Posterior Facet Load Changes in Adjacent Segments due to Moderate and Severe Degeneration in C5-C6 Disc

Abstract

Biomechanics of normal vertebral segments adjacent to a degenerated segment in the cervical spine. To test the hypothesis that posterior facet joints of adjacent segments are loaded more when degeneration occurs in the intermediate disc segment. Degeneration progression in adjacent segments is a clinical concern. Literature studies that have documented the effects of a degenerated segment on the adjacent-segment discs have not addressed these effects on adjacent-segment posterior facets. Moreover, these biomechanical studies are performed mainly on the lumbar spine or the tissue quality of the cadavers is poor because of degenerated segments. Retrospectively, it is difficult to decipher to what extent degeneration in a single disc induces biomechanical changes in facet joints along the posterior spinal column. To date, no cervical spine biomechanical study investigated the facet joints response of adjacent segments when the disc in between those segments degenerates. An earlier validated poroelastic, 3-dimensional finite element model of a normal C3-T1 segment was used. Two stages of degeneration (moderate and severe) were simulated in the C5-C6 disc. Disc geometry and tissue material properties were modified to simulate C5-C6 disc degeneration. For the 3 C3-T1 models, loads on the posterior facets at 3 levels (C4-C5, C5-C6, and C6-C7) were computed under moment loads. With progressive degeneration in the C5-C6 disc, posterior facet loading in adjacent segments and in the intermediate degenerated disc segment increased. Changes in facet loading in the inferior C6-C7 segment were greater than the corresponding changes in the superior C4-C5 segment. These changes were highest in lateral bending and lowest in axial rotation. Higher changes in facet loads along the posterior spinal column may contribute to altered biomechanics in neighboring segments. Future biomechanical experiments are required to develop a more clear understanding of the posterior facet joints response in neighboring segments because of degeneration in a cervical disc.