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Growth Guidance System for Early-Onset Scoliosis: Comparison of Experimental and Retrieval Wear

Singh, Vaneet MS*; Simpson, Joshua BS*; Rawlinson, Jeremy PhD*; Hallab, Nadim PhD†,‡

doi: 10.1097/BRS.0b013e31829991e0

Study Design. Laboratory study conducted using an in vitro wear simulator with a growth guidance system. Analysis of variance performed to compare in vitro specimens (n = 6) with in vivo retrieval components (n = 5).

Objective. To characterize the stainless steel, wear debris potential of a spinal growth guidance system by developing an in vitro model and validating tested implants with retrospectively obtained retrievals.

Summary of Background Data. Growth enabling, surgical treatments have been developed to provide fusionless options for patients with early-onset scoliosis. There exist few data regarding the wear debris associated with such spinal systems.

Methods. In this study, we determined in vitro wear from the stainless steel components of the SHILLA™ Growth Guidance System. An analogue lumbar spine model was adapted from ISO 12189:2008 to assess the growth guidance system. In a multistation wear simulator, 6 assembled constructs were tested under displacement control for 5 million cycles (Mc) with diluted bovine serum, and the wear was measured gravimetrically at end of the test. The components were compared quantitatively for wear scar depth with retrieved growth guidance implants (n = 5), and qualitatively for wear, corrosion, and other surface damage.

Results. The average total wear rate over 5 Mc was 0.39 ± 0.13 mm3/Mc (3.12 ± 1.01 mg/Mc) with an average particle size of 1.3 μm in equivalent circular diameter. Prominent wear scars were noticed on both the tested and retrieved specimens with no statistical difference in the wear scar depths of the tested and retrieved components when set and multiaxial screws when compared collectively.

Conclusion. An in vitro wear analysis for a spinal growth guidance system was conducted using a novel protocol and validated against retrieved implants. This is the first study establishing a baseline value for the wear of “growth enabling” devices for the treatment of early-onset scoliosis.

Novel in vitro wear testing was conducted for a spinal growth guidance system and compared with in vivo retrieved implants. The average total wear rate over 5 million cycles (Mc) was 0.39 ± 0.13 mm3/Mc (3.12 ± 1.01 mg/Mc) with an average particle size of 1.3 μm in equivalent circular diameter. In vitro wear scars observed on the tested components were not statistically different than retrievals.

*Medtronic Inc., Memphis, TN

Bioengineering Solutions Inc., Oak Park, IL; and

Rush University, Chicago, IL.

Address correspondence and reprint requests to Vaneet Singh, MS, Medtronic Inc, 2600 Sofamor Danek Dr, Memphis 38132, TN; E-mail:

Acknowledgment date: October 23, 2012. Revision date: March 26, 2013. Acceptance date: April 29, 2013.

The device(s)/drug(s) that is/are the subject of this manuscript is/are not FDA-approved for this indication and is/are not commercially available in the United States.

Medtronic funds were received in support of this work.

Relevant financial activities outside the submitted work: consultancy, employment.

© 2013 by Lippincott Williams & Wilkins