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Biomechanical comparison of sagittal-parallel versus non-parallel pedicle screw placement

  • Technical Note - Spine
  • Published:
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Abstract

Background

While convergent placement of pedicle screws in the axial plane is known to be more advantageous biomechanically, surgeons intuitively aim toward a parallel placement of screws in the sagittal plane. It is however not clear whether parallel placement of screws in the sagittal plane is biomechanically superior to a non-parallel construct. The hypothesis of this study is that sagittal non-parallel pedicle screws do not have an inferior initial pull-out strength compared to parallel placed screws.

Methods

The established lumbar calf spine model was used for determination of pull-out strength in parallel and non-parallel intersegmental pedicle screw constructs. Each of six lumbar calf spines (L1-L6) was divided into three levels: L1/L2, L3/L4 and L5/L6. Each segment was randomly instrumented with pedicle screws (6/45 mm) with either the standard technique of sagittal parallel or non-parallel screw placement, respectively, under fluoroscopic control. CT was used to verify the intrapedicular positioning of all screws. The maximum pull-out forces and type of failure were registered and compared between the groups.

Results

The pull-out forces were 5,394 N (range 4,221 N to 8,342 N) for the sagittal non-parallel screws and 5,263 N (range 3,589 N to 7,554 N) for the sagittal-parallel screws (p = 0.838). Interlevel comparisons also showed no statistically significant differences between the groups with no relevant difference in failure mode.

Conclusion

Non-parallel pedicle screws in the sagittal plane have at least equal initial fixation strength compared to parallel pedicle screws in the setting of the here performed cadaveric calf spine experiments.

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Authors and Affiliations

  1. Orthopaedic Surgery, Balgrist University Hospital, University of Zürich, Forchstrasse 340, 8008, Zürich, Switzerland

    Mazda Farshad & Samuel L. Schmid

  2. Institute of Diagnostic and Interventional Radiology, University Hospital of Zürich, University of Zürich, Rämistrasse 100, 8091, Zürich, Switzerland

    Nadja A. Farshad-Amacker

  3. Laboratory for Orthopaedic Biomechanics, Balgrist University Hospital, University of Zürich, Forchstrasse 340, 8008, Zürich, Zürich, Switzerland

    Elias Bachmann & Jess G. Snedeker

Authors
  1. Mazda Farshad
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  2. Nadja A. Farshad-Amacker
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  3. Elias Bachmann
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  4. Jess G. Snedeker
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  5. Samuel L. Schmid
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Correspondence to Mazda Farshad.

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Farshad, M., Farshad-Amacker, N.A., Bachmann, E. et al. Biomechanical comparison of sagittal-parallel versus non-parallel pedicle screw placement. Acta Neurochir 156, 2147–2151 (2014). https://doi.org/10.1007/s00701-014-2244-0

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  • DOI: https://doi.org/10.1007/s00701-014-2244-0

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