Actuator Size of Magnetic Controlled Growth Rod (7 cm vs. 9 cm) is Not Predictive of Unplanned Return to the Operating Room: A Retrospective Multicenter Comparative Cohort Study

Background: Magnetic controlled growth rods (MCGR) are the most common type of implant used for operative treatment of patients with early-onset scoliosis (EOS). Rods can have either a 7-cm actuator, allowing 2.8 cm of potential expansion, or a 9-cm actuator which allows 4.8 cm potential expansion. We hypothesized that the rate of unplanned return to the operating room (UPROR) will be increased when the 9-cm actuator is implanted in smaller patients. In addition, we aimed to identify a cutoff for spine length between planned upper and lower instrumented MCGR levels that best differentiated between patients having a high versus low risk of UPROR. Methods: We identified 167 patients from a prospectively collected registry of EOS patients who began MCGR treatment at 9 years of age or younger, with greater than 1 year of follow-up, and had adequate radiographs. Demographic, clinical, and surgical characteristics were analyzed for 7-cm and 9-cm actuator patients. Chi-square tests and Student t tests were used to test for differences between the 2 actuator rod groups. A predictive model for UPROR within 2 years was developed based on variables significantly predictive of UPROR. Results: The average follow-up was 2.6 years (range, 1 to 5 y) in both the 7 cm (n=74) and 9 cm (n=93) groups. Twenty-five complications in 14 patients led to UPROR within 2 years of MCGR insertion, 8% incidence (95% CI, 4%-13%). Device-related complications (n=15) were the most common reason for UPROR, followed by wound complications (n=4), pain-related complications (n=3), junctional kyphosis (n=2), and incarcerated umbilical hernia (n=1). After adjusting for age, spine height, number of spine anchors, sex, and diagnosis, there was no significant difference in UPROR rates between groups. Fewer proximal anchors, smaller T1-S1 height, and more caudal mid-point of primary coronal curvature were significantly associated with UPROR in the predictive model. Conclusion: MCGR actuator size is not a significant factor in predicted UPROR. Smaller height, fewer anchors, and caudal apex increased UPROR risk. Level of Evidence: This is a retrospective, multicenter comparative cohort study (Level III therapeutic).