How do implants overlying the spine influence "The Law of Diminishing Returns" in early-onset scoliosis patients?

Stuart L Mitchell; Jessica H Heyer; Jason B Anari; Keith D Baldwin; Pranav Kodali; Brandon S Ramo; Jack M Flynn; Ryan Fitzgerald; Walter Truong; Ying Li; Lindsay Andras; Jaysson Brooks; Patrick J Cahill

doi:10.1007/s43390-024-00885-0

How do implants overlying the spine influence "The Law of Diminishing Returns" in early-onset scoliosis patients?

Spine Deform. 2024 May 8. doi: 10.1007/s43390-024-00885-0. Online ahead of print.

Authors

Stuart L Mitchell¹, Jessica H Heyer², Jason B Anari^{3

4}, Keith D Baldwin^{3

4}, Pranav Kodali⁴, Brandon S Ramo⁵, Jack M Flynn^{3

4}, Ryan Fitzgerald⁶, Walter Truong⁷, Ying Li⁸, Lindsay Andras⁹, Jaysson Brooks⁵, Patrick J Cahill^{10

11}

Affiliations

¹ Department of Orthopaedics, University of North Carolina, Chapel Hill, NC, USA.
² Department of Pediatric Orthopaedics, Hospital for Special Surgery, New York, NY, USA.
³ Department of Orthopaedics, The Children's Hospital of Philadelphia, Philadelphia, PA, USA.
⁴ University of Pennsylvania Perelman School of Medicine, Philadelphia, PA, USA.
⁵ Department of Orthopaedic Surgery, University of Texas Southwestern Medical Center, Scottish Rite for Children, Dallas, TX, USA.
⁶ Children's Orthopaedic and Scoliosis Surgery Associates, St. Petersburg, FL, USA.
⁷ Department of Orthopaedics, Gillette Children's, St. Paul, MN, USA.
⁸ Department of Orthopaedics, University of Michigan, Michigan Medicine, Ann Arbor, MI, USA.
⁹ Department of Orthopaedics, Children's Hospital of Los Angeles, Los Angeles, CA, USA.
¹⁰ Department of Orthopaedics, The Children's Hospital of Philadelphia, Philadelphia, PA, USA. cahillp1@email.chop.edu.
¹¹ University of Pennsylvania Perelman School of Medicine, Philadelphia, PA, USA. cahillp1@email.chop.edu.

PMID: 38717696
DOI: 10.1007/s43390-024-00885-0

Abstract

Purpose: The "law of diminishing returns" (LODR) in early-onset scoliosis (EOS) is well-known. We hypothesized that previously observed variations between constructs may be related to the lateral distance that each construct lies from the spine. We therefore sought to determine whether the curve magnitude improvement and spinal length gains for distraction-based constructs in EOS are positively correlated with the collinearity of the spine and the convex-sided implant on posteroanterior radiographs.

Methods: A prospectively-collected, multicenter EOS registry was queried for all patients who underwent non-fusion, distraction-based instrumentation surgery. Post-index radiographs were graded from 1 to 5 based on amount of overlap between the convex-sided rod and the apical vertebra. Grade 1: convex rod is lateral to convex-sided pedicle; Grade 2: overlaps the convex-sided pedicle; Grade 3: lies between pedicles; Grade 4: overlaps concave-sided pedicle; Grade 5: medial to concave-sided pedicle. ANOVA assessed the correlations between post-index overlap grade and change in (a) curve magnitude and (b) T1-T12 height. Multivariable regression modeling further assessed these associations.

Results: 284 patients met all selection criteria and were included. On ANOVA, post-index grade was associated with curve magnitude (p <0.001) and T1-12 height (p = 0.028) change. Better curve correction and height change were associated with higher grade. On regression modeling, curve correction (R = 0.574) and T1-T12 height change (R = 0.339) remained significantly associated with grade when controlling for time, anchor locations, age, underlying diagnosis, and pre-index curve magnitude.

Conclusion: More apical overlap by the convex rod was associated with better spinal deformity control and improved height gain.

Level of evidence iii: Therapeutic.

Keywords: Distraction-based spine implants; Early-onset scoliosis; Growing rods; Pediatric spinal deformity.