Biomechanical Contributions of Spinal Structures with Different Degrees of Disc Degeneration

Study Design. Biomechanical cadaveric study. Objective. The aim of this study was to evaluate the effect of degeneration on biomechanical properties of the passive structures of the lumbar spine. Summary of Background Data. Although the load apportionment among the passive structures in healthy spines follows well-defined contribution patterns, it remains unknown how this load distribution and sagittal preload changes by degenerative processes of the intervertebral disc (IVD). Methods. Fifty lumbar spinal segments were tested in a displacement-controlled stepwise reduction study in flexion, extension, axial rotation, lateral bending, anterior, posterior and lateral shear. The intertransverse ligaments (ITLs), supraspinous and interspinous ligaments (ISL&SSL), facet joint capsules (FJC), facet joints (FJ), ligamentum flavum (LF), posterior longitudinal ligament (PLL), anterior longitudinal ligament (ALL), and spondylophytes were subsequently reduced. The results were set in relation to IVD-degeneration, quantified with Pfirrmann classification. Results. In flexion, a load redistribution from LF (-28% n.s.) and PLL (-13% n.s.) towards the IVD (thorn9%, n.s.) is observed comparing grade 2 to 5 IVD degeneration, whereas in all other loading directions, a reduction of IVD-contribution from -12% to -53% is recorded. In axial rotation, anterior and lateral shear, more load is shared by the FJ (+4% n.s.,+23%*, +13% n.s.). The preload of the ALL, LF, PLL, and IVD is reduced ranging from -0.06 Nm to -0.37 Nm. Conclusion. IVD degeneration is related to notable load-redistributions between the passive spinal structures. With further degeneration, reduced contribution of the LF and PLL and higher loads on the IVD are observed in flexion. In the other tested loading directions, the relative load on the IVD is reduced, whereas higher FJ-exposure in axial rotation, anterior and lateral shear is observed. Furthermore, the preload of the spinal structures is reduced. These observations can further the understanding of the degenerative cascade in the spine.