Article Abstract

Does the law of diminishing returns apply to the lengthening of the MCGR rod in early onset scoliosis with reference to growth velocity?

Authors: Adrian Gardner, Alistair Beaven, David Marks, Jonathan Spilsbury, Jwalant Mehta, Matthew Newton Ede


Background: This study is a single centre retrospective review of prospectively collected data. The ‘law of diminishing returns’ describes the number of lengthening episodes that a traditional growth rod (TGR) can undergo before stiffness across the construct prevents further increases in overall length. It is unclear whether this will affect MCGR when used in the management of EOS.
Methods: A retrospective review of prospectively collected data on the experience of MCGR lengthening in a heterogenous cohort of children with EOS from a single centre.
Results: There were 53 MCGRs in 28 patients with EOS with a number of different underlying diagnoses. The mean age of the cohort was 8 years 3 months (SD, 2 years 7 months). The mean follow-up period since primary rod implantation was 2 years 0 months (SD, 1 year 1 month). MCGR lengthening was performed on up to 13 occasions [median, 4; interquartile range (IQR), 5; range, 1–13]. There was no statistically significant difference in the amount of length achieved over then number of lengthening episodes (P=0.427). For those with at least 2 years follow up the median number of lengthening was 10 (IQR 2, range 10–13) and there was no statistically significant difference seen (P=0.438). Growth velocity of MCGRs against age was less than previously documented norms for the thoracic spine, but was maintained as age increased.
Conclusions: The ‘law of diminishing returns’ does not affect serial lengthening of MCGR in the way that has been observed using TGR. It was also demonstrated that in the MCGR group growth velocity was maintained relative to that of the normal spine.


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