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

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). 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.