Law of temporary diminishing distraction gains: the phenomenon of temporary diminished distraction lengths with magnetically controlled growing rods that is reverted with rod exchange

Abstract

Summary: This is a prospective radiographic analysis of 24 patients with Early Onset Scoliosis (EOS) who underwent magnetically controlled growing rod (MCGR) surgery and distracted monthly. We demonstrated that achieved rod lengthening gradually reduces with gain in length of the rod. With rod exchange, the original ntended distractions can be achieved. We propose such, 'Law of Temporary Diminishing Distraction Gains' is related to rod factors and not patient factors and should be distinguished from the Law of Diminishing Returns. Hypothesis: The rate of achieved length gain with MCGR reduces with rod usage but returns to baseline after rod exchange. Design: Prospective study. Introduction: MCGRs have been shown to achieve similar clinical and radiological outcomes as traditional growing rods. Divergence between targeted (lengthening amount input in the external remote controller) and achieved (measured length on radiographs) distractions has been observed with increased distractions of the MCGR. This may be related to reduced distraction forces as the rod lengthens. The relationship of this reduced rate of achieved lengthening with remaining rod length has yet to be explored. Methods: EOS patients who underwent MCGRs with minimum 2-year follow-up during the period between 12/2009 and 5/2018 were included in this study. All patients underwent a monthly distraction protocol of 2mm at each visit. Correlation (R2) between percentage of lengthening achieved from targeted length was identified, as well as its relationship with timing of rod exchanges. Results: A total of 24 patients fulfilled the inclusion criteria. The mean age at index surgery was 9.3 years and mean postoperative follow-up was 56 ± 30 months. Of these, 10 patients had at least one rod exchange which occurred at 39 ± 14 months. A decrease in rate of achieved lengthening was observed when compared with targeted distractions. The achieved lengthening drops from 86% of targeted length at the first distraction to only 50.3% at the 19th distraction episode for the first set of rods. After rod exchange, the average achieved lengthening went back up to 81.3% of the targeted length but subsequently had a gradual reduction to 35% at the 19th distraction episode. Conclusion: We propose a 'Law of Temporary Diminishing Distraction Gains' with the MCGR that users should be aware of when monitoring rod lengthening. Diminishing distraction length gains with increased length of rod is observed which is independent of patient factors. This is only temporary as the rates of achieved lengthening returns to baseline after rod exchange.