Kinematic Evaluation of Lumbar Fusion Techniques

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Abstract
Study Design Eight human cadaveric lumbosacral spines were biomechanically and kinematically tested in torsion and compression-flexion. They were retested after simulated posterolateral fusion, anterior lumbar interbody fusion, and circumferential fusion. Objectives To analyze stiffness and motion in the anterior and posterior columns of the index and contiguous spinal motion units of anterior, posterolateral, and circumferential fusions. Summary of Background Data Previous biomechanical studies have not incorporated analysis of motion with six degrees of freedom, consideration of contiguous levels, and comparisons of anterior and posterior column motion. Methods Eight human cadaveric lumbosacral spines were biomechanically tested in compression-flexion and torsion using an advanced biplanar radiography technique. Each specimen underwent either a simulated posterolateral fusion or anterior fusion followed by a circumferential fusion. Motion and stiffness at the level of the fusion and at contiguous levels were analyzed independently in the anterior and posterior columns of the spine. Results At the level of fusion, the simulated posterolateral and anterior fusions prevented more motion in torsion compared with compression-flexion. With all specimens, it was shown that circumferential fusions were stiffer than the intact specimen. Our comparison of motion in the anterior and posterior columns found no significant differences within the columns of a single vertebral motion segment. Compared with posterolateral fusions, anterior fusions were found to have the greatest effect in increasing motion at contiguous levels. The effect of circumferential fusions on adjacent level kinematics was not significantly greater than that of anterior fusions. Conclusion There are major biomechanical differences between different fusion techniques. This information should be considered in patients undergoing lumbar spinal fusion.