- •The hybrid fixation can protect the transition segment and suppress junctional problems.
- •The cable pretension of the screw-spacer system has a minor effect on the construct behavior.
- •The screw-spacer system transmits higher loads onto the transition disc.
- •The rod-rod system comparatively constrains the transition segment.
- •The rod-rod system induces higher compensations to the adjacent segments.
Hybrid fixators with quite different joint design concepts have been widely to suppress adjacent segment degeneration problems. The kinematic and kinetic responses of the adjacent and transition segments and contact behaviors at the bone-screw interfaces served as the objective of this study.
The moderately degenerated L4/L5 and mildly degenerative L3/L4 segments were respectively immobilized by a static fixator and further bridged by the rod-rod (Isobar) and screw-spacer (Dynesys) fixator.. The joint stiffness and mobility of the rod-rod system and the cable pretension of the screw-spacer system were systematically varied.
The flexion of the screw-spacer system provided higher mobility to the transition segment, reducing adjacent-segment problems. The cable pretension had a minor effect on the construct behavior. However, due to limited joint mobility, the rod-rod system showed higher constraints to the transition segment and induced more adjacent-segment compensations. The increased mobility of the rod-rod joint caused it to behave as a more dynamic fixator that increased adjacent-segment compensations at the transition segment. Comparatively, increasing the joint mobility showed more significant effects on the construct behaviors than decreasing the joint stiffness. Furthermore, increased constraint by the rod-rod joint induced higher stress and risk of loosening at the bone-screw interfaces
If the protection of the transition segment is the major concern, the rod-rod system can be used to constrain the intervertebral motion and share the higher loads through the fixator. Otherwise, the screw-spacer system is recommended in situations where higher loads onto the transition disc are allowable.
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Accepted: March 7, 2023
Received: September 9, 2022
Publication stageIn Press Journal Pre-Proof
© 2023 Published by Elsevier Ltd.