Disc degeneration normally is accompanied by structural changes in the intervertebral discs and the end plates. Reduced level of water in the disc, osteophytes formation and geometrical changes in the end plates also accompany the disc degeneration process. Structural changes in the disc due to degeneration can be classified into radial, peripheral and circumferential tears. Radial tears normally extend from inner annulus region to the outer annulus in the radial direction. These may be due to the failure of annular fibers. Peripheral tears in the annulus normally occur near the junction between the annulus and end plates. These may be due to shear loads at the end plate-annulus junction. Circumferential tears can be observed in both anterior and posterior annulus. These circumferential tears may be due to separation of annular layers due to excess shear between layers and/or due to tensile strain between layers. Although the intrinsic mechanical behavior of the annulus fibrosus as a single layer has been studied, the response of the annulus in vivo does depend on its composite multilayer structure. Biomechanical analysis of the multilayered annulus will help to understand the initiation of delamination failure in the annulus. A finite element model of a lumbar disc with annulus consisting of several layers distributed along the radial direction of the disc with interface was developed to understand the delamination process of the disc under physiological loading modes. It is hypothesized that delamination starts at the inner annular layers near the end plates.

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