At Deformity Down Under in Melbourne this year, surgeons put a question to René Castelein about the Spring Distraction System (SDS): why does its spring apply 100 newtons, and not more or less?
Cresco Spine has since given its answer: published research set the safe range, and clinical use settled the exact figure. The starting point is the doctoral thesis of Justin Lemans, The Power of Springs to Treat Scoliosis. Lemans combined published clinical and biomechanical data with finite element modelling to estimate how much distraction force a growing spine can tolerate (J Biomech 2021;124:110571).
With a safety factor of four applied, the estimates come out at roughly 200 N for children aged 5 to 6, 250 N at 7 to 8 and 300 N from age 9 onwards. Those limits matter because of how conventional growing rods work. Each lengthening produces peak forces of 300 to 500 N. Earlier studies by Sankar et al. (Spine 2011) and Cahill et al. (Spine 2010) associated these repeated distractions with a stiffer spine, spontaneous autofusion and smaller length gains after each procedure, a pattern often described as the law of diminishing returns.
The SDS uses a compressed helical spring that applies a continuous 100 N, well below the estimated limit for every age group. Rather than being stretched at intervals, the spine is guided as the child grows. The SDS force and the peak forces of conventional rods are not directly comparable, however. A sustained load and a short peak load act differently on tissue, and Cresco notes that its force graphics are schematic.
Castelein, who co-invented the system with Moyo Kruyt, was candid in Melbourne: the figure is empirical. The first patient, treated in 2015, received a 75 N spring, which turned out to be on the low side for that child. Since then, 100 N has been the working choice in the large majority of cases. The company calls it the best answer for now, open to revision as more data come in.
The force can also be adjusted. It is linear with compression, so a spring compressed less pushes less. That makes it possible to use a lower force on the convex side of the curve.
The question comes as the device enters the US market. SDS received FDA Breakthrough Device designation in 2023 and 510(k) clearance in January 2026. Cresco Spine, based in Schiedam, the Netherlands, is now running a controlled release in a limited number of children’s hospitals in the United States and Canada, with Australia and New Zealand planned at a later stage.
According to the company, the first US patient, a young child with neuromuscular scoliosis, was implanted on April 20, 2026, at Scottish Rite for Children by Dr. Amy McIntosh. The system is not CE marked.
Cresco’s presentation does not say how many patients have received the SDS since 2015, or how many were given springs below 100 N. Long-term data on correction, spinal growth and reoperation rates will show whether 100 N remains the reference value.
SOURCE: Cresco-Spine- Spring Distraction System 100-Newtons.How was the spring force chosen.pdf
About Cresco Spine
Cresco is developing Dynamic Implant Solutions™, a line of dynamic implant systems to treat scoliosis. Specifically, the dream is to establish new standards for surgical treatment of pediatric spinal deformities based on these dynamic implants. We strive to improve the treatment of children and adolescents with severe spinal deformities. Website: www.cresco-spine.com
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