Temporary reductions in distraction lengthening occurs in magnetically controlled growing rods: a phenomenon that defies the law of diminishing gains

Abstract

Background: Divergence between targeted and achieved distractions has been observed with increased distractions of magnetically controlled growing rods (MCGR). This may be related to reduced distraction forces as the rod lengthens. Relationship of this reduced rate of achieved lengthening with remaining rod length has yet to be explored. Methods: Early-onset scoliosis patients who underwent MCGR with minimum 2-year follow-up were consecutively enrolled in this prospective study. All patients underwent monthly distraction protocol of 2 mm at each visit, where targeted and achieved lengths were compared. Correlation (R2) between percentage of lengthening achieved from targeted length was identified, as well as its relationship with timing of rod exchange. Results: Twenty patients fulfilled the inclusion criteria. Mean age at index surgery was 9.5 years and mean postoperative follow-up was 68 (standard deviation=28) months. Eight patients had at least one rod exchange which occurred at 23 (standard deviation=4) months. The mean achieved lengthening dropped 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, this figure went back up to 81.3% but subsequently gradually decreased to 35% at the 19th distraction episode. Conclusions: We propose a ‘Law of Temporary Diminishing Distraction Gains’ with MCGR, independent of patient factors, where diminishing distraction length gain was observed as the rod was lengthened. Rate of achieved lengthening returned to baseline after rod exchange. This should be distinguished from the ’law of diminishing returns’ unique to traditional growing rods.