Outcomes of Primary and Conversion Magnetically Controlled Growth Rods Are Different at Two-Year Follow-up: Results of North American Release

Study design: Multicenter retrospective review. Objectives: To compare the radiographic outcomes and complication rates in patients with primary and conversion magnetically controlled growing rod (MCGR) implants at one and two years after surgery. Summary of background data: Many initial early-onset scoliosis (MCGR) implantations in the United States were conversions from other types of growth-friendly systems, and the outcome similarities and differences between primary and conversion MCGR implantation procedures are still relatively unknown. Methods: Multicenter retrospective review of EOS patients from two multicenter EOS registries identified consecutive EOS patients treated from 2014 to 2017 with a minimum of one-year follow-up. In addition, a subset of these patients who had two-year follow-up were further analyzed. Results: In total, 383 MCGR patients were identified, of which 272 (71%) were primary (P) and 111 (29%) were conversion (C). Group P patients had significantly greater coronal curves at the time of MCGR implantation and greater initial coronal correction. There was no statistically significant difference in Cobb correction at one year or between follow-up at one and two years. The preimplantation thoracic spine height was identical in both groups, with statistically greater improvement at initial implantation in P than in C patients. Significantly greater height gains were seen in P than in C patients in the one-year follow-up cohort. There was a higher rate of complications in the C group than in the P group; however, the difference was not statistically significant. Overall, most complications were implant-related. No loss of curve correction occurred in either group. Conclusions: Patients with primary MCGR insertion can be expected to have greater radiographic correction and spine length gain than those with conversion from growth-friendly instrumentation to MCGR, most likely because of increased spine stiffness in conversion patients. The rate of complications, primarily implant-related, remains higher in conversion than in primary insertion patients. Level of Evidence: Level III.