Surface strain on human intervertebral discs

Abstract

The biomechanical functions of the internal components of the intervertebral disc are not well understood. The surface deformation of 17 human cadaveric lumbar intervertebral discs was studied by photogrammetry by adhering small optical targets to the disc surface and thereby recording the length, bulge, and vertical height of lines on the disc surface representing annular fibers. Discs were studied in pure compression, flexion and extension, axial rotation, and shear. Two definitions of a fiber were investigated: first with the end-points of the fiber on the vertebra ("bone-to-bone" definition), second, where the end points of the fiber were just before the disc vertebra junction (the "disc-only" definition). Measurements were compared with a "constant-volume" physical model and with a mathematical model of the intervertebral disc. Fiber strains were 6% or less under physiological conditions. Comparison of results from the two definitions of fiber length showed greater strains for the disc-only definition in compressive loading. Fiber strains were less than in the constant-volume model of comparable dimensions in compressive loading by a factor of about two, thus suggesting fluid loss or end-plate deformations in the physiologic conditions. The mathematical model indicated that the surface strain for intervertebral discs is very sensitive to the disc-height: diameter ratio and to fluid loss from the disc but is less sensitive to the helix angle of the fibers.

The elastic fibre network of the human lumbar anulus fibrosus: architecture, mechanical function and potential role in the progression of intervertebral disc degeneration

“…Bending and compression of the intervertebral disc within the confines of each motion segment are made possible by the circumferential and axial expansion of the lamellae; this expansion is facilitated by both the direct extension of the collagen fibre bundles and their tilting relative to the transverse plane [18,31,32,51]. In circumferential expansion, direct extension of collagen fibre bundles (fibre strain, measured at the peripheral surface) as a total percentage of tissue deformation is relatively large [59,64]. In contrast, with respect to the positive axial deformations associated with bending, the percentage contribution of collagen fibre strain to total deformation (increase in disc height) is relatively small, suggesting that collagen fibre reorientation plays a more dominant role [50,64].…”

“…In circumferential expansion, direct extension of collagen fibre bundles (fibre strain, measured at the peripheral surface) as a total percentage of tissue deformation is relatively large [59,64]. In contrast, with respect to the positive axial deformations associated with bending, the percentage contribution of collagen fibre strain to total deformation (increase in disc height) is relatively small, suggesting that collagen fibre reorientation plays a more dominant role [50,64]. Analytical structure-function modelling has highlighted the relative importance of shear and normal interactions in determining the tensile mechanical response anulus fibrosus specimens, particularly in the axial direction [19], which is consistent with the idea that relative collagen fibre reorientation is the predominating deformation mechanism for this orientation.…”

“…Secondly, collagen crimp is present in other tissues which contain no documented elastic fibres, such as Achilles tendon, suggesting it is an intrinsic property determined by collagen ultrastructure [14,55]. Finally, the relatively small strains along the collagen fibre axes attributable to uncrimping [51,64] would seem to make elastic fibres redundant in consideration of their high extensibility [16].…”

Influence of extracellular osmolarity and mechanical stimulation on gene expression of intervertebral disc cells

“…2 Apart from the effects of applied load on fluid expression, various loading regimes in vivo subject the disc routinely to flexion, bending, or torsion resulting in complex load-induced signals to the cells. Changes in hydrostatic pressure, 4,8,9 cyclic strain, 10,11 or shear stress 12 have all been found to influence cellular behavior directly. Although mechanical stress in vivo in the IVD is associated with changes in hydration and therefore also with changes in osmolarity, most in vitro studies on mechanobiology in the IVD (especially when using cell culture models) have ignored the role of the osmotic environment.…”

Effects of Running on Intervertebral Disc Height

“…The nucleus pulposus is composed of a loose network of fibers in a mucopolysaccharide gel containing from 70-90% water (3, 7, 1 0, 17, 22, 25, 26, 32). It is essentially avascular and aneural (1,7,15,22,29). Nutrients reach the nucleus pulposus by diffusion from the blood vessels that lie around the periphery of the annulus fibrosus and from the vascular cavities in the central portion of the cartilagenous endplates (1,22).…”