Axes of motion of thoracolumbar burst fractures.

The neurological injury associated with thoracolumbar burst fractures may be due to the acute trauma event or due to chronic instability. For functional diagnosis and appropriate treatment, knowledge of the altered motion patterns of burst fractures may be helpful. Thirteen human cadaveric spine specimens were impacted at high speed in axial compression, resulting in 10 clinically relevant burst fractures. The specimens were subjected to a three-dimensional flexibility test (flexion, extension, bilateral lateral bending, and bilateral axial torque) before and after trauma. The vertebral motion across the burst fracture was described in terms of the helical axis of motion (HAM), a set of parameters that concisely and completely describes the three-dimensional motion. The vertebral rotations about the HAM increased significantly with burst fracture in all loading directions: flexion 8.1-17.7 degrees, extension 7.2-12.5 degrees, lateral bending 8.5-20.6 degrees (to one side), and axial torque 3.6-12.6 degrees (to one side). The HAM shifted significantly in a posterior direction with burst fracture in flexion (11-mm shift), extension (15-mm shift), and axial torque (11-mm shift). No other significant shifts in the HAM position were observed. The translation along the HAM and the orientation of the HAM did not change significantly with injury in any of the loading directions. The results provide clinically relevant information regarding the optimal treatment of thoracolumbar burst fractures. Specifically, fixation methods for burst fractures must be particularly stiff in lateral bending and axial rotation, the directions of greatest instability.