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.