Biomechanical Effects of Spinal Fusion to Adjacent Vertebral Segments

Extensive mechanical loads on the lumbar spine often lead to degenerative damages of spinal structures. In many cases surgical interventions, such as spinal fusion, are unavoidable. An appropriate method for the estimation of the mechanical effects of such interventions to adjacent vertebral segments is the mathematical computer simulation. This paper presents a 3D- Multi Body Simulation- (MBS-) model of the lumbar spine with realistic surfaces of vertebrae and correct positioning relative to each other which is based on computer tomography (CT) measurements and segmentation processes. Intersegmental discs as well as ligament structures are included. For these elements the physical behavior like force-deformation relations and characteristic curves for the torque-angle relations are formulated. The facet joints are modeled as cartilage, in order to simulate the contact between the corresponding articular surfaces. With this model the loads on the different structures, before and after spinal fusion of the functional spine unit L5-L4, are calculated to show the biomechanical effects to adjacent vertebral segments under different load cases. The comparison of the simulations shows a redistribution of loads within the spinal structures. In the simulation case of spinal fusion, the posterior structures are more loaded than in the simulated healthy state. The validation of the model was carried out by comparing the results with FE-simulations, various in vitro experiments and experimental data from biomedical literature.