How much does depth matter? Magnetically controlled growing rod distraction directly influenced by rod tissue depth.

PURPOSE: Magnetically controlled growing rod (MCGR) for the treatment of early-onset scoliosis (EOS) is a relatively innovative technique. MCGR benefits over traditional growing rods are known but limitations and complications are being revealed. The purpose of this study was to examine the importance of tissue depth on rod lengthening. METHODS: A single-institution retrospective review of 72 MCGR patients was performed. Ultrasound measured rod distraction. Differences in programmed and actual distraction, and complications were recorded. Tissue depths and achieved length were averaged and used to construct a regression to account for variability. RESULTS: Percentage of std and offset orientation rod lengthening relative to the programmed distraction was inversely proportional to rod depth (std R = 0.50, p = 0.002) (offset R = 0.60, p < 0.001). Expected std rod lengthening achieved decreased by 1.46%/mm depth. Expected offset rod lengthening achieved decreased by 1.68%/mm depth. 28 pts (38.9%) sustained complications. Age, sex, BMI, standard tissue depth, and/or offset tissue depth had no predictive ability with respect to complications sustained (overall model R = 0.31, p = 0.36). CONCLUSION: In a series of EOS surgical patients treated with MCGRs, the relationship between percentage of programmed lengthening achieved as well as total lengthening was inversely proportional to tissue depth of the rod. There was a trend towards increasing frequency of complications recorded with decreasing tissue depth though this was not significant. These data can help with surgical planning during MCGR placement.

Observed Length Increases of Magnetically Controlled Growing Rods are Lower Than Programmed.

BACKGROUND: Magnetically controlled growing rods (MCGRs) are increasingly used in the treatment of early onset scoliosis (EOS). Few studies have reported whether desired lengthening can reliably be achieved, or if prior spine instrumentation and large tissue depths affect lengthening. In this clinical study of EOS patients, it was hypothesized that increases in rod length would equal programmed increases, patients with prior spine instrumentation would lengthen less than patients without prior surgery, and larger tissue depths would decrease lengthening success. METHODS: A retrospective chart review was conducted on EOS patients with single and dual MCGRs placed between April 2014 to September 2015 and distracted at a single institution. Rod distraction was measured at each visit using ultrasound. Differences between programmed and actual distraction for each patient, and between groups with and without prior spine instrumentation, were determined by 2-tailed t tests. Regression and correlation were used to determine the relationship between tissue depth and length increases. RESULTS: Thirty-one patients were included, 18 males, 13 females, age 8.1 (±2.5) years, with major curves measuring 60 (±14.6) degrees at time of MCGR insertion. In the 12 patients with prior instrumentation, time from initial growing rod placement to MCGR insertion was 23.1 (±10.6) months. The number of surgical procedures before MCGR insertion was 2.8 (±2.0). Total length increase relative to the programmed distraction was 86% (±21) (P<0.001). Length increases for patients with and without prior surgery were 87% (±23) and 86% (±19), respectively (P>0.9). Total lengthening was inversely proportional to tissue depth (r=0.38, P<0.01); the decrease in lengthening achieved was 2.1%/mm of tissue depth. CONCLUSIONS: Increases in rod length were 14% lower than the programmed distraction. Prior instrumentation did not impact the amount of rod distraction. Greater distance between the rod and the skin surface negatively affected the magnitude of distraction.