Intraoperative biomechanical assessment of lumbar spinal instability: Validation of radiographic parameters indicating anterior column support in lumbar spinal fusion

Study Design. Lumbar spinal instability was evaluated using radiographic parameters and intraoperative biomechanical measurement. Objectives. To validate a conventional radiographic assessment using an intraoperative biomechanical measurement and to determine the radiographic parameters indicating anterior column support in lumbar spinal fusion. Background. Posterior lumbar interbody fusion is gaining acceptance in the treatment of degenerative spondylolisthesis, whereas posterolateral fusion alone may result in good clinical outcomes. A quantitative biomechanical assessment of spinal instability is essential to determine the optimal method of spinal arthrodesis. Materials and Methods. Nineteen cases with symptomatic degenerative spondylolisthesis were included in this study. All were candidates for pedicle-screw instrumented lumbar arthrodesis. A total of five L3-L4 segments and seventeen L4-L5 segments were investigated. Radiographic assessment included disc angle, range of motion ( ROM), percent of slip, and percent of posterior disc height. Distraction stiffness of the operative segment was measured intraoperatively using a strain-gauged spreader. Results. Disc angle in flexion and ROM were highly correlated to distraction stiffness. Severity of slip, disc space narrowing, and facet tropism did not influence the distraction stiffness. The cases with segmental kyphosis in flexion had significantly lower distraction stiffness than those showing segmental lordosis in flexion. Conclusions. Disc angle in flexion and ROM were the most prognostic parameters of lumbar distraction instability. Although the option of spinal arthrodesis method should be determined based on both clinical manifestation and imaging studies, the current study demonstrated that providing of anterior column support is biomechanically reasonable for degenerative spondylolisthesis with segmental kyphosis in flexion.