Nonrigid Stabilization Of The Spine - Problems Observed: Screw Loosening/Breakage/Implant Failure/Adjacent Segment Degeneration

Stabilization of a lumbar motion segment without fusion appears to be an appealing treatment option with a twofold objective: first, limit the surgical intervention by avoiding bone harvesting and bone grafting; and second, prevent adjacent level degeneration. Although several implants are on the market and claim to be dynamic, there remain important limitations as the technical failure rate remains high, about twice as high as with traditional fusion procedures. A review of a case series treated with the Dynesis system reveals implant loosening/screw migration as the main problem. Furthermore, adjacent level degeneration cannot be avoided. The Cosmic fixation system carries an inherent problem due to the monoaxial mobility on one hand, and the rigid rod connection on the other hand. The translational instability in a motion segment cannot be controlled due to the wiper effect, and furthermore, the rigid longitudinal connection is provoking high stresses between the screw – bone interface with the consequence of screw loosening. The main problem concerning dynamic stabilization remains the lack of basic knowledge regarding the critical biomechanical parameters that occur at each individual motion segment in real live. Further, we do not know exactly to what extent an implant needs to provide stability; just in general terms we may state that we would like to stabilize the spine as much as necessary but preserve motion as much as possible. The problem is given by this paradox that needs to be solved. There is definitely a need for such implants – not only for avoidance of a fusion, but for the combination of a rigid fixation with a nonfusion treatment (hybrid fixation) for complex spine problems especially in the elderly – which appears to be of outermost interest in order to get rid of the problem of adjacent segment failure.