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Research Papers

An Examination of the Influence of Strain Rate on Subfailure Mechanical Properties of the Annulus Fibrosus

[+] Author and Article Information
Diane E. Gregory

 University of Waterloo, Waterloo, ON, N2L 3G1, Canada

Jack P. Callaghan1

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

1

Corresponding author.

J Biomech Eng 132(9), 091010 (Sep 01, 2010) (8 pages) doi:10.1115/1.4001945 History: Received April 02, 2009; Revised May 24, 2010; Posted June 11, 2010; Published September 01, 2010; Online September 01, 2010

Disk herniation is often considered a cumulative injury in that repetitive stress on the posterior annulus can result in the nucleus pulposus penetrating the annulus fibrosus and eventually extruding posteriorly. Further, it has been documented that the nucleus pulposus works its way through the annulus through clefts, which form as a result of repetitive tensile strain. The annulus fibrosus is viscoelastic in nature and therefore could express different mechanical responses to applied strain at varying rates. Other viscoelastic tissues, including tendons and ligaments, have shown altered mechanical responses to different rates of applied strain, but the response of the annulus to varying rates of strain is largely unknown. The present study examined the mechanical properties of 20 two-layered samples of porcine annulus fibrosus tissue at three distinct rates of applied 20% biaxial strain (20% strain over 20 s (slow), over 10 s (medium), and over 5 s (fast)); these three rates are considered applicable to nontraumatic loading. No differences in the stiffness or maximum stress in each of the two directions of applied strain were observed between the three strain rates. Specifically, the average (standard deviation) moduli calculated at the fast, medium, and slow rates, respectively, in the axial direction were 7.42 MPa (6.06), 7.77 MPa (6.61), and 7.63 MPa (6.67) and 8.22 MPa (8.4), 8.63 MPa (9.00), and 8.49 MPa (8.69) in the circumferential direction. The maximum stress values reached during the fast, medium, and slow rates, respectively, in the axial direction were 0.40 (0.36) MPa, 0.40 (0.36) MPa, and 0.39 (0.35) MPa and 0.45 (0.47) MPa, 0.44 (0.46) MPa, and 0.43 (0.46) MPa in the circumferential direction. At submaximal strain magnitudes over a range of nontraumatic rates likely to result in clefts in the annulus and potentially leading to disk herniation, any strain rate dependence is not significant.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Dissection of each two-layered sample of the annulus fibrosus from the anterior and posterior regions of the intervertebral disk. (b) Tension was applied biaxially in the axial (compressive axis) and in the circumferential axis (perpendicular to the compressive axis).

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Figure 2

(a) Biaxial testing apparatus setup. Four rakes that are in series with actuators secure the tissue in place, which apply the strain. (b) Image of a two-layered sample of the annulus fibrosus mounted in the testing apparatus. Strain was applied in the axial and circumferential directions.

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Figure 3

Data collection time line. Strain rates A, B, and C represent the three strain rates examined. The particular strain rate, which was represented by each of the letters A, B, and C, was randomized across the 20 tissues tested. The first 20% strain test within each block was excluded from analysis to yield three repeats at each of the three strain rates examined.

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Figure 4

Mean elastic moduli values for each of the three rates of strain for each location on the disk from which tissues were obtained. Bars denote one standard deviation

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Figure 5

Mean maximum stress values for each of the three rates of strain for each location on the disk from which tissues were obtained. Bars denote one standard deviation

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Figure 6

Stress-strain profiles for a representative AF tissue sample for each of the three rates of strain: slow (20% strain/20 s), medium (20% strain/10 s), and fast (20% strain/5 s)

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Figure 7

Stress-strain profiles for the two additional tissue samples strained to failure

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