Quantifying the ‘law of diminishing returns’ in magnetically controlled growing rods

Bow

2020

Global Spine Journal

Study Design: Prospective study. Objectives: To 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. Methods: Patients 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. Results: A 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. Conclusions: We 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.

Section: Discussionmentioning

confidence: 99%

“Law of Temporary Diminishing Distraction Gains”: The Phenomenon of Temporary Diminished Distraction Lengths With Magnetically Controlled Growing Rods That Is Reverted With Rod Exchange

Bow

2020

Global Spine Journal

“…Patients were distracted at 3-month intervals and had fluoroscopic images taken immediately after the distraction. There was also a significant decrease in T/I ratio which was a gradual linear decline over time [10].…”

Section: Discussionmentioning

confidence: 91%

Does the external remote controller’s reading correspond to the actual lengthening in magnetic-controlled growing rods?

et al. 2020

Purpose Magnetic-controlled growing rods (MCGRs) are now routinely used in many centres to treat early-onset scoliosis (EOS). MCGR lengthening is done non-invasively by the external remote controller (ERC). Our experience suggests that there may be a discrepancy between the reported rod lengthening on the ERC and the actual rod lengthening. The aim of this study was to investigate this discrepancy. Methods This was a prospective series. Eleven patients who were already undergoing treatment for EOS using MCGRs were included in this study. Results One hundred and ninety-two sets of ultrasound readings were obtained (96 episodes of rod lengthening on dual-rod constructs) and compared to their ERC readings. Only 15/192 (7.8%) readings were accurate; 27 readings (14.9%) were false positive; and 8 readings (4.2%) were an underestimation while 142 readings (74.0%) were an overestimation by the ERC. Average over-reporting by the ERC was 5.31 times of the actual/ultrasound reading. When comparing interval radiographs with lengthening obtained on ultrasound, there was a discrepancy with an average overestimation of 1.35 times with ultrasound in our series. There was a significant difference between ERC and USS (p = 0.01) and ERC and XR (p = 0.001). However, there was no significant difference between USS and XR (p > 0.99). Conclusion The reading on the ERC does not equate to the actual rod lengthening. The authors would recommend that clinicians using the MCGR for the treatment of early-onset scoliosis include pre-and post-extension imaging (radiographs or ultrasound) to confirm extension lengths at each outpatient extension. In centres with ultrasound facilities, we would suggest that patients should have ultrasound to monitor each lengthening after distraction but also 6-month radiographs.

“…44 Ahmad et al identified a near linear decrease in distraction over time. 42 Their analysis also demonstrated that after initial implantation of the MAGEC, there was no significant improvement in the coronal plane deformity during follow-up. The authors suggested that spinal distraction with MAGEC also follows the "law of diminishing returns".…”

Section: Magec In Cohort Studies: Clinical and Radiological Resultsmentioning

confidence: 98%

“…40,41 In a recent meta-analysis, there was no significant difference in the change in Cobb angle or incidence of complications between EOS patients treated by MAGEC or TGRs. 41 The variation reported in true distraction achieved compared to the desired distraction with MAGEC was investigated by Ahmad et al 42 Actual distraction with MAGEC had been reported between 33% and 45.5% of intended distraction. 37,43 For TGRs, increased spinal stiffness and spontaneous fusion over the duration of treatment can lead to the "law of diminishing returns" upon repeated lengthenings, whereby repeated lengthenings yield a decrease in the true distraction achieved.…”

Section: Magec In Cohort Studies: Clinical and Radiological Resultsmentioning

confidence: 99%

<p>Magnetic Controlled Growth Rods in the Treatment of Scoliosis: Safety, Efficacy and Patient Selection</p>

Roberts

2020

MDER

Early-onset scoliosis is defined as a spinal curvature greater than 10°in children prior to 10 years of age. Untreated EOS may lead to progressively severe spinal deformity, impaired pulmonary development, restrictive lung disease, and both increased morbidity and mortality. Limitations of established conservative treatments include inability to correct severe deformity, as well as challenges with compliance when casting and bracing is applied. In addition, surgical treatment in the form of traditional growing rods requires regular surgical lengthenings and is associated with complications inherent with repeated invasive procedures and exposure to general anesthesia. MAGEC is an evolving magnetically controlled growing rod system for the treatment of EOS. After initial implantation, lengthening is achieved non-invasively by using magnetic external remote control. MAGEC offers the potential to control moderate and severe EOS, while avoiding repeated surgical procedures and associated complications. In this review, we examine the results from clinical, radiological and explant studies following the use of MAGEC, in the context of other established and emerging treatments for EOS.