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Concave rod first vs. convex rod first in AIS instrumentation with differential rod contouring: computer modeling and simulations based on ten AIS surgical cases

  • Biomechanics
  • Published:
Spine Deformity Aims and scope Submit manuscript

Abstract

Purpose

To assess biomechanical differences between AIS instrumentations using concave vs. convex rod first.

Methods

Instrumentations of ten AIS patients were simulated first with major correction maneuvers using the concave rod then with convex rod. Correction maneuvers were concave/convex rod translation, followed by apical vertebral derotation and then convex/concave rod translation. The concave/convex rods were 5.5/5.5 and 6.0/5.5 mm diameter Co–Cr and contoured to 35°/15°, 55°/15°, 75°/15° and 85°/15°, respectively.

Results

Differences in simulated thoracic Cobb angle (MT), thoracic kyphosis (TK) and apical vertebral rotation (AVR) were less than 5° between the two techniques; mean bone-screw force difference was less then 15N (p > 0.1). Increasing differential contouring angle from 35°/15° to 85°/15°, the MT changed from 14 ± 7° to 15 ± 8°, AVR from 12 ± 4° to 6 ± 5°, TK from 23 ± 4° to 42 ± 4°, and bone-screw forces from 159 ± 88N to 329 ± 170N (P < 0.05). Increasing the concave rod diameter from 5.5 to 6 mm, the mean MT correction improvement for both techniques was less than 2°, the AVR correction was improved by 2°, the TK increased by 4° and bone-screw force increased by about 25N (p < 0.05).

Conclusion

There was no significant difference in deformity corrections and bone-screw forces between the two techniques. Increasing differential contouring angle and rod diameter improved AVR and TK corrections with no significant effect on the MT Cobb angle. Although this study simplified the complexity of a generic surgical technique, the main effects of a limited number of identical steps were replicated for each case in a systematic manner to analyze the main first-order effects.

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Acknowledgements

This study was financially supported by the Natural Sciences and Engineering Research Council of Canada (Discovery Grant program and Industrial Research Chair program with Medtronic of Canada), and Approval of Research Ethics Committee was obtained to conduct this study.

Funding

The Natural Sciences and Engineering Research Council of Canada (Discovery Grant program, RGPIN-2017-05618; Industrial Research Chair program with Medtronic of Canada, IRCPJ 346145–16).

Author information

Authors and Affiliations

  1. Department of Mechanical Engineering, Polytechnique Montreal, Downtown Station, P.O. Box 6079, Montreal, QC, H3C 3A7, Canada

    Xiaoyu Wang & Carl-Eric Aubin

  2. Sainte-Justine University Hospital Center, 3175 Cote Sainte-Catherine Road, Montreal, QC, H3T 1C5, Canada

    Xiaoyu Wang & Carl-Eric Aubin

  3. Children’s Mercy Hospital, 2401 Gillham Rd, Kansas City (Missouri), 64108, USA

    Richard M. Schwend

  4. Akron Children’s Hospital, 215 West Bowery Street, Akron, OH, 44308, USA

    Todd Ritzman & Lorena Floccari

Authors
  1. Xiaoyu Wang
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  5. Carl-Eric Aubin
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Contributions

XW: design, simulations, analysis, interpretation of the data for the work, drafting work, revising, final approbation, agreeing to be accountable. RS: interpretation of the data for the work, revising, final approbation, agreeing to be accountable. TR: interpretation of the data for the work, revising, final approbation, agreeing to be accountable. LF: interpretation of the data for the work, revising, final approbation, agreeing to be accountable. CÉA: design, interpretation of the data for the work, comprehensive review, final approbation, agreeing to be accountable.

Corresponding author

Correspondence to Carl-Eric Aubin.

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Conflict of interest

Xiaoyu Wang: supported by the Natural Sciences and Engineering Research Council of Canada (Industrial Research Chair with Medtronic of Canada). Richard M. Schwend: consultant for Orthopediatrics (outside the scope of the study). Todd Ritzman: grant/research support and consultant for Medtronic, honorarium from Stryker/K2M Spine and stock/shareholder for Apto Orthopaedics (all outside the scope of the study). Lorena Floccari: none. Carl-Éric Aubin: supported by the Natural Sciences and Engineering Research Council of Canada (Industrial Research Chair with Medtronic Canada), and consultant for Medtronic (outside the scope of the study). All authors declare that they have no conflict of interest.

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Wang, X., Schwend, R.M., Ritzman, T. et al. Concave rod first vs. convex rod first in AIS instrumentation with differential rod contouring: computer modeling and simulations based on ten AIS surgical cases. Spine Deform 11, 1317–1324 (2023). https://doi.org/10.1007/s43390-023-00727-5

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