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Force Testing of Explanted Magnetically Controlled Growing Rods

Rushton, Paul R.P., FRCS (Tr & Ortho); Smith, Simon L., PhD; Forbes, Liam; Bowey, Andrew J., FRCS (Tr & Ortho); Gibson, Michael J., FRCS (Orth); Joyce, Thomas J., PhD

doi: 10.1097/BRS.0000000000002806
BIOMECHANICS

Study Design. Laboratory analysis of explanted MAGnetic Expansion Control (MAGEC) growing rods.

Objective. The aim of this study was to measure the force produced by explanted MAGEC rods compared with new rods and assess the influence of clinical variables.

Summary of Background Data. MAGEC rods are increasingly used in early-onset scoliosis. Some data are available describing the structure of explanted MAGEC rods, but to date, no study has assessed their function.

Methods. Explanted MAGEC rods were received from seven UK and one Danish center. The force produced by explanted rods on activation with the external remote controller was measured using a dedicated jig. Forces were compared with two unused rods as well as the manufacturer's defined standard (42 Ibf). Clinical variables were collected from contributing centers where possible and correlated with the force measurements.

Results. Forty-five MAGEC rods from 25 cases were received for analysis. The mean age at insertion was 8.6 years and rods were in vivo for a mean of 2.7 years in predominantly dual rod constructs. Two unused MAGEC rods produced a mean force of 45.3 (0.25) and 50.2 (1.4) Ibf, above the manufacturer's stated standard. Of the 45 explanted rods, 10 (22%) produced force greater or equal to manufacturer's standard, mean 46.7 (2.7) Ibf. Six rods (13%) produced some force but less than the manufacturer's standard, mean 34.8 (3.6) Ibf. Twenty-nine rods (64%) produced no force. The duration the rods were in vivo was significantly negatively correlated with the force produced on testing (r = −0.63, P < 0.005). Of the 12 rods implanted longer than 38 months, none produced any force.

Conclusion. This is the first study of the force, and hence likely function, of explanted MAGEC rods. The majority of explanted rods produced no force, while others produced reduced force. These findings raise questions regarding the longevity of the implant and further clinical outcome studies are recommended.

Level of Evidence: 4

45 explanted MAGEC rods from 25 patients were independently analysed to assess the force produced by the implants. The majority of explanted MAGEC rods produced no force. Less than a quarter of rods were able to produce force comparable to the pre-implantation state or manufacturer's standard.

Great North Children's Hospital, Royal Victoria Infirmary, Newcastle upon Tyne, UK

School of Mechanical and Systems Engineering, Newcastle University, Newcastle upon Tyne, UK.

Address correspondence and reprint requests to Paul R.P. Rushton, FRCS (Tr & Ortho), Great North Children's Hospital, Royal Victoria Infirmary, Newcastle upon Tyne, UK NE1 4LP; E-mail: paulrushton@doctors.org.uk

Received 1 May, 2018

Revised 27 June, 2018

Accepted 3 July, 2018

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

No funds were received in support of this work.

Relevant financial activities outside the submitted work: expert testimony.

Copyright © 2018 Wolters Kluwer Health, Inc. All rights reserved.