Nonlinear finite-element analysis of the lower cervical spine (C4-C6) under axial loading.

This study was conducted to develop a detailed, nonlinear three-dimensional geometrically and mechanically accurate finite-element model of the human lower cervical spine using a high-definition digitizer. This direct digitizing process also offers an additional method in the development of the finite-element model for the human cervical spine. The biomechanical response of the finite-element model was validated and corresponded closely with the published experimental data and existing finite-element models under axial compressive loading. Furthermore, the results indicated that the cervical spine segment response is nonlinear with increasing stiffness at higher loads. As a logical step, a parametric study was conducted by evaluating the biomechanical response related to the changes in the modeling techniques of the finite-element model and the mechanical properties of the disk annulus. Variations of the predicted horizontal disk bulge were investigated under axial compressive displacements for the normal model, the model without facet articulations, and the model without nucleus. Removal of nucleus fluids causes an inward bulge of the inner annulus layers, with the displacement magnitude dependent on external loads. The result indicates that the nucleus fluid plays an important role in cervical spine mechanics. Simulated facetectomy indicates a decrease in the stiffness of the cervical spine. The study shows that, in reality, the stiffness of the lower cervical spine depends closely on factors such as the spinal geometry and physical properties, thereby resulting in various force and displacement responses.