Rib-to-spine and rib-to-pelvis magnetically controlled growing rods: does the law of diminishing returns still apply?

Jessica H Heyer; Jason B Anari; Keith D Baldwin; Stuart L Mitchell; John M Flynn; Wudbhav N Sankar; Lindsay M Andras; David L Skaggs; John T Smith; Scott J Luhmann; Ishaan Swarup; Walter H Truong; Jaysson T Brooks; Ryan Fitzgerald; Ying Li; Pediatric Spine Study Group; Patrick J Cahill

doi:10.1007/s43390-023-00718-6

Rib-to-spine and rib-to-pelvis magnetically controlled growing rods: does the law of diminishing returns still apply?

Spine Deform. 2023 Nov;11(6):1517-1527. doi: 10.1007/s43390-023-00718-6. Epub 2023 Jul 14.

Authors

Jessica H Heyer¹, Jason B Anari², Keith D Baldwin², Stuart L Mitchell³, John M Flynn², Wudbhav N Sankar², Lindsay M Andras⁴, David L Skaggs⁵, John T Smith⁶, Scott J Luhmann⁷, Ishaan Swarup⁸, Walter H Truong⁹, Jaysson T Brooks¹⁰, Ryan Fitzgerald¹¹, Ying Li¹²; Pediatric Spine Study Group; Patrick J Cahill¹³

Affiliations

¹ Department of Pediatric Orthopaedics, Hospital for Special Surgery, New York, NY, USA.
² Department of Orthopaedic Surgery, Children's Hospital of Philadelphia, 3500 Civic Center Blvd, Philadelphia, PA, 19104, USA.
³ Department of Orthopaedics, University of North Carolina, Chapel Hill, NC, USA.
⁴ Department of Orthopaedics, Children's Hospital of Los Angeles, Los Angeles, CA, USA.
⁵ Department of Orthopaedics, Cedars Sinai, Los Angeles, CA, USA.
⁶ Department of Orthopaedics, University of Utah Health, Salt Lake City, UT, USA.
⁷ Department of Orthopaedic Surgery, Shriners Children's Pediatric Specialty Care, St. Louis, MO, USA.
⁸ Department of Orthopaedics, UCSF Benioff Children's Hospitals, San Francisco, CA, USA.
⁹ Department of Orthopaedics, Gilette Children's, St. Paul, MN, USA.
¹⁰ Department of Orthopaedics, Scottish Rite for Children, Dallas, TX, USA.
¹¹ Children's Orthopaedic and Scoliosis Surgery Associates, St. Petersburg, FL, USA.
¹² Department of Orthopaedics, Univeristy of Michigan, Michigan Medicine, Ann Arbor, MI, USA.
¹³ Department of Orthopaedic Surgery, Children's Hospital of Philadelphia, 3500 Civic Center Blvd, Philadelphia, PA, 19104, USA. CahillP1@chop.edu.

PMID: 37450222
DOI: 10.1007/s43390-023-00718-6

Abstract

Purpose: The Law Of Diminishing Returns (LODR) has been demonstrated for traditional growing rods, but there is conflicting data regarding the lengthening behavior of Magnetically Controlled Growing Rods (MCGR). This study examines a cohort of patients with early-onset scoliosis (EOS) with rib-to-spine or rib-to-pelvis-based MCGR implants to determine if they demonstrate the LODR, and if there are differences in lengthening behaviors between the groups.

Methods: A prospectively collected multicenter EOS registry was queried for patients with MCGR with a minimum 2-year follow-up. Patients with rib-based proximal anchors and either spine- or pelvis-based distal anchors were included. Patients with non-MCGR, unilateral constructs, < 3 lengthenings, or missing > 25% datapoints were excluded. Patients were further divided into Primary-MCGR (pMCGR) and Secondary-MCGR (sMCGR).

Results: 43 rib-to-spine and 31 rib-to-pelvis MCGR patients were included. There was no difference in pre-implantation, post-implantation and pre-definitive procedure T1-T12 height, T1-S1 height, and major Cobb angles between the groups (p > 0.05). Sub-analysis was performed on 41 pMCGR and 19 sMCGR rib-to-spine patients, and 31 pMCGR and 17 sMCGR rib-to-pelvis patients. There is a decrease in rod lengthenings achieved at subsequent lengthenings for each group: rib-to-spine pMCGR (rho = 0.979, p < 0.001), rib-to-spine sMCGR (rho = 0.855, p = 0.002), rib-to-pelvis pMCGR (rho = 0.568, p = 0.027), and rib-to-pelvis sMCGR (rho = 0.817, p = 0.007). Rib-to-spine pMCGR had diminished lengthening over time for idiopathic, neuromuscular, and syndromic patients (p < 0.05), with no differences between the groups (p > 0.05). Rib-to-pelvis pMCGR neuromuscular patients had decreased lengthening over time (p = 0.01), but syndromic patients had preserved lengthening over time (p = 0.65).

Conclusion: Rib-to-spine and rib-to-pelvis pMCGR and sMCGR demonstrate diminished ability to lengthen over subsequent lengthenings.

Keywords: Growing rods; Magnetic growing rods; Rib-based anchors; early onset scoliosis.