Analysing a mechanism of failure in retrieved magnetically controlled spinal rods

Vasiliki C. Panagiotopoulou, Stewart K. Tucker, Robert K. Whittaker, Harry S. Hothi, Johann Henckel, Julian J. H. Leong, Thomas Ember, John A. Skinner, Alister J. Hart

We aim to describe a mechanism of failure in magnetically controlled growth rods which are used for the correction of the early onset scoliosis. This retrieval study involved nine magnetically controlled growth rods, of a single design, revised from five patients for metal staining, progression of scoliosis, swelling, fractured actuator pin, and final fusion. All the retrieved rods were radiographed and assessed macroscopically and microscopically for material loss. Two implants were further analysed using micro-CT scanning and then sectioned to allow examination of the internal mechanism. No funding was obtained to analyse these implants. There were no potential conflicts interests. Plain radiographs revealed that three out of nine retrieved rods had a fractured pin. All had evidence of surface degradation on the extendable telescopic rod. There was considerable corrosion along the internal mechanism. We found that a third of the retrieved magnetically controlled growth rods had failed due to pin fracture secondary to corrosion of the internal mechanism. We recommend that surgeons consider that any inability of magnetically controlled growth rods to distract may be due to corrosive debris building up inside the mechanism, thereby preventing normal function.