The 'asymmetric screw sign' for magnetically controlled growing rods: a novel predictive factor for success of distraction

Abstract

Introduction: Ability to achieve magnetically controlled growing rod (MCGR) lengthening is vital for gradual spine distraction. However, both technical and mechanical factors are known to inhibit MCGR lengthening. Thus, this study aimed to identify radiological factors in the sagittal plane predicting the ability of MCGR to lengthen. Methods: 34 patients were included, with standard and offset MCGRs assessed separately. Association of kyphosis, C7 plumb line, and ratio of screw asymmetry (RSA) with distraction rate (DR) were studied. Patients with DR above and below the median were classified as ‘lower’ and ‘higher’ DR, respectively. Results: Magnitude of screw asymmetry was expressed as RSA, which when measured after initial distraction, was significantly correlated with DR among both standard (r=0.563, p<0.001) and offset MCGR (r=0.523, p<0.001). For prediction of lower DR using RSA, the area under curve of the receiver operating characteristic curve was 0.793 and 0.841 for standard and offset MCGRs, respectively. Using RSA cut-off of 0.9547, sensitivity and specificity were 72.7% for standard MCGRs; and 73.7% and 83.3%, respectively in offset MCGRs using a 0.9599 RSA cut-off. Discussion and Conclusion: We are the first to identify the asymmetric screw sign, a novel radiological sign for the prediction of MCGR lengthening ability. By classification of patients into higher and lower DR groups, RSA can guide decision making in choosing the aggressiveness of distraction regimens, while informing patient expectations on the predicted spine distraction achieved. Moreover, a prospective longitudinal study is planned to evaluate the utility of the asymmetric screw sign.