Comparative stability of the "Internal Fixator" and the "Universal Spine System" and the effect of crosslinking transfixating systems. A biomechanical in vitro study.

In this study, the three-dimensional stabilizing capabilities of the AO-Int ernal Fixator (IF) and the new Universal Spine System (USS) were investigat ed. Both devices were tested without and with the cross-link system (IF, IF C, USS, USSC). To determine biomechanical characteristics, a human thoracolumbar spine ins tability model with resection of the vertebral body Th12 was created. The v ertebral body was replaced by a spacer and transpedicular posterior stabili zation was performed from Th11 to L1. All devices reduced the range of motion (ROM) significantly compared to the values of the intact specimen. In flexion the IFC showed the highest reduc tion of ROM (85% of intact), followed by the USSC, USS and IF (79% of intac t). In extension the ROM: was restored again most by the IFC (52% of intact ), followed by the USSC, IF and USS (44% of intact). In lateral bending sta bility was provided by the USSC (right 78% and left 81% of intact), followe d in right lateral bending by the LF, IFC and USS and in left lateral bendi ng by the USS, IF and IFC. In axial rotation the ROM was reduced primary by the IFC (right 51% and left 46% of intact), followed in right axial rotati on by the USS, USSC and IF, in left axial rotation by the USSC, USS and IF. Additional stability by crosslinking has been provided in the IF and the U SS in flexion and extension, in the USS in lateral bending and in the IF in axial rotation nonsignificantly. The neutral zone (NZ) was reduced by posterior instrumentation in flexion/e xtension and right/left lateral bending significantly. In axial rotation on ly the USSC decreased the NZ below intact levels. The study showed no statistical significant differences in the stabilizing capabilities of the USS compared to the IF. For both implants the cross-lin k system increased stability in the choosen instability model insignificant ly only.