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Research Papers

The Micromechanical Role of the Annulus Fibrosus Components Under Physiological Loading of the Lumbar Spine

[+] Author and Article Information
Ugur M. Ayturk

Department of Mechanical Engineering, Orthopaedic Bioengineering Research Laboratory, and School of Biomedical Engineering, Colorado State University, Fort Collins, CO 80523-1374

Jose J. Garcia

Escuela de Ingeniería Civil y Geomática, Universidad del Valle, Cali, Colombia 25360

Christian M. Puttlitz1

Department of Mechanical Engineering, Orthopaedic Bioengineering Research Laboratory, and School of Biomedical Engineering, Colorado State University, Fort Collins, CO 80523-1374puttlitz@engr.colostate.edu

1

Corresponding author.

J Biomech Eng 132(6), 061007 (Apr 22, 2010) (8 pages) doi:10.1115/1.4001032 History: Received October 22, 2009; Revised December 19, 2009; Posted January 18, 2010; Published April 22, 2010; Online April 22, 2010

To date, studies that have investigated the kinematics of spinal motion segments have largely focused on the contributions that the spinal ligaments play in the resultant motion patterns. However, the specific roles played by intervertebral disk components, in particular the annulus fibrosus, with respect to global motion is not well understood in spite of the relatively large literature base with respect to the local ex vivo mechanical properties of the tissue. The primary objective of this study was to implement the nonlinear and orthotropic mechanical behavior of the annulus fibrosus in a finite element model of an L4/L5 functional spinal unit in the form of a strain energy potential where the individual mechanical contributions of the ground substance and fibers were explicitly defined. The model was validated biomechanically under pure moment loading to ensure that the individual role of each soft tissue structure during load bearing was consistent throughout the physiologically relevant loading range. The fibrous network of the annulus was found to play critical roles in limiting the magnitude of the neutral zone and determining the stiffness of the elastic zone. Under flexion, lateral bending, and axial rotation, the collagen fibers were observed to bear the majority of the load applied to the annulus fibrosus, especially in radially peripheral regions where disk bulging occurred. For the first time, our data explicitly demonstrate that the exact fiber recruitment sequence is critically important for establishing the range of motion and neutral zone magnitudes of lumbar spinal motion segments.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

A sagittal cut-view of the functional spinal unit. Cortical (green) and trabecular (light blue) bone, posterior elements (brown), bony (gray) and cartilaginous (red) endplates, annulus (light green), and nucleus (light brown) are demonstrated.

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Figure 2

Tensile mechanical behavior of the annulus material in the radial direction. Experimental data were adapted from graphical data published in Ref. 17.

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Figure 3

(Top) Intradiscal nuclear pressure predictions of the intact model under flexion (+moment) and extension (−moment) loading. The mean and standard deviation values reported for the model represent the nodal variation in the model’s predictions within the nucleus pulposus. The experimental data represent the median, maximum, and minimum of a pool of tested specimens, as indicated by the legend (26). (Bottom) Change in the horizontal and vertical compressive stress components in the anteroposterior direction while the FSU is under compression with the application of 2000N. Dashed lines represent the physical boundaries between the annulus and the nucleus.

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Figure 4

Maximum principal strain predictions at the anterolateral cortical surface of L4 under the application of 3 N m of pure moment. Model predictions indicate the mean and standard deviation of predictions in the anterolateral cortical region, while the experimental data indicate the mean and standard deviation in the measurements from a sample pool (n=6)(27).

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Figure 5

Moment-rotation response of the FSU with the annulus alone, annulus and nucleus, and in intact condition. Pure moments were applied under flexion/extension (+/−), lateral bending, and axial rotation. Experimental data were adapted from graphical data in Ref. 24 and indicate the median, minimum, and maximum of the measurements.

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Figure 6

Total SED predictions for the annulus ground substance and fibers, and applied moment with respect to global rotation for the model with annulus alone (left) and in the intact condition (right).

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Figure 7

Load sharing between the fibers and ground substance of the annulus (as measured using the SED) under 7.5 N m of loading in all directions with the annulus alone as the load-bearing soft tissue (top) and in the intact condition (bottom).

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Figure 8

Local annular SED distribution in the anteroposterior direction under extension (left) and flexion (middle) and in the medial-lateral direction under lateral bending (right). The white arrows indicate the positions of the nodes in the annulus corresponding to the horizontal axes of the charts below. The plots demonstrate the local SED predictions for the ground substance and the fibers with the annulus alone (middle) and the FSU in the intact condition (bottom). The dashed (ground substance) and solid (fiber) curves represent the component SED predictions under 7.5 N m of pure moment loading.

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