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Five fresh human cadavera were tested to determine range-of-motion measurements at the interspace of the first and second lumbar vertebrae after progressive disruption of the joint followed by internal and external stabilization. The disruption progressed from posterior to anterior, leaving the anterior longitudinal ligament and anterior part of the annulus fibrosus intact. Flexion-extension range of motion was most sensitive to progressive disruptions and was significant following disruption of the facets. The Taylor-Knight brace was effective for limiting lateral motion, fair for limiting flexion-extension, and not effective for rotation. The three-point hyperextension brace was fairly effective for flexion-extension only. The body cast was effective in limiting all motions. Wire loops partially cut through the spinous processes in all cases with extreme flexion. Harrington distraction rods were effective in limiting motion if under proper tension, but they dislodged in three of the five specimens. CLINICAL RELEVANCE: Data from this study show that flexion of the second lumbar vertebra on the first of 20 degrees or a lateral bend of 10 degrees seen on a routine roentgenogram without vertebral fracture indicates that all posterior ligaments and at least part of the annulus fibrosus must be disrupted. Because internal fixation failed on occasion, we strongly urge the use of external fixation and careful mobilization of the patient to prevent flexion and rotation if internal stabilization is used for disruptions of the upper lumbar spine. The body cast was the most effective in limiting motion of the external fixation devices tested.