Human intervertebral disc internal strain in compression: the effect of disc region, loading position, and degeneration

Abstract

The primary function of the disc is mechanical; therefore, degenerative changes in disc mechanics and the interactions between the annulus fibrosus (AF) and nucleus pulposus (NP) in nondegenerate and degenerate discs are important to functional evaluation. The disc experiences complex loading conditions, including mechanical interactions between the pressurized NP and the surrounding fiber-reinforced AF. Our objective was to noninvasively evaluate the internal deformations of nondegenerate and degenerate human discs under axial compression with flexion, neutral, and extension positions using magnetic resonance imaging and image correlation. The side of applied bending (e.g., anterior AF in flexion) had higher tensile radial and compressive axial strains, and the opposite side of bending exhibited tensile axial strains even though the disc was loaded under axial compression. Degenerated discs exhibited higher compressive axial and tensile radial strains, which suggest that load distribution through the disc subcomponents are altered with degeneration, likely due to the depressurized NP placing more of the applied load directly on the AF. The posterior AF exhibited higher compressive axial and higher tensile radial strains than the other AF regions, and the strains were not correlated with degeneration, suggesting this region undergoes high strains throughout life, which may predispose it to failure and tears. In addition to understanding internal disc mechanics, this study provides important new data into the changes in internal strain with degeneration, data for validation of finite element models, and provides a technique and baseline data for evaluating surgical treatments.

Figure 1

Correlation between the T_1ρ relaxation time and Pfirrmann grade for samples used in this study (filled circles) and a previous study (open circles).

Figure 2

Representative MR image of a nondegenerate sample. The dots represent the nodes from the (A) outer and (B) inner AF used to calculate the inner and outer AF radial bulge, respectively. The white polygon outlines the disc space area used to calculate the average disc height, by dividing the area by the anterior-posterior width of the polygon.

Figure 3

Internal strains and displacements across AF regions in the neutral position. A) average axial strain, B) inner AF radial displacement, C) outer AF radial displacement, and D) average radial strain in the anterior (AAF), posterior (PAF) and lateral (LAF) AF. *p< 0.05.

Figure 4

The median and interquartile range of strain in the flexion (F), neutral (N) and extension (E) loading positions for the average A) axial, B) radial, and C) shear strain components for all samples (n = 20) in the anterior AF (AAF), NP, and posterior AF (PAF). * denotes significantly different from neutral position, p < 0.05.

Figure 5

Internal displacements in the flexion (F), neutral (N), and extension (E) loading positions for A) axial deformation, B) inner AF radial bulge, and C) outer AF radial bulge. * denotes significance compared to the neutral position, p < 0.05.

Figure 6

The median maximum and minimum strain for the A) axial and B) radial strain components for the anterior AF (□), NP (▧), and posterior AF (■). The interquartile range for peak strains is not shown. Significant differences with the neutral position are represented by † for the maximum strains and for the minimum strain.

Figure 7

A) Correlation with degeneration of the initial disc height in the reference image. B) Correlation of axial deformation with degeneration. Axial deformation data for flexion and extension are shown in the supplementary figure available online (S – Figure 7). F: Flexion position is shown as a dashed line, N: neutral position is represented by the squares with solid line, and E: extension position is shown by the dashed line. * p < 0.05.

Figure 8

Average axial strain in the anterior AF (AAF – filled) and posterior AF (PAF – open) in the A) neutral, B) extension, and C) flexion position. * p < 0.05.

Figure 9

Average radial strain in the anterior AF (AAF – filled) and posterior AF (PAF – open) in the A) neutral and B) extension, and C) flexion position. * p < 0.05.