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Biomechanical performance of bicortical versus pericortical bone trajectory (CBT) pedicle screws

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Abstract

Purpose

The cortical bone trajectory (CBT) is an alternative to the traditional pedicle screw trajectory (TT) in posterior spinal instrumentation, enhancing screw contact with cortical bone and therefore increasing fixation strength. Additional to the trajectory, insertion depth (pericortical vs. bicortical placement) could be a relevant factor affecting the fixation strength. However, the potential biomechanical benefit of a bicortical placement of CBT screws is unknown. Therefore, the aim of this study was to quantify the fixation strength of pericortical- versus bicortical-CBT (pCBT versus bCBT) screws in a randomized cadaveric study.

Methods

Pedicle screws were either placed pericortical or bicortical with a CBT in 20 lumbar vertebrae (2 × 20 instrumented pedicles) from four human spine cadavers by using patient-specific templates. Instrumented specimens underwent physiological cyclic loading testing (1′800′000 cycles, 10 Hz), including shear and tension loads as well as bending moments. Translational and angular displacements of the screws were quantified and compared between the two techniques.

Results

There was a slight decrease in translational (0.2 mm ± 0.09 vs. 0.24 mm ± 0.11) and angular displacements (0.06° ± 0.05 vs. 0.13° ± 0.11) of bCBT screws when compared with pCBT screws after 1′800′000 cycles. However, the results were non-significant (p > 0.05).

Conclusion

The authors do not recommend placing CBT screws bicortically, as no relevant biomechanical advantage is gained while the potential risk for iatrogenic injury to structures anterior to the spine is increased.

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Data availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Code availability

MATLAB R2018a, MathWorks, Natick, USA.

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Acknowledgements

The authors gratefully acknowledge the contribution of Beda Rutishauser and Eleonora Croci for their support with the mechanical test set-up. They also thank Natalie Hinterholzer and Daniel Nanz from the Swiss Center for Musculoskeletal Imaging (SCMI) for their technical support during radiological imaging.

Funding

The vertebra-specific drill guides and the pedicle screws were provided to us by MySpine, Medacta SA International, Switzerland.

Author information

Author notes

  1. José M. Spirig, Elin Winkler, Jonas Widmer and Mazda Farshad these authors contributed equally.

Authors and Affiliations

  1. Department of Orthopaedics, Balgrist University Hospital, University of Zürich, Zürich, Switzerland

    José M. Spirig, Elin Winkler, Frédéric Cornaz, Marie-Rosa Fasser, Michael Betz, Jess G. Snedeker, Jonas Widmer & Mazda Farshad

  2. Institute for Biomechanics, ETH Zürich, Zürich, Switzerland

    Marie-Rosa Fasser, Jess G. Snedeker & Jonas Widmer

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  1. José M. Spirig
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Correspondence to José M. Spirig.

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Conflict of interest

Author Prof. Mazda Farshad owns stocks at PrognoSyst AG and Incremed AG. He further receives research support from Medacta SA International and fellowship support from Johnson and Johnson. All other authors declare that they have no conflict of interest.

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Spirig, J.M., Winkler, E., Cornaz, F. et al. Biomechanical performance of bicortical versus pericortical bone trajectory (CBT) pedicle screws. Eur Spine J 30, 2292–2300 (2021). https://doi.org/10.1007/s00586-021-06878-1

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