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Technical Brief

The Effect of Local Hydration Environment on the Mechanical Properties and Unloaded Temporal Changes of Isolated Porcine Annular Samples

[+] Author and Article Information
Kristina M. Gruevski

Department of Kinesiology,
University of Waterloo,
200 University Avenue West,
Waterloo, ON N2L 3G1, Canada
e-mail: kmgruevs@uwaterloo.ca

Chad E. Gooyers

Department of Kinesiology,
University of Waterloo,
200 University Avenue West,
Waterloo, ON N2L 3G1, Canada
e-mail: cgooyers@giffinkoerth.com

Thomas Karakolis

Department of Kinesiology,
University of Waterloo,
200 University Avenue West,
Waterloo, ON N2L 3G1, Canada
e-mail: Thomas.Karakolis@drdc-rddc.gc.ca

Jack P. Callaghan

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

1Present address: Giffin Koerth Forensic Engineering and Science, 40 University Avenue, Suite 800, Toronto, ON M5J 1T1, Canada.

2Present address: Human Systems Integration Section, Defence Research and Development Canada, 1133 Sheppard Avenue West, Toronto, ON M3K 2C9, Canada.

3Corresponding author.

Manuscript received November 10, 2015; final manuscript received July 18, 2016; published online August 18, 2016. Assoc. Editor: James C. Iatridis.

J Biomech Eng 138(10), 104502 (Aug 18, 2016) (6 pages) Paper No: BIO-15-1571; doi: 10.1115/1.4034335 History: Received November 10, 2015; Revised July 18, 2016

Preventing dehydration during in vitro testing of isolated layers of annulus fibrosus tissue may require different test conditions than functional spine units. The purpose of the study was twofold: (A) to quantify changes in mass and thickness of multilayer annulus samples in four hydration environments over 120 min; and (B) to quantify cycle-varying biaxial tensile properties of annulus samples in the four environments. The environments included a saline bath, air, relative humidity control, and misting combined with controlled humidity. The loading protocol implemented 24 cycles of biaxial tensile loading to 20% strain at a rate of 2%/s with 3-, 8-, and 13-min of intermittent rest. Specimen mass increased an average (standard deviation) 72% (11) when immersed for 120 min (p < 0.0001). The air condition and the combined mist and relative humidity conditions reduced mass by 45% (15) and 25% (23), respectively, after 120 min (p < 0.0014). Stress at 16% stretch in the air condition was higher at cycle 18 (18 min of exposure) and cycle 24 (33 min of exposure) compared to all other environments in both the axial and circumferential directions (p < 0.0460). There was no significant change in mass or thickness over time in the relative humidity condition and the change in circumferential stress at 16% stretch between cycles 6 and 24 was a maximum of 0.099 MPa and not statistically significant. Implementation of a controlled relative humidity environment is recommended to maintain hydration of isolated annulus layers during cyclic tensile testing.

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Figures

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Fig. 4

Sample stress–stretch plot, which illustrates the selected stretch magnitudes (2% and 16%) in the toe and linear elastic regions of the curve

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Fig. 3

Time course of the mechanical testing protocol including the duration of rest breaks

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Fig. 1

Anterolateral samples were dissected from levels C23, C34, and C45 (a). The anterior annulus was excised along each end plate (b) and intermediate layers viewed at 10× magnification were further dissected for testing (c).

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Fig. 5

Change in thickness (a) and mass (b) relative to baseline across hydration conditions over time

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Fig. 6

Stress in the circumferential (a) and axial (b) directions at 2% actuator stretch across hydration condition in preload cycle 1 and loading cycles 1, 6, 12, 18, and 24

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Fig. 7

Stress in the circumferential (a) and axial (b) directions at 16% actuator stretch across hydration condition in loading cycles 1, 6, 12, 18, and 24

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