The biomechanical impact of facet tropism on the intervertebral disc and facet joints in the cervical spine

Background Context Facet tropism is defined as the angular difference between the left and the right facet orientation. Facet tropism was suggested to be associated with the disc degeneration and facet degeneration in the lumbar spine. However, little is known about the relationship between facet tropism and pathologic changes in the cervical spine and the mechanism behind. Purpose This study was conducted to investigate the biomechanical impact of facet tropism on the intervertebral disc and facet joints. Study Design A finite element analysis study. Methods The computed tomography (CT) scans of a 28-year-old male volunteer was used to construct the finite element model. First, a symmetrical cervical model from C2 to C7 was constructed. The facet orientations at each level were simulated using the data from our previously published study. Second, the facet orientations at the C5–C6 level were altered to simulate facet tropism with respect to the sagittal plane. The angular difference of the moderate facet tropism model was set to be 7 degrees, whereas the severe facet tropism model was set to be 14 degrees. The inferior of the C7 vertebra was fixed. A 75 N follower loading was applied to simulate the weight of the head. A 1.0 N⋅m moments was applied on the odontoid process of the C2 to simulate flexion, extension, lateral bending, and axial rotation. Results The intradiscal pressure (IDP) at the C5–C6 level of the severe facet tropism model increased by 49.02%, 57.14%, 39.06%, and 30.67%, under flexion, extension, lateral bending, and axial rotation moments, in comparison with the symmetrical model. The contact force of the severe facet tropism model increased by 35.64%, 31.74%, 79.26%, and 59.47% from the symmetrical model under flexion, extension, lateral bending, and axial rotation, respectively. Conclusions Facet tropism with respect to the sagittal plane at the C5–C6 level increased the IDP and facet contact force under flexion, extension, lateral bending, and axial rotation. The results suggested that facet tropism might be the anatomic risk factor of the development of cervical disc degeneration or facet degeneration. Future clinical studies are in need to verify the biomechanical impact of facet tropism on the development of degenerative changes in the cervical spine.