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. Fo r functional diagnosis and appropriate treatment, knowledge of the alt ered motion patterns of burst fractures may be helpful. Thirteen human cadaveric spine specimens were impacted at high speed in axial compre ssion, resulting in 10 clinically relevant burst fractures. The specim ens were subjected to a three-dimensional flexibility test (flexion, e xtension, bilateral lateral bending, and bilateral axial torque) befor e and after trauma. The vertebral motion across the burst fracture was described in terms of the helical axis of motion (HAM), a set of para meters that concisely and completely describes the three-dimensional m otion. The vertebral rotations about the HAM increased significantly w ith 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 shift ed significantly in a posterior direction with burst fracture in flexi on (11-mm shift), extension (15-mm shift), and axial torque (11-mm shi ft). No other significant shifts in the HAM position were observed. Th e translation along the HAM and the orientation of the HAM did not cha nge significantly with injury in any of the loading directions. The re sults provide clinically relevant information regarding the optimal tr eatment of thoracolumbar burst fractures. Specifically, fixation metho ds for burst fractures must be particularly stiff in lateral bending a nd axial rotation, the directions of greatest instability.