Investigation on the biomechanical behaviour of the lower cervical spine induced by facet tropism with respect to the sagittal plane.

Clinic observations have shown that facet tropism with respect to the sagittal plane is associated with cervical degeneration, but their mechanisms haven't been clearly revealed. This study developed different levels of facet tropism musculoskeletal multi-body dynamics cervical models to investigate the biomechanical effect on the intervertebral compressive force (ICF), intervertebral shear force (ISF), facet joint force (FJF) and intervertebral disc displacement (IDD) during head flexion, extension, lateral bending and axial rotation movements. Results showed that the ICFs at the asymmetrical levels of the severe facet tropism models increased by 9.33% and 15.34% respectively during extension and right lateral bending, but did not change significantly during axial rotation. The ISFs at the asymmetrical levels of the severe facet tropism models increased by 56.64% and 164.40% respectively during right lateral bending and right axial rotation. The corresponding IDDs in medial-lateral direction at asymmetrical level also increased greatly during extension, right lateral bending and right axial rotation. The FJFs at asymmetrical level of the severe facet tropism models decreased by 3.41%, 10.55% and 9.19% during extension, right lateral bending and left axial rotation, but increased by 22.62% during right axial rotation. Facet tropism increased the ICFs, ISFs and IDDs of the asymmetrical level, but reduced the protection against cervical excessive motion during certain head movements. The results suggested that facet tropism may contribute to the initiation or enhancement of the process of intervertebral disc degeneration, especially intervertebral disc herniation.