Effects of rod stiffness and fusion mass on the adjacent segments after floating mono-segmental fusion: a study using finite element analysis

Purpose The aims of the present study were to compare the biomechanical effects on the adjacent segments after mono-segmental floating fusion with posterior semi-rigid or rigid stabilization, and to evaluate the effect of the amount of fusion mass on the biomechanical differences. Methods A detailed, nonlinear L1-S1 finite element model had been developed and validated. Then five models were reconstructed by different fixation techniques on the L3-L4 level: rigid fixation with an interbody spacer (Ti + IS), rigid fixation with a large interbody spacer (Ti + IS_all), semi-rigid fixation with an interbody spacer (PEEK + IS), semi-rigid fixation with a large interbody spacer (PEEK + IS_all), and semi-rigid fixation only (PEEK). Analyses were conducted for the case of erect standing position, flexion, and extension motion. Results At L1-L2 and L2-L3, PEEK + IS demonstrated less inter-segmental rotation and nucleus pressure increments from the intact model compared with Ti + IS. The L4-L5 and L5-S1 levels showed slightly higher values with PEEK + IS, but these differences among the instrumented models were not significant. The motion difference based on the fusion mass at the adjacent levels was at most 3 %. All instrumentation cases generated a 55 % higher facet contact force at the lower adjacent level (L4-L5) compared to that of the intact model during 26 degrees extension and the largest increment was detected at the upper adjacent level (L2-L3) in the Ti + IS. Instrumentation with Ti + IS markedly increased the stress in the intervertebral disk at the upper adjacent level, while the stress with PEEK + IS appeared largest at the lower adjacent level. Conclusions Posterior instrumentation with semi-rigid rods may lower the incidence of disk and facet degeneration in the upper adjacent segment compared to rigid rods. On the other hand, the possibility of facet degeneration will be similar for all instrumentation devices in the lower adjacent segment in the long-term. The stiffness difference between rigid and semi-rigid rods on the changes in the adjacent motion segments was more crucial than amount of fusion mass.