Comparison of mechanical motion profiles following instrumented fusion and non-instrumented fusion at the L4-5 segment.

PURPOSE: To investigate the difference in motion profiles between instrumented and non-instrumented fusion of the lumbar spine. METHOD: In vivo retrospective radiological analysis of dynamic (flexion-extension) lateral plain films was performed in different lumbar spine fusion types. Twenty-eight patients underwent lumbar fusion surgery at the L4/5 level. Fourteen patients underwent anterior fusion surgery without implantation, and the others underwent posterior instrumented fusion. Segmental angular motion was measured at the fused and adjacent levels using dynamic plain lateral film 2 years after operation. RESULTS: The anterior uninstrumented fusion group showed mean 2.0 degrees of segmental angular motion at the fused level compared with mean of 0.8 degrees in the posterior instrumented fusion group (P < 0.05). In contrast, at the proximal adjacent level, decreased angular motion (mean 7.7 degrees ) was noted in the anterior uninstrumented fusion group compared with mean 11.6 degrees in the posterior instrumented fusion group (P < 0.05). CONCLUSION: This study suggests that differing stiffness of fusion segments could cause different mechanical motion profiles at adjacent segments.