We studied 100 consecutive patients with potentially unstable fractures and fracture-dislocations by multiplane computed tomography. The mechanism of failure of the middle osteoligamentous complex of the spine (posterior longitudinal ligament, posterior part of the vertebral body, and posterior annulus fibrosus) was determined by three-dimensional analysis. Three modes of middle-column failure were used to classify the injuries: axial compression (seventy-three patients), axial distraction (fifteen patients), and translation within the transverse plane (twelve patients). Fifty of eighty-six patients who were evaluated in the acute phase of injury underwent operative stabilization, and the mechanism of middle-column disruption determined the type of instrumentation that was used. Compression and distraction injuries of the middle complex could be appropriately treated by Harrington distraction and compression instrumentation, respectively. However, in translational injuries (torn posterior longitudinal ligament) routine Harrington instrumentation was contraindicated due to the risk of overdistraction. Translational injuries were associated with the greatest degree of instability and often had complete ligament discontinuity at the level of the affected vertebrae. Patients with a translational injury had the most severe neural deficits (six of eleven patients studied acutely having a complete spinal cord lesion). Translational injuries of the middle column were treated by segmental spinal instrumentation to provide strong fixation with minimum risk of neural sequelae from passing sublaminar wires. Moreover, postoperative use of a cast over insensate skin was not required. Computed tomography was more sensitive than any other modality in the diagnosis of disruption of the posterior elements in unstable burst fractures, and computer-reconstructed sagittal images were accurate in evaluating the nature of facet-joint failure in distraction injuries. Computed tomography with metrizamide proved superior to either conventional tomography or myelography alone in localizing the site of neural canal compromise in acute thoracolumbar injuries. The mode of failure of the middle osteoligamentous complex as visualized by computed tomography determined the pattern of spinal injury, the severity of the neural deficit, the degree of instability, and the type of instrumentation required.