A Fiber-Reinforced Multi-Shell Composite Finite Element Model of the Human Annulus Fibrosus

In a healthy intervertebral disc, the axial compressive loads applied to the spine are largely carried by the hydrostatic pressure of the nucleus and the hoop stresses induced in the collagen fibers of the annulus fibrosus. In a degenerated disc, the loss of nuclear hydration results in a transfer of the axial compression from the nucleus to the annulus, for which it is not as well suited (Kurowski and Kubo, 1986). To examine the role of compression on the response of the annulus, an experimental protocol has been developed to directly test the annulus in axial tension and compression (Lotz and Ashford, 1994). In these experiments, the individual lamellae were observed to buckle into a mode shape, which suggested that the lamellae possessed an intrinsic bending stiffness. The objective of this study was to develop a finite element model (FEM) of an 8 mm circumferential segment of the annulus modelled by a series of fiber reinforced shells to represent the individual lamellae. The predicted radial strains of this shell based model were compared with current models and experimental measurements.