Contouring the magnetically controlled growing rods: impact on expansion capacity and proximal junctional kyphosis

Abstract

Objective

To quantitatively determine the relationship between the contouring of the magnetically controlled growing rods (MCGR), their expansion capacity and the risk of developing proximal junctional kyphosis in early onset scoliosis (EOS).

Summary of background data

MCGRs allow gradual expansion and correction of the spinal deformity in EOS while reducing the need for repeated surgeries. As the expansion of the MCGRs is controlled externally, several factors can impact the discrepancy between the intended and actual expansions of the rods. The expansion capacity of the growing rods as a function of the expanded length has been tested in experimental setups; however, no study has evaluated the role of contouring of the MCGRs on its function and long-term surgical outcome.

Methods

A total of 25 EOS patients, a total of 48 MCGRs, with right thoracic curves, were studied retrospectively. All patients had two view spinal radiographs at pre-operative, after MCGR implantation, and after 6 lengthening visits. The first post-operative radiographs were used to calculate the 3D contour of the MCGR at the proximal end. 2D ultrasound images before and after lengthening visits were used to measure the rod lengthening at each visit. The relationship between the increase in the rod length and rod curvature was determined. Finally, the rod curvature was correlated to the changes in proximal junctional kyphosis (PJK) angle between the pre-operative and the most recent follow-up, i.e., after 6 visits.

Results

The average rod 3D angle at the proximal end was 13.5° ± 9.7° [0°-37.2°]. The overall increased length after six lengthening visits for the rod at the concave side was 18.8 mm and at the convex side was 16.9 mm. 62% of the patients with a contoured rod at the proximal end developed a PJK exceeding 10° whereas in patients with a straight rod PJK occurred in 9.1%.

Conclusions

Contouring the MCGR impacts both the mechanics of the rod expansion and the prevalence of PJK in EOS patient population.

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Correspondence to Saba Pasha.

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No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. SP received funding from the scoliosis research society outside of scope of this work. PFS received personal fees from DePuy, Medtronic Sofamor Danek, and NuVasive outside of scope of this work.

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Pasha, S., Sturm, P.F. Contouring the magnetically controlled growing rods: impact on expansion capacity and proximal junctional kyphosis. Eur J Orthop Surg Traumatol 31, 79–84 (2021). https://doi.org/10.1007/s00590-020-02743-x

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Keywords

  • Early onset scoliosis
  • Magnetically controlled growing rod
  • Three-dimensional
  • Proximal junctional kyphosis
  • Rod contour