Effects of the level of mono-segmental dynamic stabilization on the whole lumbar spine

Little biomechanical data has been gathered for the biomechanical effects of pedicle-based dynamic stabilization system (PBDS) and fusion (a conventional titanium rod and cage) to the whole lumbar spine according to the instrumentation level. A previously validated three-dimensional, intact osteoligamentous L1-S1 finite element model was modified to incorporate three different PBDS (Dynesys, Nflex, or PEEK) and fusion at three different levels (L3-L4, L4-L5, and L5-S1). A new loading method that can create the segmental motion similar to an in-vivo measurement was applied to the model. The biomechanical changes in the stabilized models were compared with those of the intact model during sagittal plane motion. The simulation results demonstrated that Dynesys generated relatively larger motion when it was instrumented at the L3-L4 segment, whereas the Nflex was the most appropriate device for L4-L5 stabilization. Depending on the stabilization device and instrumented level, whole-lumbar segmental motion also varied. During flexion, stabilization at the L3-L4 level or L4-L5 level produced a relatively higher increase in the motion at all cranial levels. Stabilization at the L5-S1 level generated a slight decrease in the motion at the adjacent cranial level without respect to the type of fixation. In cases of fusion, the change in the motion was higher relative to that with PBDS. Given the biomechanical change at each level after stabilization, adjacent segment degeneration was expected cranially rather caudally, and the probability of this degeneration differed depending on the stabilization level and device.