Analysing the mechanism of fracture in drive pins used in magnetically controlled growth rods

Abstract

The MAGnetic Expansion Control (MAGEC) spinal implant has transformed the treatment of early-onset scoliosis (i.e. abnormal curvature of the spine in children). Despite the innovative solution, the MAGEC device has encountered various performance issues. One component that has been frequently reported amongst the failure issues is the drive pin and the cause of these reoccurring fractures may be associated with multiple factors including over-loading, fatigue, and corrosion. This study aimed to examine the failure of eight broken drive pins from explanted MAGEC rods through fractography analysis, to better understand the factors leading to their fracture. The fracture surface of the drive pins was examined using Scanning Electron Microscopy (SEM). The clinical data were compared against observed failure modes. The fracture surfaces of four drive pins showed low cycle (high stress) or high cycle (low stress) fatigue failure mechanisms. The other four drive pins displayed environment assisted stress corrosion cracking. Almost all samples had evidence of corrosion after fracture with the amount of corrosion implying fracture occurred a relatively long time before explantation. Since multiple failure mechanisms of low cycle fatigue, high cycle fatigue or simultaneous effect of tension and corrosive environment have been observed, the fracture mechanisms illustrate the complexity of loading in these spinal growing rods. Evidence of corrosion after the failure recognises that the pins have snapped long before the device was explanted.