Influence of connecting plate on construct stiffness of spinal implants and motion of adjacent segments

An in vitro biomechanical study compared the influence of connecting plates on construct stiffness and motion of adjacent segments. Twelve porcine lumbar spines were destabilized by laminectomy and instrumented at the L3 and L4 vertebrae by one of three transpedicular screw fixation systems: Cotrel-Dubousset, Diapason, and a newly designed implant (FPI). The current study demonstrated that connecting plates significantly increased the construct compression and flexion stiffness and added resistance to axial rotation. The upper intact disc had greater rotational displacement than the unfixed intact spine, but the lower intact disc had more anterior translational displacement. When the construct was flexed with a I-second period, the upper intact disc (L2/L3) flexed much more than with a 5-second period (P<0.05). This study also showed a significant correlation between flexion stiffness and compensatory rotational displacement at the upper intact disc.