Combined effect of artificial cervical disc replacement and facet tropism on the index-level facet joints: a finite element study

Abstract

Abstract Objective The purpose of this study was to assess the impact of facet tropism on the facet contact force and facet capsule strain after artificial cervical disc replacement (ACDR). Methods A finite element model was constructed from computed tomography (CT) scans of a 28-year-old male volunteer. Symmetrical, moderate asymmetrical (7 degrees tropism), and severe asymmetrical (14 degrees tropism) models were created at the C5/C6 level. C5/C6 ACDR was simulated in all models. A 75 N follower load and 1 N⋅m moment was applied to the odontoid process to initiate flexion, extension, lateral bending, and axial rotation, and the range of motions, facet contact forces, and facet capsule strains were recorded. Results In the severe asymmetrical model, the right-side FCF increased considerably under extension, right bending, and left rotation compared with the symmetrical model after C5/C6 ACDR. The ride-side FCFs of the severe asymmetrical model under extension, right bending, and left rotation were about 1.7, 3.1, and 1.8 times of those of the symmetrical model, respectively. The facet capsule strains of both the moderate and severe asymmetrical models increased significantly compared with those of the symmetrical model after C5/C6 ACDR. The left-side capsule strains of the severe asymmetrical model were 2.1, 2.4, 1.6, and 8.5 times of those of the symmetrical model under left bending, right bending, left rotation, and right rotation, respectively. The right-side capsule strains of the severe asymmetrical model were 6.3, 1.6, 3.7, and 2.2 times of those of the symmetrical model under left bending, right bending, left rotation, and right rotation, respectively. The stress distribution on the facet surface in the asymmetrical models was different from that in the symmetrical model. Conclusions The existence of facet tropism could considerably increase facet contact force and facet capsule strain after ACDR, especially under extension, lateral bending, and rotation. Facet tropism also could result in abnormal stress distribution on the facet joint surface and facet joint capsule. Such abnormality might be a risk factor for post-operative facet joint degeneration progression after ACDR, making facet tropism noteworthy when ACDR was considered as the surgical option.