Kinematic and Mechanical Comparisons of Lumbar Hybrid Fixation Using Dynesys and Cosmic Systems

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Abstract

Study Design.

The biomechanical effects of Dynesys and Cosmic fixators on transition and adjacent segments were evaluated using 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 been 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 bone-screw 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 bone-screw causes the screw and hinge prone to pullout and fatigue failures.

Conclusion.

Cosmic fixation can better 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 be 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.

Conclusion.

Level of Evidence: N/A

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