Theoretical and Uniaxial Experimental Evaluation of Human Annulus Fibrosus Degeneration

Abstract

Background The highly organized structure and composition of the annulus fibrosus provides the tissue with mechanical behaviors that include anisotropy and nonlinearity. Mathematical models are necessary to interpret and elucidate the meaning of directly measured mechanical properties, to understand the structure-function relationships of the tissue components, namely the fibers and extrafibrillar matrix. This study models the annulus fibrosus as a combination of strain energy functions describing the fibers, matrix, and their interactions. The objective was to quantify the behavior of both nondegenerate and degenerate annulus fibrosus tissue using uniaxial tensile experimental data. Method of Approach Mechanical testing was performed with samples oriented along the circumferential, axial, and radial directions. Results For samples oriented along the radial direction, the toe-region modulus was 2.7X stiffer with degeneration. However, no other differences in measured mechanical properties were observed with degeneration. The constitutive model fit well to samples oriented along the radial and circumferential directions (R2 ≥ 0.97). The fibers supported the highest proportion of stress for circumferential loading at 60%. There was a 70% decrease in the matrix contribution to stress from the toe- to the linear-region of both nondegenerate and degenerate tissue. The shear fiber-matrix interaction contribution increased 125% with degeneration in the linear-region. Samples oriented along the radial and axial direction behaved similarly under uniaxial tension (modulus = 0.32MPa versus 0.37MPa), suggesting that uniaxial testing in the axial direction is not appropriate for quantifying the mechanics of a fiber reinforcement in the annulus. Conclusions In conclusion, the structurally motivated nonlinear, anisotropic hyperelastic constitutive model help to further understand the effect of microstructural changes with degeneration, suggesting that a remodeling in the subcomponents (i.e. the collagen fibers and FMI) may minimize the overall change in tissue mechanics with degeneration.

Modeling Degenerative Disk Disease in the Lumbar Spine: A Combined Experimental, Constitutive, and Computational Approach

“…These efforts were generally focused on the simulation of global spinal kinematics rather than accurately predicting the local mechanics of the intervertebral disk. More recent experimental studies have quantified the mechanical behavior of the annulus fibrosus tissue through the use ' of uniaxial and biaxial loading protocols, providing a basis for constitutive modeling attempts [2,3,[7][8][9][10][11]]. The continuum modeling framework suggested by Spencer [12] for specific applications of fiber-reinforced composites has typically been utilized.…”

Biaxial mechanics and inter‐lamellar shearing of stem‐cell seeded electrospun angle‐ply laminates for annulus fibrosus tissue engineering

“…2 With degeneration, structural organization, and mechanical properties of the AF decrease, resulting in abnormal function. 3,4 Current treatments options, including discectomy and spinal fusion, do not restore mechanical function to the disk, and as such, often result in degeneration of adjacent disks. 5 This indicates a need for regenerative strategies that restore mechanical function for all relevant loading regimes, ideally matching native tissue benchmarks.…”

The Biomechanics of eyelid tarsus tissue

“…However, again there are significant differences in the biomechanical properties of the two tissues, with the stiffness of sclera previously shown to vary range from 2.8 to 3.3 MPa (Eilaghi et al, 2010) and elastic modulus averaging 2.9 71.4 MPa with a maximum strain of approximately 20% (Friberg and Lace, 1988). Previous studies of human annulus fibrosis, composed of 50% collagen, 5% proteoglycan and a small percentage of elastin have reported circumferential tensile modulus ranging between 14 and 49 MPa (Acaroglu et al, 1995;Ebara et al, 1996;Elliott and Setton, 2001;Guerin and Elliott, 2006;O'Connell et al, 2009;Skrzypiec et al, 2007). There have been many previous studies investigating the biomechanical properties of human skin both in vivo and in vitro, and significant variation exists between studies due to numerous factors including, but not limited to: differences in specimen location, age, biological variability and test conditions.…”