A Parametric Study of the Spinal Motion Segment Unit Fixation Using the Finite Element Method

Metal screws and rods are used in intervertebral disk fixation. The most common in mechanical failure clinically is at the lower screw. In order to reduce or eliminate this mechanical failure, it is useful to study the mechanical behavior of the implant under different loading conditions. Experimental work has shown the effects of varying rod diameters on the bending moments and stresses in the rods, but stresses in the screws could not be addressed by experimental techniques. In the current paper, a 3-D finite element biomechanical model was developed using ANSYS software. The model included two vertebral bodies and the implant, which consists of two screws, a rod, and a nut on each side of the vertebrae. The intervertebral disc was completely removed and a metal cage of diameter of 16 mm was provided on each side. In this work an extension moment of 10 N.m with a compressive force of 1000 N was studied. In the model, a 10-node tetrahedral element was used to represent the vertebral body, screws, the rods, the nuts, and the graft bone, while an 8-node shell element was used to model the cage. Several parameters were investigated to study their effects on the normal stress distributions in the screws, rods, cage, and graft bone. These parameters include the rod diameter (4mm, 5mm, and 6.3 mm), the presence of a cage with and without bone graft, and the location of the cage (anterior versus posterior). A total of 12 cases for each parameter will be reported.

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