Analysis of 280 Magnetically Controlled Growing Rod Lengthenings Comparing External Remote Control Readouts and Radiographic Measurements: Impact of Patient and Deformity Factors

Background:This study aimed to assess the accuracy of the external remote control (ERC) digital readout to the radiographic measurement of magnetically controlled growing rod (MCGR) actuators undergoing lengthenings/expansions and to analyze the impact of patient and spinal deformity factors on the accuracy of the ERC digital readout.Methods:Single-surgeon database at 2 tertiary-care pediatric hospitals identified early-onset scoliosis patients with MCGR constructs. There were 14 males and 16 females with a minimum of 2 lengthenings included in the analysis. ERC readouts at each lengthening were recorded.Results:Sixty-six MCGR actuators were assessed in 30 patients which underwent 280 lengthenings (mean of 5.7 lengthenings). At individual lengthening sessions the ERC and radiographic measurements were correlated (r = 0.69, P<0.001). The summed radiographic measurements averaged 15% less than the summed ERC readouts (SD = 19%). The difference between the sum of radiographic measurements and ERCs was positively correlated with subsequent lengthenings (r = 0.94, P = 0.015). The accuracy of the ERC did not improve with increased grouped sessions (r = -0.54, P = 0.388) with no difference between the group with 2 and the group with ≥5 lengthenings (P = 0.670). In patients with dual rods, the difference was correlated between rods (r = 0.66, P<0.001) and the distraction of each rod was not significantly different from the other (P = 0.124). Patient weight correlated with the difference between the radiographs and ERC readouts (r = 0.27, P = 0.032). In patients with multiple grouped lengthenings, increased weight was positively correlated with an increase in measurement difference (mean r = 0.972).Conclusions:Summed radiographic measurements during lengthening sessions averaged 15% less than the summed ERC readouts. The accuracy of radiographic measurements did not increase when sessions were grouped together (2 vs. ≥5). There was no difference in the ERC accuracy over lengthening sessions early versus late. Patient weight, but not body mass index, was positively correlated with the discrepancy between the ERC and radiographic measures.Level of Evidence:Level III.