Internal strains in healthy and degenerated lumbar intervertebral discs

AB Study Design. A biomechanical study investigating the intradiscal mechanics of human lumbar intervertebral discs (IVDs). Objectives. To assess the relationship between nucleuspulposus migration and intradiscal strains as a function of degeneration. Summary of Background Data. Intradiscal deformation studies have documented the nucleus pulposus migration capabilities during bending but without assessing subsequent intradiscal strains of the anulus fibrosus. Degenerated IVDs show higher anular laxity, hypermobility, and, perhaps, segmental instability. It is unknown if nucleus pulposus migration might be the cause of increased intradiscal anular strains and if such a phenomenon is modulated by IVD degeneration. Methods. Eighteen healthy and degenerated IVDs were subjected to compression, extension, flexion, and lateral bending. Craniocaudal radiographs at unloaded and loaded steps documented positions of wires placed within and beads glued to the external surface in the mid-transverse plane. Circumferential and radial strains from the anterior, lateral, and posterolateral regions during load were compared between healthy and degenerated IVDs. Results. The nucleus pulposus migrated to the opposite side of bending regardless of bending direction and significantly more in degenerated IVDs. The highest nucleus pulposus migration was observed during lateral bending. Circumferential tensile strains were significantly higher in the posterolateral regions of degenerative IVDs during all loads. Degeneration significantly increased radial tensile and compressive strains during all bending loads. Conclusions. Increased nucleus pulposus migration in degenerated IVDs may result in increased shifting of the IVD pivot point during bending movements as well as intradiscal anular strains, particularly in the posterolateral anulus. This phenomenon may explain the segmental instability observed in degenerated segments as well as the associated anular tears present in the posterolateral region before IVD failure.