Kinematic and mechanical comparisons of lumbar hybrid fixation using dynesys and cosmic systems

Study Design: The biomechanical effects of Dynesys and Cosmic fixators on transition and adjacent segments were evaluated usinag the finite-element method. Objective. This study investigated the load-transferring mechanisms of 2 dynamic fixators and the fixator-induced effects on the junctional problem of the adjacent segments. Summary of Background Data: The mobility and flexibility of Dynesys screw-spacer and Cosmic screw-hinge joints preserve motion and share loads for the transition segment. However, the differences in tissue responses and fixator mechanisms among these 2 fixators have not/en investigated extensively. Methods: A lumbosacral model from L1 to S1 levels was developed and subjected to muscular contraction, ligamentous interconnection, compressive force, and trunk moment. A static fixator was instrumented at the moderately degenerative L4-L5 segment to serve as a comparison baseline. Subsequently, the 2 fixators were instrumented at the mildly degenerative L3-L4 segment. The tissue responses of the adjacent segments and the load transmission at the screw-spacer and bonescrew interfaces were compared. Results: Both systems show the ability to protect the transition segment but deteriorate the adjacent segments. The screw-hinge joint and the stiffer rod of the Cosmic system significantly constrained the motion pattern of the transition segment. Comparatively, the Dynesys screw-spacer interfaces make contact with and depart from each other during motion; thus providing higher mobility to the transition segment. However, the highly stressed distribution at the Cosmic bonescrew causes the screw and hinge prone to pullout and fatigue failures. Conclusion: Cosmic fixation can/tter protect the disc and facet joint of the transition segment than can the Dynesys. However, the screw-hinge joint strictly constrains intersegmental motion and deteriorates the junctional problem. The Cosmic system can/chosen to treat more severely degenerative transition segments. With higher flexibility, the Dynesys system is recommended for the transition segment that is healthy or mildly degenerative.