BIOMECHANICAL ANALYSIS OF LUMBOSACRAL FIXATION

Study Design. The authors sought to measure the stiffness at the lumbo sacral junction when it has been immobilized by means of two different posterior fixation systems in conjunction with three different anteri or 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 impo rtant anatomic site. Objectives. To determine which spinal instrumenta tion systems, alone or in combination, provide the most stability at t he lumbosacral junction. Summary of Background Data. In the clinical s etting, several different posterior instrumentation systems are curren tly 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 jun ction may be enhanced by use of appropriate anterior interbody fixatio n techniques. Methods. Two posterior and three anterior fusion constru cts were tested in eight fresh-frozen human cadaveric specimens in axi al compression and torsion, flexion, extension, and lateral bending. R esults. The isolated posterior instrumentation constructs produced gre ater intervertebral stiffness than the isolated anterior constructs te sted. The Jackson instrumentation was stiffer than the Galveston in ax ial torsion and flexion. Anteriorly, the femoral ring was stiffer than either the threaded interbody fusion device in flexion or the cortico cancellous bone graft in flexion and compression. The threaded interbo dy fusion device was the stiffest anterior construct in axial torsion. Conclusions. The authors' data demonstrate that the threaded interbod y fusion device may be an effective system for immobilization of the L 5-S1 disc space. Rigidity of fixation at the lumbosacral junction may be enhanced by use of appropriate anterior interbody fusion techniques .