Magnetically controlled growing rods (MCGR’s) have revolutionized the treatment of early-onset scoliosis (EOS) because painless lengthenings can be done in the outpatient clinic without anesthesia. Untreated EOS leads to respiratory insufficiency and reduced life expectancy. However, MCGR’s have inherent complications like non-functioning of the lengthening mechanism. We quantify an important failure mechanism and give advice on how to avoid this complication. The magnetic field strength was measured on new/explanted rods at different distances between the external remote controller and the MCGR and likewise in patients before/after distractions. The magnetic field strength of the internal actuator decayed fast with increasing distances and plateaued at 25–30 mm approximating zero. Two new and 12 explanted MCGRs was used for the lab measurements of the elicited force using a forcemeter. At a distance of 25 mm, the force was reduced to approximately 40% (ca. 100 N) compared to zero distance (ca. 250 N), most so for explanted rods. This is used to point out the importance of minimizing the implantation depth to ensure proper functionality of the rod lengthening in clinical use for EOS patients. A distance of 25 mm from skin to MCGR should be considered a relative contraindication to clinical use in EOS patients.