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Clinically relevant biomechanical properties of three different fixation techniques of the upper instrumented vertebra in deformity surgery

  • Biomechanics
  • Published:
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Abstract

Objective

Adjacent segment disease, junctional kyphosis/failure and pseudarthrosis can negatively impact the mid to long-term outcome in spinal deformity surgery. These complications might be influenced by upper instrumented vertebra (UIV) fixation techniques. In this study we analyze key biomechanical properties of three different UIV fixation techniques and define their ideal clinical use based on patient-specific risk profiles using a finite element analysis (FEA) model.

Methods

A T9-pelvis posterior instrumented spinal fusion was assumed. Three different FEA models were created based on the UIV fixation technique: T9 pedicle screws (PS); T9 cortical bone screws (CBS); T9 transverse process hooks (TPH). The three FEA models consisted of T8–T10 bone and ligamentous anatomy derived from a CT scan of a healthy patient as well as spinal implants consisting of either pedicle screws, cortical bone screws or transverse process hooks as well as cobalt chromium rods. The FEA models were constrained at T10, axial load as assumed for a healthy 80 kg male during flexion, extension and lateral bending were applied. As surrogate markers for risk of proximal junctional kyphosis, proximal junctional failure, adjacent segment disease and pseudarthrosis the following biomechanical parameters were calculated: UIV range of motion (ROM); intradiscal stress at UIV/UIV + 1; UIV intravertebral stress and screw pull out forces. One-way ANOVA analyses have been performed to compare biomechanical outcome parameters between the three construct variants under investigation.

Results

UIV-ROM was restricted during flexion/extension/lateral bending by: PS: 73%/80%/86%, CBS: 71%/81%/85% and TPH: 62%/76%/85%. Average intradiscal stress at UIV/UIV + 1 during flexion/extension/lateral bending was (Mega Pascal, MPa): PS 0.42/0.44/0.38, CBS 0.49/0.4/0.44, TPH 0.66/0.51/0.58; average intravertebral stress of the UIV superior endplate during flexion/extension/lateral bending was (MPa): PS 2.23/2.12/2.21, CBS 1.87/1.98/1.8, TPH 1.67/0.98/1.53. Screw pull-out forces (N) at UIV during flexion/extension/lateral bending were: PS 476/320/375, CBS 444/245/308. Statistically significant differences were found for intradiscal stress as well as vertebral body average stress (p = 0.02 and p = 0.02).

Conclusion

Different UIV fixation techniques carry different biomechanical properties. Pedicle screw fixation is the most rigid, leading to the highest UIV stress and UIV screw pull out forces. Cortical bones screw fixation is similarly rigid; however, UIV stress and UIV screw pull out is significantly lower. Transverse process hook fixation is the least rigid, with the lowest UIV stress, however highest intradiscal stress at UIV/UIV + 1. Thus, these biomechanical differences may help select optimal UIV fixation techniques according to patient specific risk factors.

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Funding

Funding for this study was received by EN and MM from Medtronic.

Author information

Authors and Affiliations

  1. Department of Neurosurgery, Cantonal Hospital Lucerne, Luzern, Switzerland

    Edin Nevzati & Manuel Moser

  2. Department of Neurosurgery, University of Louisville, Louisville, KY, USA

    Nick Dietz

  3. Department of Orthopedic Surgery, Rady’s Children Hospital San Diego, San Diego, USA

    Burt Yaszay

  4. Department of Orthopedic Surgery, Columbia University, New York, USA

    Lawrence G. Lenke

  5. Department of Orthopedics, Balgrist University Hospital, University of Zurich, Zurich, Switzerland

    Mazda Farshad

  6. Department of Orthopedic Surgery, Icahn School of Medicine, Mount Sinai Hospital, New York, USA

    Varun Arvind & Samuel K. Cho

  7. Department of Neurosurgery, North Shore University Hospital, 300 Community Drive, Manhasset, NY, 11030, USA

    Alexander Spiessberger

Authors
  1. Edin Nevzati
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  2. Manuel Moser
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  3. Nick Dietz
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  4. Burt Yaszay
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  5. Lawrence G. Lenke
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  7. Varun Arvind
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  8. Samuel K. Cho
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  9. Alexander Spiessberger
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Contributions

EN, MM, ND, BY, LGL, MF, VA, SKC: conception, drafting, data analysis, manuscript preparation. AS: conception, drafting (original draft and final approval), data collection, manuscript preparation.

Corresponding author

Correspondence to Alexander Spiessberger.

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Nevzati, E., Moser, M., Dietz, N. et al. Clinically relevant biomechanical properties of three different fixation techniques of the upper instrumented vertebra in deformity surgery. Spine Deform 10, 1017–1027 (2022). https://doi.org/10.1007/s43390-022-00506-8

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  • DOI: https://doi.org/10.1007/s43390-022-00506-8

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