Biomechanical Analysis of Lumbosacral Fixation

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Fluxion testing was performed until failure on 66 lumbosacral bovine spinal segments comparing ten different lumbosacral Instrumentation techniques. Maximum flexion moment at failure, flexural stiffness, and maximum angulation of the lumbosacral joint at failure were determined as well as strain measurements across the anterior aspect of the lumbosacral intervertebral disc using an extensometer. The maximum moment at failure was significantly greater for the only two devices that extended fixation into the ilium anterior to the projected image of the middle osteoligamentous column: ISOLA Galveston and ISOLA Iljac screws (F = 12.2, P < 0.001). The maximum stiffness at failure reinforced these findings (F = 23.7, P < 0.001). A second subset of stability showed the advantages of S2 pedicle fixation by increasing the flexural lever arm (Cotrel-Dubousset butterfly plate, and Cotrel-Dubousset Chopin block, P < 0.05). This exhaustive in vitro biomechanical study introduces the concept of a pivot point at the lumbosacral joint at the intersection of the middle osteoligamentous column (sagittal plane) and the lumbosacral intervertebral disc (transverse plane) A spinal surgeon can increase the stability of lumbosacral instrumentation by extending fixation through the anterior sacral cortex [Steffee plate group with pedicle screws that medially converge in a triangular fashion). A means of enhancing this fixation was to achieve more interior purchase by extending the fixation down to the S2 pedicle (Cotrel-Dubousset Chopin and Cotrel-Dubousset butterfly groups). However, the best fixation was achieved by obtaining purchase between the iliac cortices down into the superior acetabular bone. This extends the fixation a greater distance anterior to the projected lateral image of the middle column: ISOLA Galveston and ISOLA iliac screw techniques (P < 0.05). Crossing the SI joint with fixation is only biomechanically justified if the construct obtains purchase of the iliac creast anterior to the projected image of the middle osteoligamentous column.

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