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Analysis of Explanted Magnetically Controlled Growing Rods From Seven UK Spinal Centers

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

doi: 10.1097/BRS.0000000000002221
BIOMECHANICS

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

Objective. To analyze explanted MAGEC rods used in management of early onset scoliosis and identify the mode of failure in such cases.

Summary of Background Data. Magnetically controlled growing rods are increasingly used as the option of choice for early onset scoliosis. However, being more complex than conventional growing rods they are perhaps more likely to succumb to multifarious failure modes. In addition, metallosis has been reported around failed MAGEC rods.

Methods. Explanted MAGEC rods from seven UK spinal centers were obtained for independent analysis. Thirty-four MAGEC rods, from 18 children, explanted for reasons including failure of rod lengthening and maximum rod distraction reached, were cut open to allow internal components to be evaluated and assessed.

Results. Externally, all MAGEC rods showed localized marks, which were termed “growth marks” as they indicated growth of the rod in vivo, on the extending bar component. After cutting open, titanium wear debris was found inside all 34 (100%) MAGEC rods. Ninety-one percent (31/34) of MAGEC rods showed measurable wear of the extending bar, towards the magnet end. Substantial damage to the radial bearing was seen inside 74% (25/34) of MAGEC rods while O-ring seal failure was seen in 53% (18/34) of cases. In 44% (15/34) of MAGEC rods the drive pin was fractured but this was felt to be an effect of rod failure, not a cause.

Conclusion. The combination of high volumes of titanium wear debris alongside O-ring seal damage likely accounts for the metallosis reported clinically around some MAGEC rods. Based on this explant data, a failure mechanism in MAGEC rods due to the natural off axis loading in the spine was proposed. This is the largest data set reporting a complete analysis of explanted MAGEC rods to date.

Level of Evidence: 4

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

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

Address correspondence and reprint requests to Thomas J. Joyce, PhD, School of Mechanical and Systems Engineering, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK; E-mail: Thomas.joyce@ncl.ac.uk

Received 25 January, 2017

Revised 24 March, 2017

Accepted 29 March, 2017

The device is FDA-approved or approved by corresponding national agency for this indication.

Newcastle University/Engineering and Physical Sciences Research Council ‘Prosperity Outcomes’ award (EP/P511201/1) funds were received in support of this work.

No relevant financial activities outside the submitted work.

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