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Biomechanical cadaver study of proximal fixation in a minimally invasive bipolar construct

Study design

Biomechanical human cadaver study.

Objective

To determine the three-dimensional intervertebral ranges of motion (ROMs) of intact and hook-instrumented thoracic spine specimens subjected to physiological loads, using an in vitro experimental protocol with EOS biplane radiography.

Summary of background data

Pedicle screws are commonly used in thoracic instrumentation constructs, and their biomechanical properties have been widely studied. Promising clinical results have been reported using a T1–T5 thoracic hook–claw construct for proximal rod anchoring. Instrumentation stability is a crucial factor in minimizing mechanical complications rates but had not been assessed for this construct in a biomechanical study.

Methods

Six fresh-frozen human cadaver C6–T7 thoracic spines were studied. The first thoracic vertebrae were instrumented using two claws of supra-laminar and pedicle hooks, each fixed on two adjacent vertebrae, on either side of a single free vertebra. Quasi-static pure-moment loads up to 5 Nm were applied to each specimen before and after instrumentation, in flexion–extension, right and left bending, and axial rotation. Five steel beads impacted in each vertebra allowed 3D tracking of vertebral movements on EOS biplanar radiographs acquired after each loading step. The relative ranges of motion (ROMs) of each pair of vertebras were computed.

Results

Mean ROMs with the intact specimens were 17° in flexion–extension, 27.9° in lateral bending, and 29.5° in axial rotation. Corresponding values with the instrumented specimens were 0.9°, 2.6°, and 7.3°, respectively. Instrumentation significantly (P < 0.05) decreased flexion–extension (by 92–98%), lateral bending (by 87–96%), and axial rotation (by 68–84%).

Conclusion

This study establishes the biomechanical stability of a double claw–hook construct in the upper thoracic spine, which may well explain the low mechanical complication rate in previous clinical studies.

Level of evidence

Not applicable, experimental cadaver study.

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Funding

Part of this study was financed by EUROS, including the spinal implants. We are grateful to the BiomecAM chair program on musculoskeletal modeling for financial support.

Author information

MG: study design, data acquisition and interpretation, engineering, manuscript drafting and revision for important intellectual content, approval of the version to be published. SP: study design, engineering, manuscript drafting, and approval of the final version to be published. CV: data interpretation, engineering, manuscript drafting, and approval of the final version to be published. CG: data analysis, manuscript revision for important intellectual content, and approval of the final version to be published. WS: study design, manuscript drafting, and approval of the final version to be published. LM: study design, specimen instrumentation, manuscript drafting, and approval of the final version to be published.

Correspondence to M. Gaume.

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Conflict of interest

The author declares that they have no conflict of interest.

Ethics/IRB approval

CPP n° ID-RCB/ EUDRACT: 2014-A01043-44.

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Gaume, M., Persohn, S., Vergari, C. et al. Biomechanical cadaver study of proximal fixation in a minimally invasive bipolar construct. Spine Deform 8, 33–38 (2020). https://doi.org/10.1007/s43390-019-00014-2

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Keywords

  • Thoracic fixation
  • Biomechanical cadaver study
  • Neuromuscular scoliosis
  • Minimally invasive surgery
  • Bipolar construct