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Research Papers

Biomechanical Influence of Disk Properties on the Load Transfer of Healthy and Degenerated Disks Using a Poroelastic Finite Element Model

[+] Author and Article Information
Amélie Chagnon, Carl-Éric Aubin

Department of Mechanical Engineering, Ecole Polytechnique de Montreal, P.O. Box 6079, Station “Centre-Ville,” Montréal, QC, H3C 3A7, Canada; Sainte-Justine University Hospital Center, 3175 Côte-Ste-Catherine Road, Montréal, QC, H3C 1C5, Canada

Isabelle Villemure1

Department of Mechanical Engineering, Ecole Polytechnique de Montreal, P.O. Box 6079, Station “Centre-Ville,” Montréal, QC, H3C 3A7, Canada; Sainte-Justine University Hospital Center, 3175 Côte-Ste-Catherine Road, Montréal, QC, H3C 1C5, Canadaisabelle.villemure@polymtl.ca

1

Corresponding author.

J Biomech Eng 132(11), 111006 (Oct 20, 2010) (8 pages) doi:10.1115/1.4002550 History: Received December 03, 2009; Revised August 26, 2010; Posted September 15, 2010; Published October 20, 2010; Online October 20, 2010

Spine degeneration is a pathology that will affect 80% of the population. Since the intervertebral disks play an important role in transmitting loads through the spine, the aim of this study was to evaluate the biomechanical impact of disk properties on the load carried by healthy (Thompson grade I) and degenerated (Thompson grades III and IV) disks. A three-dimensional parametric poroelastic finite element model of the L4/L5 motion segment was developed. Grade I, grade II, and grade IV disks were modeled by altering the biomechanical properties of both the annulus and nucleus. Models were validated using published creep experiments, in which a constant compressive axial stress of 0.35 MPa was applied for 4 h. Pore pressure (PP) and effective stress (SE) were analyzed as a function of time following loading application (1 min, 5 min, 45 min, 125 min, and 245 min) and discal region along the midsagittal profile for each disk grade. A design of experiments was further implemented to analyze the influence of six disk parameters (disk height (H), fiber proportion (%F), drained Young's modulus (Ea,En), and initial permeability (ka,kn) of both the annulus and nucleus) on load-sharing for disk grades I and IV. Simulations of grade I, grade III, and grade IV disks agreed well with the available published experimental data. Disk height (H) had a significant influence (p<0.05) on the PP and SE during the entire loading history for both healthy and degenerated disk models. Young’s modulus of the annulus (Ea) significantly affected not only SE in the annular region for both disk grades in the initial creep response but also SE in the nucleus zone for degenerated disks with further creep response. The nucleus and annulus permeabilities had a significant influence on the PP distribution for both disk grades, but this effect occurred at earlier stages of loading for degenerated than for healthy disk models. This is the first study that investigates the biomechanical influence of both geometrical and material disk properties on the load transfer of healthy and degenerated disks. Disk height is a significant parameter for both healthy and degenerated disks during the entire loading. Changes in the annulus stiffness, as well as in the annulus and nucleus permeability, control load-sharing in different ways for healthy and degenerated disks.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Finite element models of L4/L5 motion segment for Thompson grade I, grade III, and grade IV disks

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Figure 2

Comparison of creep responses between the FE model predictions and the experimental tests by Chiu (21) for the three lumbar disks submitted to a compressive stress of 0.35 MPa for 4 h

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Figure 3

Comparison of total stress profiles across the healthy (GR.I) disk midsagittal diameter between the FE model prediction at 1 min following load application and the experimental study by McNally and Adams (5)

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Figure 4

Sagittal pore pressure distribution for Thompson grade I, grade III, and grade IV lumbar disks at 1 min, 45 min, and 245 min following load application. ANT=anterior region of the disk; POST=posterior region of the disk

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Figure 5

Profiles of pore pressure (PP), effective (SE), and total stresses (ST) across the midsagittal diameter for Thompson grade I, grade III, and grade IV lumbar disks at 1 min, 5 min, 45 min, 125 min, and 245 min following load application. Positive values for effective and total stresses correspond to a compressive stresses.

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Figure 6

Relative contributions of effective stress (black) and pore pressure (white) with respect to total stress acting on midsagittal profile for Thompson grade I, grade III, and grade IV lumbar disks at 1 min, 45 min, and 245 min following load application. ANT=anterior region; AAB=anterior annulus border; ANB=anterior nucleus border; NC=nucleus region; PNB=posterior nucleus border; PAB=posterior annulus border; POST=posterior region.

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