Effect of instrumented lumbar fusion on the flexibility and viscoelastic properties of the adjacent segment: An in vivo canine study

The altered mechanical environment due to a rigid fixation is considered as the cause of degeneration at the adjacent levels. A canine in vivo study was performed to investigate the long-term effects of spinal fusion with instrumentation across the L5-sacrum on the flexibility and viscoelastic properties of the adjacent L4-5 disc. Five experimental dogs underwent a posterior fusion with Isola transpedicular instrumentation from L5 to sacrum and were euthanized 30 weeks postoperatively. A solid fusion was achieved across L5-L7 levels, but substantial motion persisted at the L7-sacrum segment. Biomechanical tests were performed to measure the flexibility and viscoelastic properties of the intervertebral disc above the instrumented spinal fusion in the experimental and nonoperated control groups. No statistically significant differences in the flexibility and viscoelastic responses of the adjacent L4-L5 segment were found between the control and experimental groups. Fusion of the L5-L7 segments did not significantly affect the flexibility and viscoelastic properties of the L4-5 disc at 30 weeks postoperatively in the mongrel dog spine.