Flexion-distraction injuries in the thoracolumbar spine: an in vitro study of the relation between flexion angle and the motion axis of fracture.

A new concept, the motion axis of fracture (MAF), which is defined as the transitional point from anterior compressive to posterior splitting failure on a lateral radiograph, has provided a true understanding of the mechanisms of flexion-distraction injuries in clinical cases. This study was designed to produce in vitro injuries that have MAFs and to clarify the relation between the flexion angle and the MAF location. Adolescent porcine thoracolumbar spines were exposed to a vertical compressive load to failure at three different flexion angles and then examined radiographically. The MAF location was recorded as the distance from the anterior border to the MAF expressed as a percentage of the anteroposterior diameter of the vertebral body. All specimens showed similar injuries, with MAFs consisting of anterior compression fractures in the vertebral bodies and posterior disruptions. A significant negative correlation emerged between the flexion angle and the MAF location (r = -0.890; p < 0.0001). These results suggest that even a vertical compressive load contributes to the production of a flexion-distraction injury with an MAF in the thoracolumbar spine. They also indicate that the flexion angle of the spine at which the vertical compressive load is applied is an important factor in determining the MAF location; that is, the larger the flexion angle, the more anterior the MAF.