A Biomechanical Analysis of Spinal Instrumentation Systems in Thoracolumbar Fractures

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Abstract
A total of 61 biomechanical tests were performed on 25 cadaveric spinal segments to investigate the comparative strengths of three instrumentation systems: 1) conventional Harrington distraction instrumentation (HRI), 2) segmentally wired Harrington distraction rods, and 3) Luque segmental spinal instrumentation (SSI). In type I testing in which axial preload was applied to normal specimens, and then progressive rotation until ultimate failure followed, five of six Harrington systems failed at the bone-metal Interface. In contrast, all six Luque SSI vertebral segments disrupted in a location removed from the bone-metal Interface. In Type-II testing (six specimens) in which axial loading of experimentally produced unstable burst fractures was applied, the most stable fixation In resisting compressive loads was segmentally wired Harrington distraction rods (P < 0.001). In Type-III testing (six specimens), there was axial preloading, then progressive rotation applied to translational fracture-dislocations and this snowed that the ability to resist torsion was lowest with plain HRI, slightly improved by segmentally wired HRI, and the stiffest system was Luque SSI (P < 0.05). The three methods of testing cadaveric segments provided a relevant laboratory model for investigation of spinal instrumentation systems in thoracolumbar fracture stabilization. The results compare favorably with other biomechanical studies, information derived from In vitro and ex vivo animal models and clinical experience with failures of fixation. The biomechanical advantages of segmentally wired Harrington distraction instrumentation in resisting axial loads seem to justify this method of fixation in unstable burst fractures. Similarly, the use of Luque segmental spinal instrumentation with L-rods coupled together is the best method of achieving rotational stability in translational injuries (fracture-dislocations). However, the above biomechanical considerations should be balanced against the increased operative time, more exacting technical expertise required, and possible risk of iatrogenic neurologic sequelae in implementing segmental fixation in unstable thoracolumbar fracture management.