Dynamic analysis of the Harrington system using a spinal simulator. The authors constructed a spinal simulator for the thoracolumbar spine with mechanical properties similar to those of cadaver specimens, and studied the effectiveness of the Harrington system in unstable fractures of the thoracolumbar spine. Distraction and compression rods were applied under various conditions. External bending moment was applied to the model to measure the internal bending moment on the vertebral body and ligaments. Optimal spinal stability was obtained when the distraction system was combined with the compression system with hooks at laminas three levels above and below the fracture. 