Biomechanical comparisons of dynamic fixators with rod-rod and screw-spacer joints on lumbar hybrid fixation

Yu-Kun Xu; Pei-Wei Weng; Shih-Hao Chen; Shang-Chih Lin

doi:10.1016/j.clinbiomech.2023.105943

Biomechanical comparisons of dynamic fixators with rod-rod and screw-spacer joints on lumbar hybrid fixation

Clin Biomech (Bristol, Avon). 2023 Apr:104:105943. doi: 10.1016/j.clinbiomech.2023.105943. Epub 2023 Mar 11.

Authors

Yu-Kun Xu¹, Pei-Wei Weng², Shih-Hao Chen³, Shang-Chih Lin⁴

Affiliations

¹ Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan; Graduate Institute of Biomedical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan.
² Department of Orthopedics, Shuang Ho Hospital, Taipei Medical University, Taipei, Taiwan; International Ph.D. Program in Biomedical Engineering, College of Biomedical Engineering, Taipei Medical University, Taipei, Taiwan; Department of Orthopaedics, School of Medicine, College of Medicine, Taipei Medical University, Taipei, Taiwan; Research Center of Biomedical Device, Taipei Medical University, Taipei, Taiwan.
³ Tzu Chi General Hospital at Taichung and Tzu Chi University, Taiwan.
⁴ Graduate Institute of Biomedical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan. Electronic address: xyz@mail.ntust.edu.tw.

PMID: 37040676
DOI: 10.1016/j.clinbiomech.2023.105943

Abstract

Background: 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.

Methods: 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.

Findings: 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 INTERPRETATION: 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.

Keywords: Dynamic; Dynesys; Hybrid fixation; Isobar; Junctional problem.

MeSH terms

Biomechanical Phenomena / physiology
Bone Screws
Humans
Internal Fixators
Lumbar Vertebrae* / surgery
Lumbosacral Region
Range of Motion, Articular / physiology
Spinal Fusion*
Stress, Mechanical