Effect of Annulus Fibrosus Collagen Orientation on Intervertebral Disc Torsional Mechanical Behavior

INTRODUCTION: Intervertebral disc experiences large complex deformations including compression, rotation, and bending under daily activity. For example, twisting while carrying a weight, loads the disc under axial compression and axial rotation. Previous work showed that the rate of degeneration and disc injury is higher for in workers that experience repetitive twisting and lifting [1]. Disc degeneration and injury alters fiber orientation, mechanical behavior (i.e. compressive and torsional stiffness), and load distribution between the disc and surrounding tissues [2]. Furthermore, axial rotation of individual disc joints has been shown to increase with degeneration [3]. Previous work in our laboratory demonstrated that torsional mechanics of healthy bovine discs are greatly dependent on axial compressive stress and the maximum rotation angle applied [4]. However, the effect of degeneration, injury and fiber orientation on compression-torsional mechanics is not well understood. A validated finite element model (FEM) can provide additional information about internal stress distributions that are difficult to measure experimentally. Therefore, the objective of this study is to develop and validate a FEM model of the disc joint under combined compression and torsion loading. Once the response was validated with experimental data, the model was used to determine the effect of AF fiber orientation on torsional mechanics. We hypothesize that the alternating orientation of collagen fibers through the AF plays an important role in stress distribution during axial rotation.