Biomechanical Analysis of Lumbosacral Fixation

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Abstract
The authors sought to measure the stiffness at the lumbosacral junction when it has been immobilized by means of two different posterior fixation systems in conjunction with three different anterior interbody fixation techniques. The information obtained provides a foundation for determining how methods of lumbosacral spinal fixation can maximize rigidity and improve fusion rates at this clinically important anatomic site. To determine which spinal instrumentation systems, alone or in combination, provide the most stability at the lumbosacral junction. In the clinical setting, several different posterior instrumentation systems are currently used, but the stability of these systems, when used in conjunction with anterior interbody fixation, has not been adequately tested. The authors hypothesized that rigidity of fixation at the lumbosacral junction may be enhanced by use of appropriate anterior interbody fixation techniques. Two posterior and three anterior fusion constructs were tested in eight fresh-frozen human cadaveric specimens in axial compression and torsion, flexion, extension, and lateral bending. The isolated posterior instrumentation constructs produced greater intervertebral stiffness than the isolated anterior constructs tested. The Jackson instrumentation was stiffer than the Galveston in axial torsion and flexion. Anteriorly, the femoral ring was stiffer than either the threaded interbody fusion device in flexion or the corticocancellous bone graft in flexion and compression. The threaded interbody fusion device was the stiffest anterior construct in axial torsion. The authors' data demonstrate that the threaded interbody fusion device may be an effective system for immobilization of the L5-S1 disc space. Rigidity of fixation at the lumbosacral junction may be enhanced by use of appropriate anterior interbody fusion techniques.