• CheungJason Pui YinJPYhttps://orcid.org/0000-0002-7052-087525809The University of Hong Kong, Pokfulam, Hong Kong, SAR, China., Cora Bow, and CheungKenneth M CKMC25809The University of Hong Kong, Pokfulam, Hong Kong, SAR, China..
• 25809The University of Hong Kong, Pokfulam, Hong Kong, SAR, China.
• Global Spine J. 2020 Aug 17: 2192568220948475.

Study DesignProspective study.ObjectivesTo study the increasing divergence between targeted and achieved distractions observed with magnetically controlled growing rod (MCGR)lengthening, and the relationship of this reduced rate of achieved lengthening with remaining rod length.MethodsPatients with early onset scoliosis (EOS) who underwent MCGRs with minimum 2-year follow-up were consecutively enrolled. Targeted and achieved lengths were compared. Correlation between percentage of lengthening achieved from targeted length was identified with the timing of rod exchanges.ResultsA total of 20 patients fulfilled the inclusion criteria. The mean age at index surgery was 9.5 years and mean postoperative follow-up was 68 ± 28 months. Of these, 8 patients had at least one rod exchange that occurred at 23 ± 4 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 58.8% at the 21st 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.ConclusionsWe propose a "law of temporary diminishing distraction gains" that MCGR users should be aware of when monitoring rod lengthening. Diminishing distraction length gains is observed as the rod is lengthened and is a phenomenon independent of patient factors. It is only temporary as the rates of achieved lengthening returns to baseline after rod exchange.

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