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Research Papers

The Effect of Creep on Human Lumbar Intervertebral Disk Impact Mechanics

[+] Author and Article Information
David Jamison

Department of Mechanical Engineering,
Villanova University,
Villanova, PA 19085
e-mail: david.jamison@villanova.edu

Michele S. Marcolongo

Department of Materials
Science and Engineering,
Drexel University,
Philadelphia, PA 19104

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received May 9, 2013; final manuscript received October 31, 2013; accepted manuscript posted November 25, 2013; published online February 17, 2014. Assoc. Editor: James C. Iatridis.

J Biomech Eng 136(3), 031006 (Feb 17, 2014) (6 pages) Paper No: BIO-13-1224; doi: 10.1115/1.4026107 History: Received May 09, 2013; Revised October 31, 2013; Accepted November 25, 2013

The intervertebral disk (IVD) is a highly hydrated tissue, with interstitial fluid making up 80% of the wet weight of the nucleus pulposus (NP), and 70% of the annulus fibrosus (AF). It has often been modeled as a biphasic material, consisting of both a solid and fluid phase. The inherent porosity and osmotic potential of the disk causes an efflux of fluid while under constant load, which leads to a continuous displacement phenomenon known as creep. IVD compressive stiffness increases and NP pressure decreases as a result of creep displacement. Though the effects of creep on disk mechanics have been studied extensively, it has been limited to nonimpact loading conditions. The goal of this study is to better understand the influence of creep and fluid loss on IVD impact mechanics. Twenty-four human lumbar disk samples were divided into six groups according to the length of time they underwent creep (tcreep = 0, 3, 6, 9, 12, 15 h) under a constant compressive load of 400 N. At the end of tcreep, each disk was subjected to a sequence of impact loads of varying durations (timp = 80, 160, 320, 400, 600, 800, 1000 ms). Energy dissipation (ΔE), stiffness in the toe (ktoe) and linear (klin) regions, and neutral zone (NZ) were measured. Analyzing correlations with tcreep, there was a positive correlation with ΔE and NZ, along with a negative correlation with ktoe. There was no strong correlation between tcreep and klin. The data suggest that the IVD mechanical response to impact loading conditions is altered by fluid content and may result in a disk that exhibits less clinical stability and transfers more load to the AF. This could have implications for risk of diskogenic pain as a function of time of day or tissue hydration.

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Figures

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Fig. 1

X-ray of disk (sagittal section) showing measurements of anterior, middle, and posterior heights. The radiopaque object at the left is a reference metal rod with a diameter of 2.8 mm.

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Fig. 2

Load-displacement response of a 1000-ms impact event showing calculation methods for NZ, ΔE, ktoe, and klin

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Fig. 3

Neutral zone (NZ) was positively correlated with (a) tcreep (some time points omitted for clarity) and negatively correlated with (b) timp

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Fig. 4

Energy dissipation (ΔE) was negatively correlated with both (a) tcreep (some time points omitted for clarity) and (b) timp

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Fig. 5

Toe-region stiffness (ktoe) was negatively correlated with both (a) tcreep (some time points omitted for clarity) and (b) timp

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Fig. 6

Linear-region stiffness (klin) was positively correlated with (a) tcreep (some time points omitted for clarity) but showed no strong correlation with (b) timp

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Fig. 7

Axial strain versus tcreep. Results showed a significant positive correlation between the two parameters.

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Fig. 8

Plots of NZ versus (a) ΔE and (b) ktoe showed a negative correlation with both pairings

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