In-vitro flexibility characteristics of dynamic pedicle screw systems

Abstract

In recent years, nonfusion stabilization of the lumbar spine has gained more and more popularity. This nonfusion systems intend to maintain or restore the intersegmental motions to magnitudes of the intact spine and have no negative effects on the segments adjacent to the stabilized once. This study investigates posterior dynamic stabilization devices with different implant stiffness in comparison to the gold standard of dorsal spinal instrumentation, namely fusion. To determine the magnitude of stabilization and the effect of the stabilization on the adjacent segment, six human lumbar cadaver spines were fixed in a spinal simulator and loaded with pure moments alone, and with pure moments and additional preload in the three motion planes. For each spine, five different stages were tested: native, rigid fixation, dynamic fixation with two Prototypes and the Dynesys®. Intersegmental motions were measured at all levels. For the bridged segment, the dynamic systems stabilized the spine and were more flexible than the rigid fixation. The results showed that there is a strong dependency of implant stiffness and intersegmental rotation in flexion/ extension and lateral bending. The motion in the adjacent segments was not influenced by either stabilization method. Our results suggests that dynamic stabilization provides substantial stability in case of degenerative spinal disorders and can therefore be considered as an alternative method to fusion surgery in this indications while the motion segment is preserved. But it is still unclear to which amount motion should be resrticted or allowed.