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Technical Briefs

[+] Author and Article Information
Alicia R. Jackson, Francesco Travascio

Department of Biomedical Engineering, Tissue Biomechanics Laboratory, University of Miami, Coral Gables, FL 33146

Wei Yong Gu1

Department of Biomedical Engineering, Tissue Biomechanics Laboratory, University of Miami, Coral Gables, FL 33146wgu@miami.edu

1

Corresponding author. Also at Department of Biomedical Engineering, College of Engineering, University of Miami, P.O. Box 248294, Coral Gables, FL 33124-0621.

J Biomech Eng 131(5), 054505 (Apr 13, 2009) (5 pages) doi:10.1115/1.3116152 History: Received August 22, 2008; Revised December 11, 2008; Published April 13, 2009

Abstract

The intervertebral disk (IVD), characterized as a charged, hydrated soft tissue, is the largest avascular structure in the body. Mechanical loading to the disk results in electromechanical transduction phenomenon as well as altered transport properties. Electrical conductivity is a material property of tissue depending on ion concentrations and diffusivities, which are in turn functions of tissue composition and structure. The aim of this study was to investigate the effect of mechanical loading on electrical behavior in human IVD tissues. We hypothesized that electrical conductivity in human IVD is strain-dependent, due to change in tissue composition caused by compression, and inhomogeneous, due to tissue structure and composition. We also hypothesized that conductivity in human annulus fibrosus (AF) is anisotropic, due to the layered structure of the tissue. Three lumbar IVDs were harvested from three human spines. From each disk, four AF specimens were prepared in each of the three principal directions (axial, circumferential, and radial), and four axial nucleus pulposus (NP) specimens were prepared. Conductivity was determined using a four-wire sense-current method and a custom-designed apparatus by measuring the resistance across the sample. Resistance measurements were taken at three levels of compression (0%, 10%, and 20%). Scanning electron microscopy (SEM) images of the human AF tissue were obtained in order to correlate tissue structure with conductivity results. Increasing compressive strain significantly decreased conductivity for all groups ($p<0.05$, analysis of variance (ANOVA)). Additionally, specimen orientation significantly affected electrical conductivity in the AF tissue, with conductivity in the radial direction being significantly lower than that in the axial or circumferential directions at all levels of compressive strain ($p<0.05$, ANOVA). Finally, conductivity in the NP tissue was significantly higher than that in the AF tissue ($p<0.05$, ANOVA). SEM images of the AF tissues showed evidence of microtubes orientated in the axial and circumferential directions, but not in the radial direction. This may suggest a relationship between tissue morphology and the anisotropic behavior of conductivity in the AF. The results of this investigation demonstrate that electrical conductivity in human IVD is strain-dependent and inhomogeneous, and that conductivity in the human AF tissue is anisotropic (i.e., direction-dependent). This anisotropic behavior is correlated with tissue structure shown in SEM images. This study provides important information regarding the effects of mechanical loading on solute transport and electrical behavior in IVD tissues.

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Figures

Figure 1

Schematic of intervertebral disk showing sites and orientations of test specimens. AF indicates annulus fibrosus and NP indicates nucleus pulposus.

Figure 2

Schematics of (a) compression apparatus and (b) apparatus for measuring strain-dependent electrical conductivity

Figure 3

Variation of electrical conductivity with increasing compressive strain for AF and NP tissues. For the AF tissue, the results for conductivity in three directions are shown: axial (A), circumferential (C), and radial (R). Variation bars signify standard deviation (SD). Statistical analysis showed a significant difference (p<0.05) between conductivity values for all groups except axial AF and circumferential AF (at all levels of compressive strain).

Figure 4

Model prediction of normalized conductivity in human AF and NP tissues compared with experimental data. The value of FCD was assumed to be 0.2 mEq/ml at water volume fraction of 0.72. The values for α and β were taken from literature (16).

Figure 5

SEM images showing obviously (a) no obviously visible microtubes in the radial section of human AF, (b) the clear presence of microtubes in the axial and (c) circumferential sections, and (d) a magnified view of the microtube

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