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Current benchtop protocols are not appropriate for the evaluation of distraction-based growing rods: a literature review to justify a new protocol and its development

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Abstract

Purpose

Although distraction-based growing rods (GR) are the gold standard for the treatment of early onset scoliosis, they suffer from high failure rates. We have (1) performed a literature search to understand the deficiencies of the current protocols, (2) in vitro evaluation of GRs using our proposed protocol and performed a finite element (FE) model validation, and (3) identified key features which should be considered in mechanical testing setups.

Methods

PubMed, Embase, and Web of Science databases were searched for articles published on (a) in vivo animal, in vitro cadaveric, and biomechanical studies analyzing the use of GRs as well as (b) failure mechanisms and risk factors for GRs. Both FE and benchtop models of a proposed TGR test construct were developed and evaluated for two cases, long tandem connectors (LT), and side-by-side connectors (SBS). The test construct consisted of five polymer blocks representing vertebral bodies, joined with springs to simulate spinal stiffness. The superior and inferior blocks accepted the pedicle screw anchors, while the three middle blocks were floating. After the pedicle screws, rods, and connectors were assembled onto this construct, distraction was performed, mimicking scoliosis surgery. The resulting distracted constructs were then subjected to static compression-bending loading. Yield load and stiffness were calculated and used to verify/validate the FE results.

Results

From the literature search, key features identified as significant were axial and transverse connectors, contoured rods, and distraction, distraction being the most challenging feature to incorporate in testing. The in silico analyses, once they are validated, can be used as a complementing technique to investigate other anatomical features which are not possible in the mechanical setup (like growth/scoliosis curvature). Based on our experiment, the LT constructs showed higher stiffness and yield load compared to SBS (78.85 N/mm vs. 59.68 N/mm and 838.84 N vs. 623.3 N). The FE predictions were in agreement with the experimental outcomes (within 10% difference). The maximum von Mises stresses were predicted adjacent to the distraction site, consistent with the location of observed failures in vivo.

Conclusion

The two-way approach presented in this study can lead to a robust prediction of the contributing factors to the in vivo failure.

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Funding

The work was supported in part by NSF Industry/University Cooperative Research Center at The University of California at San Francisco, San Francisco, CA, The University of Toledo, Toledo, OH, and The Ohio State University, Columbus, OH (www.nsfcdmi.org).

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Authors and Affiliations

  1. Departments of Bioengineering and Orthopaedic Surgery, Engineering Center for Orthopedic Research Excellence (E-CORE), Colleges of Engineering and Medicine, University of Toledo, 2801 West Bancroft Street, MS 303, NI Hall, Room 5046, Toledo, OH, 43606, USA

    Niloufar Shekouhi, Amey Kelkar, David Dick & Vijay K. Goel

  2. DePuy Synthes Spine, 325 Paramount Drive, Raynham, MA, 02767, USA

    Derek Shaw

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  1. Niloufar Shekouhi
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  4. Vijay K. Goel
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Contributions

All authors have read and approved the final submitted manuscript. NS and AK acquired the data, reviewed the literature, and drafted the manuscript. DD provided feedback, assisted with data acquisition, and revised the manuscript. VKG and DS edited the manuscript and served as mentors to NS.

Corresponding author

Correspondence to Vijay K. Goel.

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Appendix

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See Table

Table 5 In vitro and in vivo animal studies on the distraction-based growing rods. TGR, MCGR, and SMA represent traditional growing rods, magnetically controlled growing rods, and shape memory alloy
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5.

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Shekouhi, N., Kelkar, A., Dick, D. et al. Current benchtop protocols are not appropriate for the evaluation of distraction-based growing rods: a literature review to justify a new protocol and its development. Eur Spine J 31, 963–979 (2022). https://doi.org/10.1007/s00586-022-07113-1

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