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Research Papers

# Spatially Resolved Streaming Potentials of Human Intervertebral Disk Motion Segments Under Dynamic Axial Compression

[+] Author and Article Information
James C. Iatridis1

School of Engineering, Department of Orthopaedics and Rehabilitation, University of Vermont, 33 Colchester Avenue, Burlington, VT 05405james.iatridis@uvm.edu

Masaru Furukawa, Ian A. F. Stokes, Mack G. Gardner-Morse

Department of Orthopaedics and Rehabilitation, University of Vermont, Burlington, VT 05405

Jeffrey P. Laible

School of Engineering, University of Vermont, Burlington, VT 05405

1

Corresponding author.

J Biomech Eng 131(3), 031006 (Jan 06, 2009) (6 pages) doi:10.1115/1.3005164 History: Received October 05, 2007; Revised August 20, 2008; Published January 06, 2009

## Abstract

Intervertebral disk degeneration results in alterations in the mechanical, chemical, and electrical properties of the disk tissue. The purpose of this study is to record spatially resolved streaming potential measurements across intervertebral disks exposed to cyclic compressive loading. We hypothesize that the streaming potential profile across the disk will vary with radial position and frequency and is proportional to applied load amplitude, according to the presumed fluid-solid relative velocity and measured glycosaminoglycan content. Needle electrodes were fabricated using a linear array of $Ag∕AgCl$ micro-electrodes and inserted into human motion segments in the midline from anterior to posterior. They were connected to an amplifier to measure electrode potentials relative to the saline bath ground. Motion segments were loaded in axial compression under a preload of $500N$, sinusoidal amplitudes of $±200N$ and $±400N$, and frequencies of $0.01Hz$, $0.1Hz$, and $1Hz$. Streaming potential data were normalized by applied force amplitude, and also compared with paired experimental measurements of glycosaminoglycans in each disk. Normalized streaming potentials varied significantly with sagittal position and there was a significant location difference at the different frequencies. Normalized streaming potential was largest in the central nucleus region at frequencies of $0.1Hz$ and $1.0Hz$ with values of approximately $3.5μV∕N$. Under $0.01Hz$ loading, normalized streaming potential was largest in the outer annulus regions with a maximum value of $3.0μV∕N$. Correlations between streaming potential and glycosaminoglycan content were significant, with $R2$ ranging from 0.5 to 0.8. Phasic relationships between applied force and electrical potential did not differ significantly by disk region or frequency, although the largest phase angles were observed at the outermost electrodes. Normalized streaming potentials were associated with glycosaminoglycan content, fluid, and ion transport. Results suggested that at higher frequencies the transport of water and ions in the central nucleus region may be larger, while at lower frequencies there is enhanced transport near the periphery of the annulus. This study provides data that will be helpful to validate multiphasic models of the disk.

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## Figures

Figure 1

Schematic representation of specimen preparation with two different boundary conditions in order to adjust vertebral endplate permeability

Figure 2

Construction detail of the linear array of eight Ag∕AgCl micro-electrodes

Figure 3

Electrodes were inserted from anterior to posterior in the disk using 18gauge needles and connected to amplifiers

Figure 4

Sample data of amplified electrode potentials relative to bath potential

Figure 5

Mean±SEM(n=9) for (a) streaming potential normalized by applied force amplitude, (b) phase angle between streaming potential and applied force as a function of electrode position with relative sagittal position on the upper x-axis. (c) GAG measurements with relative sagittal position. For streaming potential data, * indicates significant (p<0.05) difference from electrode 8 for all frequencies; although at 0.1Hz electrode seven streaming potentials are not significantly different from electrode 8. For GAG measurements, * indicates significant difference from value at 100%.

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