Internal spinal fixator stiffness has only a minor influence on stresses in the adjacent discs

Study Design. Stresses in vertebral endplates and discs were calculated using the th ree-dimensional nonlinear finite-element model of a lumbar spine with an internal spinal fixation device. Objective. To determine the influence of fixator stiffness on stresses in the adjacent discs. Summary of Background Data, There are few computer models of the lumbar spine with a fixator. Most of these models neglect the muscle forces. Fixator stiffness is assumed to influence greatly the stresses in the adjacent discs. Methods. Two three-dimensional nonlinear finite-element models were used to determine stresses in the lumbar spine for standing and 60 degrees flexion of the upper body. One model had an internal spinal fixator, the other did not. In a parameter study, the diameters of the longitudinal rod of the fixator were assumed to be 3, 5, 7, and 10 mm. In the computer model, the forces of the trunk muscles were simulated. Results. The diameter of the longitudinal rod strongly affected the fixator loads but hardly influenced the stresses in the vertebral endplates. The stresses in the bridged discs were strongly reduced. However, the internal fixator had only a minor influence on the stresses in the anulus fibrosus and the pressure in the nucleus pulposus of the adjacent discs. Conclusions. The stiffness of an internal spinal fixation device has only a minor influence on stresses in the adjacent discs.