J Korean Soc Spine Surg.  2010 Dec;17(4):177-183. 10.4184/jkss.2010.17.4.177.

Comparison of Mechanical Property of Conventional Rods versus Growing Rods for Pediatric Early Onset Scoliosis

• Kim JY
• Moon ES
• Chong HS
• Lee SJ
• Kim HS

• KMID: 2209580
• DOI: http://doi.org/10.4184/jkss.2010.17.4.177

Abstract

STUDY DESIGN: This is a mechanical study.
OBJECTIVES
We wanted to investigate the mechanical properties of newly developed dual growing rods for obtaining approval for their clinical application. SUMMARY OF LITERATURE REVIEW: The current expandable spinal implant system appears effective for controlling progressive early onset scoliosis, and it allows for spinal growth and improving lung development.
MATERIALS AND METHODS
We investigate the yield load and ultimate load during compression, tension and torsion of the growing rods and the conventional rods assembly using UHMWPE blocks, and the diameter of the rods was 6.0 mm and they expanded 5cm long. We also performed a fatigue test with growing rods, and the diameter of which was 6.0 mm and it expanded 2.5cm long. The guideline for the American Society for Testing Materials was followed during the entire mechanical test. With the growing rods and conventional rods, we tested for each mechanical property7 times with the new rods and blocks.
RESULTS
The yield load of the growing rods and conventional rods were 845.2+/-18.2 (N) and 812.9+/-29.9 (N), respectively, and the ultimate load of the growing rods and conventional rods were 961.9+/-31.1 (N) and 914.9+/-25.6 (N), respectively, when compression force was applied. The yield load and ultimate load of the growing rods were statistically higher than those of the conventional rods (p<0.05). The ultimate load of the growing rods and conventional rods were 3281.7+/-41.5 (N) and 3678.5+/-447.9 (N), respectively when tension force was applied. The ultimate load was similar for both types of rods (p>0.05). The yield loads of the growing rods and conventional rods were 11.56+/-0.59 (Nm) and 12.46+/-0.71 (Nm), respectively, the ultimate loads of the growing rods and conventional rods were 16.97+/-0.94 (Nm) and 17.42+/-2.66 (Nm) during the torsion, respectively. The yield load and ultimate load of the growing rods were statistically lower than that of the conventional rods (p<0.05).
CONCLUSIONS
The newly developed growing rods have a higher yield load and ultimate load under compression, a similar ultimate load under tension and a lower yield load and ultimate load under torsion. The differences of the yield load and ultimate load under torsion were minimal, and so the growing rods and conventional rods have similar mechanical properties.

Keyword

Early onset scoliosis; Growing rod; Mechanical study

MeSH Terms

Fatigue
Lung
Polyethylenes
Scoliosis
Polyethylenes

Figure

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  Fig.1. This picture show the standard dimension of test block follows the standards of The American Society for Testing Materials (ASTM) F1717-04.

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  Fig. 2These pictures show the basic assembly of the growing rods for static test (A), the growing rods for fatigue test (B) and the conventional rods for static test (C) with UHMWPE(Ultra-High Molecular Weight Polyethylene) blocks through the mechanical testing and this setup follows the standards of The American Society for Testing Materials (ASTM) F1717-04.

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  Fig. 3. These pictures show the bilateral construct test setup for screws and rods.

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  Fig. 4. These pictures show the test results of growing rods construct during the extension test (A), the compression test (B), and the torsion test(C).

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  Fig. 5. These diagrams show load – displacement curve of the growing rods (A) and the conventional rods (B) during compression test.

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  Fig. 6. These diagrams show load – displacement curve of the growing rods (A) and the conventional rods (B) during extension test.

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  Fig. 7. These diagrams show torque – angular curve of the growing rods (A) and the conventional rods (B) during torsion test.

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