. 2022 Jul 8;2022:8273853.
doi: 10.1155/2022/8273853.
eCollection 2022.
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Biomed Res Int.
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Abstract
Biomechanical performance of longitudinal component in dynamic hybrid devices was evaluated to display the load-transfer effects of Dynesys cord spacer or Isobar damper-joint dynamic stabilizer on junctional problem based on various disc degenerations. The dynamic component was adapted at the mildly degenerative L3-L4 segment, and the static component was fixed at the moderately degenerative L4-L5 segment under a displacement-controlled mode for the finite element study. Furthermore, an intersegmental motion behavior was analyzed experimentally on the synthetic model under a load-controlled mode. Isobar or DTO hybrid fixator could reduce stress/motion at transition segment, but compensation was affected at the cephalic adjacent segment more than the caudal one. Within the trade-off region (as a motion-preserving balance between the transition and adjacent segments), the stiffness-related problem was reduced mostly in flexion by a flexible Dynesys cord. In contrast, Isobar damper afforded the effect of maximal allowable displacement (more than peak axial stiffness) to reduce stress within the pedicle and at facet joint. Pedicle-screw travel at transition level was related to the extent of disc degeneration in Isobar damper-joint (more than Dynesys cord spacer) attributing to the design effect of axial displacement and angular rotation under motion. In biomechanical characteristics relevant to clinical use, longitudinal cord/damper of dynamic hybrid lumbar fixators should be designed with less interface stress occurring at the screw-vertebral junction and facet joint to decrease pedicle screw loosening/breakage under various disc degenerations.
Copyright © 2022 Shih-Hao Chen et al.
Conflict of interest statement
The authors declare that there is no conflict of interest regarding the publication of this article and there has been no financial support for this work that could have influenced its outcome.
Figures
Figure 1
Under a “displacement-controlled” mode, the performances of (a, b) 2 hybrid and (c, d) 2 static lumbar fixation models were normalized by the corresponding degenerative conditions without implantation for comparison.
Figure 2
(a) A uniaxial material testing system equipped with a custom-made holder was performed under pure compression and distraction of 700 N load displacement. The distance from Isobar device to epoxy resin surface was fixed with four screws on both sides and mounted by universal joint on a X–Y table for calculating stiffness under +700 N loading. (b) Numerical Isobar model was set and validated to reveal (c) reaction force averaged 27% lower in compression and 3% higher in distraction than those of experimental results.
Figure 3
(a) Intersegmental motion behavior was analyzed experimentally on the synthetic model modified from ISO 12189 standards under a “load-controlled” mode. Infrared light emitting diodes were attached to each vertebral body with two landmarkers installed for measuring the 3-dimensional coordinates in (b) DTO and (c) Isobar devices. Initial 2 mm precompression on the assembled construct guaranteed stability and (d) increased axial vertical load of 500 N for overall construct; then, 10 Nm bending moment of flexion or lateral bending was applied to measure (e) interpedicular travel vector (r).
Figure 4
All the DTO, Isobar, 1-level, and 2-level static fixator models revealed various extents of junctional problems at the cephalic segment (L2-L3) more than the caudal one (L5-S1). Both the DTO and Isobar dynamic hybrid fixators had better performance than the 2-level static fixator to increase (a) motion and decrease (b) disc stress at the L3-L4 transition segment for balancing junctional problems.
Figure 5
At L3-L4 transition segment, the ROMs, disc stresses, and FCF in the model implanted with dynamic Isobar damper or Dynesys were compared with those of no fixator. The dynamic Isobar damper afforded the decreased FCF, ranging from 15% (bending) to 41% (rotation), more than the Dynesys did.
Figure 6
The trade-off region of Dynesys cord stiffness was set as 50-200 N/mm because the convergent value of disc stress decussated around 50 N/mm and approached zero around 200 N/mm. In terms of a 100% degenerative model without instrumentation, the changes of ROM at L3-L4 transition segment ranged from 26% to 43%, and disc stresses ranged from 23% to -3% in flexion to reduce stiffness-related problem.
Figure 7
The upper screw-bone interface stress distribution at transition level was shown from the bound screw tip to posterior screw hub of the original dynamic devices. Dynesys screw exhibited a linear pattern of pedicle travel path with peak stress located at the posterior pedicle orifice of screw hub revealing 98% in flexion and 47% in extension, respectively, higher than those of the Isobar integral damper.
Figure 8
Schematic diagrams illustrated the performances of dynamic hybrid fixators. (a) Dynesys cord pretension made spacer-screw contact but induced high stress at pedicle screw-bone junction. (b) Isobar integral damper provided axial rotation and distraction coupling to afford stress-shielding and reduce loading on the facet joint and pedicle screw. (c) A 70-year-old male patient received Isobar hybrid fixation at L3-4-5 and got back pain at 8 months postoperatively due to screw loosening at L3 (arrow). (d) Subsequently, radicular pain was noticed at four years due to adjacent segment disease at L2-3 (arrow).
References