J Korean Soc Spine Surg.  2001 Jun;8(2):130-135. 10.4184/jkss.2001.8.2.130.

Bending Stiffness of Rod in Pedicle Screw Systems

• Suk SI
• Kim JH
• Lee SM
• Chung ER
• Jin GH
• Cho ST
• Lee SJ
• Kim BJ
• Lee JH
• Sohn HM

• KMID: 2097752
• DOI: http://doi.org/10.4184/jkss.2001.8.2.130

Abstract

PURPOSE: To measure bending stiffness of rod in pedicle screw systems and identify the optimal rod for surgical correction of spinal deformities.
MATERIALS AND METHODS
Bending stiffness of six different rods -7 mm stainless steel, 6 mm titanium alloy, 6.35 mm titanium, 5.5 mm titanium, 6 mm Shape Memory Alloy after martensite temperature, 7 mm Shape Memory Alloy after martensite temperature were measured by MTS 858 Bionix test system according to the ASTM standards. The specimen number was 8 for each rod. Young's modulus of elasticity was calculated from load-displacement data.
RESULTS
Seven-mm stainless steel rod was stiffer than any other rods with bending stiffness of 143.7 +/- 3.8 N/mm, and also showed largest Young's modulus of elasticity of 135.1 +/- 3.0 GPa. Six-mm Shape Memory Alloy rod after martensite temperature was most flexible with bending stiffness of 58.1 +/- 2.8 N/mm, and showed smallest Young's modulus of elasticity of 68.0 +/-2.1 GPa. Seven-mm Shape Memory Alloy rod after martensite temperature was similar to the 6.35 mm titanium and 6 mm titanium alloy rod in bending stiffness and Young's modulus of elasticity.
CONCLUSION
Seven-mm stainless steel rod was the stiffest rod tested, which is necessary to improve correction rate and maintain achieved correction. However, the rod selection should be individulized since stiffer rod might increase the chance of acute failure. Seven-mm Shape Memory Alloy could be more useful in deformity correction than 6 mm Shape Memory Alloy since it was not only similar to other rods in stiffness but also had shape memory function which would be necessary to establish the desired position of the spine.

Keyword

Pedicle screw; Rod; Bending stiffness; Shape memory alloy

MeSH Terms

Alloys
Congenital Abnormalities
Elastic Modulus
Elasticity
Memory
Spine
Stainless Steel
Titanium
Alloys
Stainless Steel
Titanium

Figure

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  Fig. 1-A. Schematic diagram of three point bending test. D is the diameter of rod specimen tested. Fig. 1-B. Before bending test. Fig. 1-C. After bending test.

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  Fig. 2-A. Bending stiffness. Fig. 2-B. Young's modulus of elasticity.

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