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A Comparison of Uniaxial and Biaxial Mechanical Properties of the Annulus Fibrosus: A Porcine Model

[+] Author and Article Information
Diane E. Gregory

Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada

Jack P. Callaghan1

Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canadacallagha@uwaterloo.ca

1

Corresponding author.

J Biomech Eng 133(2), 024503 (Jan 31, 2011) (5 pages) doi:10.1115/1.4003327 History: Received November 05, 2010; Revised December 01, 2010; Posted December 22, 2010; Published January 31, 2011; Online January 31, 2011

Abstract

The annulus fibrosus of the intervertebral disk experiences multidirectional tension in vivo, yet the majority of mechanical property testing has been uniaxial. Therefore, our understanding of how this complex multilayered tissue responds to loading may be deficient. This study aimed to determine the mechanical properties of porcine annular samples under uniaxial and biaxial tensile loading. Two-layer annulus samples were isolated from porcine disks from four locations: anterior superficial, anterior deep, posterior superficial, and posterior deep. These tissues were then subjected to three deformation conditions each to a maximal stretch ratio of 1.23: uniaxial, constrained uniaxial, and biaxial. Uniaxial deformation was applied in the circumferential direction, while biaxial deformation was applied simultaneously in the circumferential and compressive directions. Constrained uniaxial consisted of a stretch ratio of 1.23 in the circumferential direction while holding the tissue stationary in the axial direction. The maximal stress and stress-stretch ratio (S-S) moduli determined from the biaxial tests were significantly higher than those observed during both the uniaxial tests (maximal stress, 97.1% higher during biaxial; $p=0.002$; S-S moduli, 117.9% higher during biaxial; $p=0.0004$) and the constrained uniaxial tests (maximal stress, 46.8% higher during biaxial; S-S moduli, 82.9% higher during biaxial). These findings suggest that the annulus is subjected to higher stresses in vivo when under multidirectional tension.

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Figures

Figure 3

Representative stress-stretch ratio curve in the circumferential direction. Tissues were strained to a stretch ratio of 1.23, The S-S modulus (E) was determined by taking the slope of the linear portion of the stress-stretch ratio curve. This linear portion, on average, started at a stretch ratio of 1.19, or approximately 82.5% of the final stretch magnitude.

Figure 2

(a) BioTester 5000 biaxial material testing apparatus. (b) Four rakes secure the tissue during biaxial tension, and two rakes are used for uniaxial tension. (c) Biaxial configuration with four rakes inserted into the tissue. (d) Uniaxial configuration with two rakes inserted into the tissue.

Figure 1

(a) Depiction of dissection procedures to obtain annulus fibrosus tissue samples with only two adjacent lamellae. (b) Direction of axial and circumferential tension. Axial tension is in the direction of spinal compression, while circumferential tension is perpendicular to the compressive axis.

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