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 the 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.