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Research Papers

The Mechanical Role of the Radial Fiber Network Within the Annulus Fibrosus of the Lumbar Intervertebral Disc: A Finite Elements Study

[+] Author and Article Information
Mirit Sharabi, Aviad Levi-Sasson, Roza Wolfson

The Fleischman Faculty of Engineering,
School of Mechanical Engineering,
Tel Aviv University,
Tel Aviv 69978, Israel

Kelly R. Wade, Hans-Joachim Wilke

Institute of Orthopaedic
Research and Biomechanics,
University of Ulm,
Ulm 89081, Germany

Fabio Galbusera

Institute of Orthopaedic
Research and Biomechanics,
University of Ulm,
Ulm 89081, Germany;
IRCCS Galeazzi Orthopaedic Institute,
Milan 20161, Italy

Dafna Benayahu

Department of Cell and Developmental Biology,
Sackler School of Medicine,
Tel Aviv University,
Tel Aviv 69978, Israel

Rami Haj-Ali

Professor
The Fleischman Faculty of Engineering,
School of Mechanical Engineering,
Tel Aviv University,
Tel Aviv 69978, Israel
e-mail: rami98@tau.ac.il

1Corresponding author.

Manuscript received April 29, 2018; final manuscript received October 4, 2018; published online December 5, 2018. Assoc. Editor: Kyle Allen.

J Biomech Eng 141(2), 021006 (Dec 05, 2018) (11 pages) Paper No: BIO-18-1208; doi: 10.1115/1.4041769 History: Received April 29, 2018; Revised October 04, 2018

The annulus fibrosus (AF) of the intervertebral disc (IVD) consists of a set of concentric layers composed of a primary circumferential collagen fibers arranged in an alternating oblique orientation. Moreover, there exists an additional secondary set of radial translamellar collagen fibers which connects the concentric layers, creating an interconnected fiber network. The aim of this study was to investigate the mechanical role of the radial fiber network. Toward that goal, a three-dimensional (3D) finite element model of the L3–L4 spinal segment was generated and calibrated to axial compression and pure moment loading. The AF model explicitly recognizes the two heterogeneous networks of fibers. The presence of radial fibers demonstrated a pronounced effect on the local disc responses under lateral bending, flexion, and extension modes. In these modes, the radial fibers were in a tensile state in the disc region that subjected to compression. In addition, the circumferential fibers, on the opposite side of the IVD, were also under tension. The local stress in the matrix was decreased in up to 9% in the radial fibers presence. This implies an active fiber network acting collectively to reduce the stresses and strains in the AF lamellae. Moreover, a reduction of 26.6% in the matrix sideways expansion was seen in the presence of the radial fibers near the neutral bending axis of the disc. The proposed biomechanical model provided a new insight into the mechanical role of the radial collagen fibers in the AF structure. This model can assist in the design of future IVD substitutes.

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Figures

Grahic Jump Location
Fig. 1

The geometry and fibers architecture in the L3-L4 FSU-FE model. (a) The FSU in lateral view; (b) discrete fiber networks embedded in the AF matrix in the FSU-FE model. The circumferential fiber network with decreasing FVFs from the outer to inner AF (dark to bright colors) and radial fiber network with 20 deg circumferential distribution. The IVD geometry (c) and the IVD with partial removal of the AF matrix revealing theembedded fibers with (d) and without radial fibers (20 deg circumferential distribution) (e).

Grahic Jump Location
Fig. 2

Moment-rotation results of the FSU-FE models with (case 1, Table 1) and without radial fibers in different loading modes compared with in vitro measurements of Yamamoto et al. [61], Panjabi et al. [60] and Guan et al. [62]: for flexion-extension (a), lateral bending (b), torsion (c), and load–displacement results under compression compared with Markolf and Morris, 1974 [63] (d)

Grahic Jump Location
Fig. 3

The max principal stresses and strains map of the radial fibers, circumferential fibers and AF matrix in the model with radial fibers under 10 N·m (for the moments loading cases: flexion, extension, right and left bending and right and left torsion) and 1.6 mm displacement (for the compression). Matrix contours were taken in the midheight of the AF matrix.

Grahic Jump Location
Fig. 4

While the radial fibers were tensioned and the AF matrix was compressed in the loading direction (active region), the circumferential fibers and AF matrix were tensioned in the opposite direction for flexion, extension and lateral bending. The figure presents the average max principal stresses and strains in the different regions of the IVD: anterior, posterior, right and left lateral for the circumferential fibers, radial fibers, and AF matrix under a moment of 10 N·m in the loading direction (active region) versus the opposite region. The stresses and strains were calculated for the anterior and posterior regions in flexion and extension and for left and right lateral regions for the lateral bending.

Grahic Jump Location
Fig. 5

The change in stresses due to the presence of radial fibers in different cross section and circumferential distributions at different loading modes. (a) Relative decrease in volume-average max principal stress on the circumferential fibers. (b) Relative decrease in average max. principal stress on the AF matrix.

Grahic Jump Location
Fig. 6

The effect of the radial fibers presence, under different loading modes at 5 deg (case 3, Table 1), on (a) the decrease in averaged annulus height together with matrix sideways expansion and (b) the decrease in annulus matrix expansion (radial distance)

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