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Mechanical Performance of Posterior Spinal Instrumentation and Growing Rod Implants

Experimental and Computational Study

Foltz, Mary H. MS∗,†,‡,§; Freeman, Andrew L. MS; Loughran, Galyna PhD; Bechtold, Joan E. PhD∗,†,‡; Barocas, Victor H. PhD; Ellingson, Arin M. PhD†,§,¶; Polly, David W. Jr MD

doi: 10.1097/BRS.0000000000003061
BIOMECHANICS
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Study Design. Experimental and computational study of posterior spinal instrumentation and growing rod constructs per ASTM F1717-15 vertebrectomy methodology for static compressive bending.

Objective. Assess mechanical performance of standard fusion instrumentation and growing rod constructs.

Summary of Background Data. Growing rod instrumentation utilizes fewer anchors and spans longer distances, increasing shared implant loads relative to fusion. There is a need to evaluate growing rod's mechanical performance. ASTM F1717-15 standard assesses performance of spinal instrumentation; however, effects of growing rods with side-by-side connectors have not been evaluated.

Methods. Standard and growing rod constructs were tested per ASTM F1717-15 methodology; setup was modified for growing rod constructs to allow for connector offset. Three experimental groups (standard with active length 76 mm, and growing rods with active lengths 76 and 376 mm; n = 5/group) were tested; stiffness, yield load, and load at maximum displacement were calculated. Computational models were developed and used to locate stress concentrations.

Results. For both constructs at 76 mm active length, growing rod stiffness (49 ± 0.8 N/mm) was significantly greater than standard (43 ± 0.4 N/mm); both were greater than growing rods at 376 mm (10 ± 0.3 N/mm). No significant difference in yield load was observed between growing rods (522 ± 12 N) and standard (457 ± 19 N) constructs of 76 mm. Growing rod constructs significantly decreased from 76 mm (522 ± 12 N) to 376 mm active length (200 ± 2 N). Maximum load of growing rods at 76 mm (1084 ± 11 N) was significantly greater than standard at 76 mm (1007 ± 7 N) and growing rods at 376 mm active length (392 ± 5 N). Simulations with active length of 76 mm were within 10% of experimental mechanical characteristics; stress concentrations were at the apex and cranial to connector–rod interaction for standard and growing rod models, respectively.

Conclusion. Growing rod constructs are stronger and stiffer than spinal instrumentation constructs; with an increased length accompanied a decrease in strength. Growing rod construct stress concentration locations observed during computational simulation are consistent with clinically observed failure locations.

Level of Evidence: 5

This study compared mechanical performance of posterior spinal instrumentation and growing rods instrumentation using vertebrectomy model methodology. Growing rods were stiffer and stronger. Mechanical was within 10% of computational simulations for both models with 76 mm active lengths. Maximum stress concentrations were observed at the rod–connector interface on growing rods.

Department of Biomedical Engineering, College of Science and Engineering, University of Minnesota, Minneapolis, Minnesota

Department of Orthopaedic Surgery, Medical School, University of Minnesota, Minneapolis, Minnesota

Excelen Center for Bone & Joint Research and Education, Minneapolis, Minnesota

§Division of Rehabilitation Science, Department of Rehabilitation Medicine, Medical School, University of Minnesota, Minneapolis, Minnesota

Division of Physical Therapy, Department of Rehabilitation Medicine, Medical School, University of Minnesota, Minneapolis, Minnesota.

Address correspondence and reprint requests to David W. Polly, Jr, MD, Department of Orthopaedic Surgery, University of Minnesota, 2450 Riverside Avenue South, Suite R200, Minneapolis, MN 55454; E-mail: pollydw@umn.edu

Received 11 December, 2018

Revised 26 February, 2019

Accepted 14 March, 2019

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

Product support toward this study was received from Medtronic Sofamor Danek USA. Research reported in this publication was supported by the Catherine Mills Davis Endowed Chair and the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) under Award Number K12HD073945.

No relevant financial activities outside the submitted work.

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