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Are precontoured cobalt–chromium spinal rods mechanically superior to manually contoured rods?

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

Abstract

Study design

Laboratory based study.

Objective

To compare reduction force and plastic deformation of cobalt–chromium (Co–Cr) spinal rods using a rigid, thoracolumbar spinal deformity model.

Summary of background data

Pre-contoured spinal rods are growing in their utilization for spinal deformity. Although there are theoretical advantages to pre-contouring rods, no previous studies have compared pre-contoured and manually contoured rods for their ability to maintain sagittal contour and resist mechanical load.

Methods

A spinal deformity model was utilized, simulating a rigid, thoracolumbar spinal deformity fixated with pedicle screws. Roll-formed pre-contoured and manually contoured 5.5 mm and 6.0 mm Co–Cr rods were reduced to the model with a load cell attached to the apical screw to measure corrective force. Rods remained reduced in the model for 20 min and change in contour was assessed to characterize plastic deformation.

Results

Twenty-four rods were tested with six rods per group (Table 1). The load to reduction was significantly lower in the 5.5 mm rods compared to the 6.0 mm rods (95% CI -254.0 to -61.42; p = 0.008). Although there was no difference in the corrective forces for manual and pre-contoured 5.5 mm rods (p = 0.722), the 6.0 mm rod produced significantly less corrective force compared to the manually contoured 6.0 mm rods (95% CI -134.42 to -5.317; p = 0.039). Additionally, rod contour for the manual group showed significantly less plastic deformation than the pre-contoured group in both 5.5 mm and 6.0 mm rods (5.5 mm: 57.1% vs. 61.6%, p = 0.006; 6.0 mm: 54.3% vs. 62.28%, p = 0.003).

Conclusions

Roll formed, pre-contoured Co–Cr rods demonstrated significantly greater plastic deformation when compared with manually contoured rods of the same diameter. Furthermore, 6.0 mm pre-contoured rods required significantly lower load for rod reduction, the equivalent of 15 lb-force. Post-manufacturing, roll-formed pre-contouring of larger diameter Co–Cr may impair the rods mechanical properties.

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Funding

Implants for this study were provided by OrthoPediatrics.

Author information

Authors and Affiliations

  1. Department of Orthopedic Surgery, Dwight D. Eisenhower Army Medical Center, 300 East Hospital Road, Fort Gordon, GA, 30905, USA

    K. Aaron Shaw

  2. Department of Pediatric Orthopedic Surgery, Children’s Healthcare of Atlanta, Scottish Rite Campus, Atlanta, GA, USA

    Dennis P. Devito, Michael L. Schmitz & Joshua S. Murphy

Authors
  1. K. Aaron Shaw
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  2. Dennis P. Devito
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  3. Michael L. Schmitz
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  4. Joshua S. Murphy
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Contributions

KAS: study design, data acquisition, data analysis, data interpretation, manuscript drafting, and manuscript approval. DPD, MLS, and JSM: study design, data interpretation, manuscript editing, and manuscript approval.

Corresponding author

Correspondence to K. Aaron Shaw.

Ethics declarations

Conflict of interest

Dr. Shaw is a committee member for NASS; Dr Devito receives royalties from Astura Spine, Medicrea, and SeaSpine, and is a paid consultant for Sea Spine, Medicrea, and receives research support from K2M, Mazor surgical technologies, received material support from K2M, Medtronic, and is a paid speaker for Medicrea; Dr. Schmitz is a consultant for Stryker, Orthopediatrics, and a board member for Journal of Pediatric Orthopedics, POSNA, and Scoliosis Research Society; Dr. Murphy is a consultant for Depuy and OrthoPediatrics, receives research support from OrthoPediatrics, and board member for Journal of Pediatric Orthopedics, POSNA, Spine Journal, Journal of Spine Deformity, and Scoliosis Research Society.

Ethical approval

This study was exempted from IRB review.

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Shaw, K.A., Devito, D.P., Schmitz, M.L. et al. Are precontoured cobalt–chromium spinal rods mechanically superior to manually contoured rods?. Spine Deform 8, 871–877 (2020). https://doi.org/10.1007/s43390-020-00133-1

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  • DOI: https://doi.org/10.1007/s43390-020-00133-1

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