BIOMECHANICS OF INDIRECT REDUCTION OF BONE RETROPULSED INTO THE SPINAL-CANAL IN VERTEBRAL FRACTURE

The biomechanics of indirect reduction of bone fragments retropulsed i nto the spinal canal in a burst fracture were investigated. In this mo del, tunnels were created in vertebrae L1 and C5 oriented anterior-to- posterior, allowing access to the posterior longitudinal ligament. A p robe containing a load-sensing tip was passed through the tunnel. Both the location of the tip and the load acting on it by posterior deflec tion of the posterior longitudinal ligament were measured. In the lumb ar spine, distraction was applied by spinal instrumentation that also permitted independent kyphotic-lordotic alignment of the vertebrae, In the cervical spine, axial traction was applied through direct loading . Several clinically relevant observations were made. It was not possi ble to produce an anteriorly directed force in the posterior longitudi nal ligament at less than 35% canal occlusion, partly because the post erior longitudinal ligament stands away from the midbody of the verteb ra. Distractive forces of up to 150 N were applied in the lumbar spine , which were nearly equal to the tensile breaking strength of the isol ated posterior longitudinal ligament. Regardless of the relative sagit tal plane angulation of the vertebrae, distraction was the governing f actor in generating force in the posterior longitudinal ligament. Beca use positioning the vertebrae in lordosis before applying distraction significantly slackens the posterior longitudinal ligament, it is sugg ested that distraction be applied before angular positioning of the ve rtebrae is performed.