Biomechanical Effect of Annular Tears in Healthy and Degenerated Lumbar Discs

Structural alterations in the disc are accompanied by changes in disc tissue structure and composition during degeneration processes. As the lumbar disc degenerates, the disc height decreases. At the same time due to the development of fissures and cracks in the annulus, its material behavior changes. The degeneration process also includes cleft formation in the nucleus and reduction in water content in the nucleus and annulus. Porosity and drained modulus of the disc tissues are also affected by disc degeneration. The clear demarcation between nucleus and surrounding annulus slowly disappear as the disc degeneration progresses thus increasing the annulus volume while reducing the nucleus volume. Both in-vitro and finite element studies exist in literature which deals with the biomechanical effect of disc degeneration. The changes in biomechanical behavior due to degeneration are a combined effect of change in geometrical parameters of the disc, change in material characteristics of the disc components and presence of annular tears. It is not known how much of the change in disc biomechanics is due to change in disc tissue composition and due to additional tears present within the degenerated disc. The current study is thus related to the effect of such progressive changes on lumbar disc biomechanics.