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Research Papers

Peak Stress in the Annulus Fibrosus Under Cyclic Biaxial Tensile Loading

[+] Author and Article Information
Chad E. Gooyers

Giffin Koerth Forensic Engineering and Science,
40 University Avenue, Suite 800,
Toronto, ON M5J 1T1, Canada
e-mail: cgooyers@giffinkoerth.com

Jack P. Callaghan

Professor
Canada Research Chair in Spine Biomechanics
and Injury Prevention,
Department of Kinesiology,
Faculty of Applied Health Sciences,
University of Waterloo,
Waterloo, ON N2L 3G1, Canada
e-mail: jack.callaghan@uwaterloo.ca

1Corresponding author.

Manuscript received July 20, 2015; final manuscript received March 8, 2016; published online March 29, 2016. Assoc. Editor: James C. Iatridis.

J Biomech Eng 138(5), 051006 (Mar 29, 2016) (7 pages) Paper No: BIO-15-1365; doi: 10.1115/1.4032996 History: Received July 20, 2015; Revised March 08, 2016

Numerous in vitro studies have examined the initiation and propagation of fatigue injury pathways in the annulus fibrosus (AF) using isolated motion segments; however, the cycle-varying changes to the AF under cyclic biaxial tensile loading conditions have yet to be examined. Therefore, the primary objective of this study was to characterize the cycle-varying changes in peak tensile stress in multilayer AF tissue samples within a range of physiologically relevant loading conditions at subacute magnitudes of tissue stretch up to 100 loading cycles. A secondary aim was to examine whether the stress-relaxation response would be different across loading axes (axial and circumferential) and whether this response would vary across regions of the intervertebral disk (IVD) (anterior and posterior–lateral). The results from the study demonstrate that several significant interactions emerged between independent factors that were examined in the study. Specifically, a three-way interaction between the radial location, magnitude of peak tissue stretch, and cycle rate (p = 0.0053) emerged. Significant two-way interactions between the magnitude of tissue stretch and cycle number (p < 0.0001) and the magnitude of tissue stretch and loading axis (p < 0.0001) were also observed. These findings are discussed in the context of known mechanisms for structural damage, which have been linked to fatigue loading in the IVD (e.g., cleft formation, radial tearing, increased neutral zone, disk bulging, and loss of intradiscal pressure).

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Figures

Grahic Jump Location
Fig. 6

Average peak unnormalized tensile stress in AF tissue samples excised from the posterior–lateral region of the IVD, across experimental conditions. Note: y-axis scale doubles when moving left to right between subplots.

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Fig. 5

Average peak unnormalized tensile stress in AF tissue samples excised from the anterior region of the IVD, across experimental conditions. Note: y-axis scale doubles when moving left to right between subplots.

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Fig. 4

Profile plot of average (+ standard deviation) unnormalized peak stress data across the magnitude of peak stress and loading axis. Average data collapsed across anterior and posterior regions of the IVD and cycle number are presented.

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Fig. 3

Profile plot of average (+ standard deviation) unnormalized peak stress data across the magnitude of peak stress and loading cycle number

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Fig. 2

Profile plot of average (+ standard deviation) unnormalized peak stress data across the magnitude of peak stretch and cycle rate for (a) anterior and (b) posterior–lateral regions. Average data across cycle number 1, 10, and 100 is presented.

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Fig. 1

Representative cycle-varying changes of stress stretch-ratio loading curves for (a) circumferential and (b) axial loading axes (c34, anterior sample, 10 cycles per minute stretch at 12–20%)

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