Mechanical properties of the extra-fibrillar matrix of human annulus fibrosus are location and age dependent

Abstract

The mechanical behavior of the annulus fibrosus (AF) of the intervertebral disc can be modeled as a mixture of fibers, extra-fibrillar matrix (EFM), ions, and fluid. However, the properties of the EFM have not been measured directly. We measured mechanical properties of the human EFM at several locations, determined the effect of age and degeneration, and evaluated whether changes in EFM properties correspond to AF compositional changes. EFM mechanical properties were measured using a method that combines osmotic loading and confined compression. AF samples were dissected from several locations, and mechanical properties were correlated with age, degeneration, and composition. EFM modulus was found to range between 10 and 50 kPa, increasing nonlinearly with compression magnitude and being highest in the AF outer-anterior region. EFM properties were not correlated with composition or degeneration. However, the EFM modulus, its relative contribution to tissue modulus, and model parameters were correlated with age. These measurements will result in more accurate predictions of deformations in the intervertebral disc. Additionally, parameters such as permeability and diffusivity used for biotransport analysis of glucose and other solutes depend on EFM deformation. Consequently, the accuracy of biotransport simulations will be greatly improved.

Keywords: annulus fibrosus; extra-fibrillar matrix; mechanical properties; osmotic pressure.

Figure 1

Schematic showing harvest sites for AF samples. OAF: outer anterior. IAF: inner anterior. PAF: posterior. NP: nucleus pulposus. (Samples were dissected as full cylinders)

Figure 2

Circumferential (Circ.), axial, and radial free-swelling stretch at different locations in the AF. Free swelling stretch was higher in the radial direction, followed by axial and circumferential directions. Within each, no effect of location in the disc was found. OAF: outer anterior. IAF: inner anterior. PAF: posterior. (* p < 0.05)

Figure 3

The aggregate modulus of the AF and EFM, and EFM contribution (A, B, and C, respectively) for different locations in the disc as function of applied EFM stretch. The EFM contribution is defined as the ratio between EFM and AF moduli. EFM stretch lower than 1.0 represents compression, and higher than 1.0 represents tension. EFM modulus of the OAF region was higher than the other two regions when EFM stretch was lower than 0.7. The EFM contribution was higher for the OAF region in tension (OAF: outer anterior, IAF: inner anterior, PAF: posterior) (* p < 0.05).

Figure 4

EFM aggregate modulus in compression (A) and tension (B), and EFM contribution in compression (C) for the OAF region as a function of age (A: r² = 0.611, p < 0.05; B: r² = 0.745, p < 0.05; C: r² = 0.823, p < 0.05). Changes were not significant for other AF regions.

Figure 5

Location-averaged values of the Holmes-Mow model parameters for the EFM as function of age. c₁ and c₂ increased with age (A and B) while there was a trend for a decrease in c₃ (C). c₁ represents the stiffness of the tissue in the reference configuration, while c₂ and c₃ control the non-linearity of the tissue.

Figure 6

Location-averaged values of the Holmes-Mow model parameters for the EFM as function of degeneration measured using T1ρ relaxation time. No correlation was found between parameters c₁ and c₂ and degeneration (A and B). However, c₃ was correlated with degeneration (r = 0.785, p < 0.05) (C).