“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

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.

The evolving stall rate of magnetically controlled growing rods beyond 2 years follow-up

“…The mechanistic reason for MCGR stalling is not well understood in the literature. A postulate is that the magnetic actuator generates an insufficient force to continue to lengthen the spine over time with successive lengthening sessions resulting in decrease yield or complete failure to lengthen [ 9 , 11 , 12 ]. This “law of diminishing returns” has been proposed to represent progressive stiffness or even auto-fusion of spanned spinal segments over the treatment course [ 11 ], as has been reported in the TGR literature [ 13 , 14 ].…”

“…This “law of diminishing returns” has been proposed to represent progressive stiffness or even auto-fusion of spanned spinal segments over the treatment course [ 11 ], as has been reported in the TGR literature [ 13 , 14 ]. However, it has also been reported that this diminished distraction gain is rather a progressive failure of magnetic actuator that is reversed with MCGR exchange [ 12 ]. Explant analysis of MCGR implants have identified that duration of implantation is directly correlated with the extent of lengthening with implanted MCGRs of greater duration being significantly less likely to be functional at the time of explant [ 6 ].…”

Premature spinal fusion after insertion of magnetically controlled growing rods for treatment of early-onset scoliosis: illustrative case

“…14 Long-term studies have shown a similar concept of diminishing returns that is inherent to the rod rather than the patient. 15 Cheung et al 15 illustrated increasing mismatch between the intended and observed lengthening of the rods with increasing lengthening, which is reset with rod exchange. In addition, Rushton et al 16 tested the force generated by the rods with correlation to the time from implantation.…”

A biomechanical study on the effect of lengthening magnitude on spine off-loading in magnetically controlled growing rod surgery: Implications on lengthening frequency

“…Furthermore, Cheung et al proposed the “law of temporary diminishing distraction gains” for monthly distracted MCGRs, which suggests diminishing distraction length gain is independent of patient factors, and normal distraction length is again achieved after rod exchange. 14,54 Whether these findings apply to clinicians who distract on a three- or six-monthly basis remains to be seen. Our study demonstrated that neither lengthening regimen increases off-loading from the spine compared to the other; however, damage to the spine’s tissues was observed when larger lengthening regimens were applied.…”