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Risk of Implant Loosening After Cyclic Loading of Fusionless Growth Modulation Techniques

Nitinol Staples Versus Flexible Tether

Yaszay, Burt MD; Doan, Joshua D. MEng; Parvaresh, Kevin C. MD; Farnsworth, Christine L. MS

doi: 10.1097/BRS.0000000000001809

Study Design. Biomechanical evaluation using porcine spines.

Objective. Compare the fixation strength of two currently used fusionless adolescent idiopathic scoliosis correction techniques following cyclic loading using porcine spines.

Summary of Background Data. The ability of fusionless implants to control or correct scoliosis in a growing patient requires such implants to maintain spinal fixation. Because they cross the disc, motion may weaken fixation over time.

Methods. Eight pig spines were divided into cycled segments (T10-T13) and uncycled segments (T7-T8, L2-L3). Initial range of motion (ROM) was determined in torsion, flexion-extension, and lateral bending (0.5°/s to 1.75 N·m).

Staple group (n = 4): 6 mm parallel staples were inserted on the right anterolateral spine across each intervertebral disc. Cycled segments received six staples (three adjacent discs) and uncycled segments received four staples (two separate discs).

Tether group (n = 4): 5.35 × 35 mm right anterolateral vertebral body screws were placed into each vertebra. Cycled segments received four screws and uncycled segments received four screws. Screws in cycled segments were connected with a flexible tether tensioned to straight alignment.

ROM of instrumented cycled segments was measured, and then segments were loaded to the measured ROM in flexion-extension (2000 cycles), lateral bending (1000 cycles), and axial rotation (2000 cycles). Implants were axially loaded to failure. Parametric tests compared pre- to postimplant ROM; nonparametric tests compared staple to screw pullout strength; P < 0.05 was significant.

Results. There were no differences in ROM before instrumentation between groups. ROM was not changed except tethers decreased left lateral bending (–6.2°). Although staple pullout was less than screw pullout for cycled and uncycled segments (P < 0.05 and P = 0.057, respectively), there was no difference in pullout strength with and without cyclic loading for either group (P = 0.4).

Conclusion. Tethers decreased lateral bending away from the tether. Screws had almost five times greater load to failure than staples. Five thousand cycles did not result in loosening of either staple or tether screws.

Level of Evidence: N/A

Division of Orthopedics, Rady Children's Hospital-San Diego, San Diego, CA

Department of Orthopedic Surgery, University of California, San Diego, CA.

Address correspondence and reprint requests to Burt Yaszay, MD, Division of Orthopedics, Rady Children's Hospital-San Diego, 3030 Children's Way, Suite 410, San Diego, CA 92123; E-mail:

Received 19 May, 2016

Accepted 23 June, 2016

The device(s)/drug(s) that is/are the subject of this manuscript is/are exempt from FDA or corresponding national regulations because this is a biomechanical study and does not involve use in patients.

Pediatric Orthopaedic Society of North America, 2014 Globus Medical QSV Research Grant funds were received in support of this work.

Relevant financial activities outside the submitted work: board membership, consultancy, grants, royalties, payment for lectures.

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