Degeneration Affects the Structural and Tissue Mechanics of the Intervertebral Disc

The intervertebral disc is comprised of mechanically and structurally unique substructures including the annulus fibrosus and the nucleus pulposus, which function together to support and absorb the large complex spinal loads. Disc degeneration causes drastic changes in the composition and may lead to altered mechanics; however, the changes in the internal mechanical function of the nucleus and annulus are not well understood. Moreover, herniation of nucleus pulposus material through the posterior annulus causes low back pain and may alter the mechanical function of the disc. Discectomy, a procedure to relieve the pain by removing the herniated material; however, the effect of discectomy on the mechanical function of nondegenerate and degenerate discs are not known. Therefore, the overall goal of this study was to evaluate the mechanical behavior of the intervertebral disc in its intact form, as excised tissue and via constitutive modeling. Degeneration increases the tensile radial and compressive axial strain in the nucleus pulposus and annulus fibrosus; with the internal mechanics being most sensitive to degeneration in the neutral position. These increases in strain may be the cause of, result of microfractures that may lead to future herniations. Discectomy affected nondegenerate and degenerate discs differently. Generally, the axial strains of nondegenerate discs were similar to intact discs, while degenerate discs experienced larger compressive axial strains and inward bulging of the inner annulus. Alterations in the radial strains of the lateral and posterior annulus were observed in both nondegenerate and degenerate discs; suggesting that nondegenerate discs may experience an advanced progression of degeneration following discectomy. Mechanical testing of the annulus in uniaxial and biaxial extension demonstrated the importance of the loading condition utilized to evaluate musculoskeletal tissues. Constitutive modeling, based on biaxial experimental results, was able to accurately describe the tissue behavior in uniaxial tension and simple shear; however, uniaxial experimental data was unable to describe the tissue function in other loading modalities. This study provides valuable information that can be utilized to understand and design of future treatments for degeneration and herniation.