Anterior cervical fusion has been the standard treatment following anterior cervical discectomy and provides sufficient short-term symptomatic relief, but growing evidence suggests that fusion contributes to adjacent-segment degeneration. Motion-sparing disc replacement implants are believed to reduce adjacent-segment degeneration by preserving motion at the treated level. Such implants have been shown to maintain the mobility of the intact spine, but the effects on load transfer between the anterior and posterior elements remain poorly understood.
In order to investigate the effects of disc replacement on load transfer in the lower cervical spine, a finite element model was generated using cadaver-based Computed Tomography (CT) imagery. The thickness distribution of the cartilage on the articular facets was measured experimentally, and material properties were taken from the literature. Mesh resolution was varied in order to establish model convergence, and cadaveric testing was undertaken to validate model predictions.
The validated model was altered to include a disc replacement prosthesis at the C4/C5 level. The effect of disc-replacement on range of motion, antero-posterior load distribution, total contact forces in the facets, as well as the distribution of contact pressure on the facets were examined, and the effect of different facet cartilage thickness models on load sharing and contact pressure distribution predictions were examined.
Model predictions indicate that the properly-sized implant retains the mobility, load sharing, and contact force magnitude and distribution of the intact case. Mobility, load sharing, nuclear pressures, and contact pressures at the adjacent motion segments were not strongly affected by the presence of the implant, indicating that disc replacement may not be a significant cause of post-operative adjacent-level degeneration.
Variation in articular cartilage distribution did not substantially affect mobility, contact forces, or load sharing. However, mean and peak contact pressure, contact area, and center of pressure predictions were strongly affected by the cartilage distribution used in the model. These results indicate that oversimplification of the cartilage thickness distribution will negatively affect the ability of the model to predict facet contact pressures, and thus subsequent cartilage degeneration.