Design and mechanical evaluation of a novel dynamic growing rod to improve the surgical treatment of Early Onset Scoliosis

This paper describes the development of a dynamic implant designed to correct three dimensional spinal deformities in children with Early Onset Scoliosis (EOS). The device is a distraction-based implant that provides two innovative features: (1) a dynamic-viscoelastic system, which gives some flexibility to the implant, to avoid potential device fractures; and (2) a lengthening mechanism that allows minimally invasive elongation surgery to reduce infection. The novel device was designed and evaluated with finite element analysis. Five working prototypes were produced and mechanically tested according to a modified version of the ASTM F1717 Standard. Results demonstrated that the maximum load achieving run-out at 5 million cycles was 180 N. One prototype withstood almost two tests in a row (9 million cycles) with the peak load of 180 N and showed no sign of failure. Three tests performed with higher loads (190 and 200 N) did not achieve full runout due to pedicle screw fracture with only one device fracture along the weld. These results demonstrated an improved fatigue performance in comparison with data from the commercially available VEPTR I device (Depuy Synthes Spine, Raynham, USA). Therefore, the novel device proposed has the clinical potential to improve the surgical treatment of EOS.