Flexion-distraction injury of the lumbar spine: influence of load, loading rate, and vertebral mineral content.

The biomechanical response of the lumbar spine to combined flexion-shear loading was determined experimentally. The injury model simulated flexion-distraction trauma to the lumbar spine. Forty-eight lumbar functional spinal units (FSUs) were subjected to dynamic loading to injury with six different load types. The reactive forces and moments and resulting deformity were determined. Static physiologic loading was performed before and after the injurious loading to assess residual injury. The biomechanical response of FSUs was dependent on the amount of load and loading rate. The vertebral bone mineral content explained most of the biologic variation of the results. An osteoporotic or severely degenerated spine will be more easily rendered unstable after trauma with lower deformity. Injury at high loading rates will create instability with lower deformity. In vitro experiments should be performed on entire spinal units and with combined loads.