lower for both experimental groups compared to control in each of radial and circumferential loading directions in the posterolateral region (p < 0.032). Within each experimental group in both loading directions, the ILM failure stress was significantly lower by 36% (pre-herniation), and 59% (herniation), compared to the lamella (p < 0.029). In pre-herniated compared to control discs, microstructural imaging revealed significant tissue stretching and change in orientation (p < 0.003), resulting in a loss of distinction between respective lamellae and ILM boundaries.

Relative Nucleus Pulposus Area and Position Alter Disk Joint Mechanics

“…The Nucleotomy model demonstrated greater outward bulging of the outer AF, larger tensile radial strains, and inward bulging of the inner AF, agreeing well with experimental observations [5-7, 9, 43]. Large tensile radial strains occurred more frequently in the mid-AF and may lead to annular delamination, which is known to be more prevalent in older or degenerated discs [43][44][45]. Moreover, herniated discs have also been shown to have thicker lamellae, which may be a result of a tissue remodeling or permanent deformations from larger radial tensile strains [43,46,47].…”

“…Large tensile radial strains occurred more frequently in the mid-AF and may lead to annular delamination, which is known to be more prevalent in older or degenerated discs [43][44][45]. Moreover, herniated discs have also been shown to have thicker lamellae, which may be a result of a tissue remodeling or permanent deformations from larger radial tensile strains [43,46,47].…”

New insights into the viscoelastic and failure mechanical properties of the elastic fiber network of the inter-lamellar matrix in the annulus fibrosus of the disc

“…Investigation of the radial cohesion of AF lamellae identified a complex hierarchy of interconnecting fibers in the ILM that demonstrates the role of elastic fibers in lamellae connectivity [17,18]. It is thought that elastic fibers in the ILM assist lamellae to return to their original position after deformation.…”

Spatial mapping of collagen content and structure in human intervertebral disk degeneration

“…Loss of microstructural components can contribute to a decrease in structural integrity of the AF, 48 and this mechanical damage accumulates in the IVD during degeneration. 49 Furthermore, the loss of orientation entropy could be associated with the microfailure of the AF interlamellar matrix, which was shown to be the weakest AF structure prone to microfailure under high loading conditions. 50 We note that, to our knowledge, a comparison of type I and type II collagen has not been performed in terms of their effects on intensity entropy, thus we cannot make compositional assumptions based on these data.…”