Reduction in segmental flexibility because of disc degeneration is accompanied by higher changes in facet loads than changes in disc pressure: a poroelastic C5-C6 finite element investigation

BACKGROUND CONTEXT: Nerve fiber growth inside the degenerative intervertebral discs and facets is thought to be a source of pain, although there may be several other pathological and clinical reasons for the neck pain. It, however, remains difficult to decipher how much disc and facet joints contribute to overall degenerative segmental responses. Although the biomechanical effects of disc degeneration (DD) on segmental flexibility and posterior facets have been reported in the lumbar spine, a clear understanding of the pathways of degenerative progression is still lacking in the cervical spine. PURPOSE: To test the hypothesis that after an occurrence of degenerative disease in a cervical disc, changes in the facet loads will be higher than changes in the disc pressure. STUDY DESIGN: To understand the biomechanical relationships between segmental flexibility, disc pressure, and facet loads when the C5-C6 disc degenerates. METHODS: A poroelastic, three-dimensional finite element (FE) model of a normal C5-C6 segment was developed and validated. Two degenerated disc models (moderate and severe) were built from the normal disc model. Biomechanical responses of the three FE models (normal, moderate, and severe) were further studied under diurnal compression (at the end of the daytime activity period) and moment loads (at the end of 5 seconds) in terms of disc height loss, angular motions, disc pressure, and facet loads (average of right and left facets). RESULTS: Disc deformation under compression and segmental rotational motions under moment loads for the normal disc model agreed well with the corresponding in vivo studies. A decrease in segmental flexibility because of DD is accompanied by a decrease in disc pressure and an increase in facet loads. Biomechanical effects of degenerative disc changes are least in flexion. Segmental flexibility changes are higher in extension, whereas changes in disc pressure and facet loads are higher in lateral bending and axial rotation, respectively. CONCLUSIONS: The results of the present study confirmed the hypothesis of higher changes in facet loads than in disc pressure, suggesting posterior facets are more affected than discs because of a decrease in degenerative segmental flexibility. Therefore, a degenerated disc may increase the risk of overloading the posterior facet joints. It should be clearly noted that only after degeneration simulation in the disc, we recorded the biomechanical responses of the facets and disc. Therefore, our hypothesis does not suggest that facet joint osteoarthritis may occur before degeneration in the disc. Future cervical spine based experiments are warranted to verify the conclusions presented in this study. (C) 2010 Elsevier Inc. All rights reserved.