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Research Papers

A Novel Method for Repeatable Failure Testing of Annulus Fibrosus

[+] Author and Article Information
Benjamin Werbner

Mechanical Engineering Department,
University of California, Berkeley,
2162 Etcheverry Hall, #1740,
Berkeley, CA 94720-1740
e-mail: benwerbner@berkeley.edu

Minhao Zhou

Mechanical Engineering Department,
University of California, Berkeley,
2162 Etcheverry Hall, #1740,
Berkeley, CA 94720-1740
e-mail: minhao.zhou@berkeley.edu

Grace O'Connell

Mechanical Engineering Department,
University of California, Berkeley,
5122 Etcheverry Hall, #1740,
Berkeley, CA 94720-1740
e-mail: g.oconnell@berkeley.edu

1Corresponding author.

Manuscript received April 28, 2017; final manuscript received September 1, 2017; published online September 27, 2017. Assoc. Editor: Kyle Allen.

J Biomech Eng 139(11), 111001 (Sep 27, 2017) (7 pages) Paper No: BIO-17-1183; doi: 10.1115/1.4037855 History: Received April 28, 2017; Revised September 01, 2017

Tears in the annulus fibrosus (AF) of the intervertebral disk can result in disk herniation and progressive degeneration. Understanding AF failure mechanics is important as research moves toward developing biological repair strategies for herniated disks. Unfortunately, failure mechanics of fiber-reinforced tissues, particularly tissues with fibers oriented off-axis from the applied load, is not well understood, partly due to the high variability in reported mechanical properties and a lack of standard techniques ensuring repeatable failure behavior. Therefore, the objective of this study was to investigate the effectiveness of midlength (ML) notch geometries in producing repeatable and consistent tissue failure within the gauge region of AF mechanical test specimens. Finite element models (FEMs) representing several notch geometries were created to predict the location of bulk tissue failure using a local strain-based criterion. FEM results were validated by experimentally testing a subset of the modeled specimen geometries. Mechanical testing data agreed with model predictions (∼90% agreement), validating the model's predictive power. Two of the modified dog-bone geometries (“half” and “quarter”) effectively ensured tissue failure at the ML for specimens oriented along the circumferential-radial and circumferential-axial directions. The variance of measured mechanical properties was significantly lower for notched samples that failed at the ML, suggesting that ML notch geometries result in more consistent and reliable data. In addition, the approach developed in this study provides a framework for evaluating failure properties of other fiber-reinforced tissues, such as tendons and meniscus.

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Figures

Grahic Jump Location
Fig. 1

Schematic of (a) circumferential-radial and (b) circumferential-axial specimen orientations. Schematic of (c) half and (d) quarter notch geometries with dimensions given for experimental specimens. Model was created at 40% of experimental dimensions. (e) Quarter notch sample glued into sandpaper grips for mechanical testing.

Grahic Jump Location
Fig. 2

Representative stress–strain curve. Toe- and linear-region moduli were calculated using a custom linear-regression optimization technique. The point of failure was defined by the maximum stress achieved (star), and this point was used to define the failure stress and failure strain. Inset: computational model fit versus experimental stress–strain curves.

Grahic Jump Location
Fig. 3

ML grip strain ratio versus global engineering strain. X's denote predicted failure, which was defined as the point when the average local strain reached 65% at either the ML or the grip-line.

Grahic Jump Location
Fig. 4

Strain map at the time-step when failure initiation was predicted by the FEM. Arrows indicate peak local strains. Model simulations in A, B, and D predict bulk tissue failure at the grip-line (black arrows). Model simulations in C and E predict failure at the notch site (white arrows).

Grahic Jump Location
Fig. 5

Simulation results showing the effect of ML cross-sectional area (normalized by gripped area) on (a) the stress–strain response and (b) the linear-region modulus [9,11,29,3638]

Grahic Jump Location
Fig. 6

Representative circumferential-radial sample with a half notch geometry failing at the ML

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Fig. 8

Coefficient of variation for each measured mechanical property

Grahic Jump Location
Fig. 7

Mechanical properties of circumferential-axial specimens. Data are reported as the mean plus or minus the standard deviation.

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