599 - Upper-most Instrumented Vertebra Mechanical Loading Correlates with th...

General Session: Biomechanics

Presented by: J. Elysée - View Audio/Video Presentation (Members Only)


J. Elysée(1), R. Lafage(1), B. Line(2), F. Schwab(1), J. Smith(3), I. Obeid(4), R. Hart(5), J. Gum(6), C. Ames(7), G. Mundis(8), T. Albert(1), S. Bess(9), M. Gupta(10), V. Lafage(1), International Spine Study Group (ISSG)

(1) Hospital for Special Surgery, Spine Service, New York, NY, United States
(2) Rocky Mountain Hospital for Children Denver, Denver, CO, United States
(3) University of Virginia Medical Center, Department of Neurosurgery, Charlottesville, VA, United States
(4) Centre Hospitalier, Universitaire de Bordeaux, Bordeaux, France
(5) Oregon Health Sciences University, Department of Orthopaedic Surgery, Portland, OR, United States
(6) Norton Leatherman Spine Center, Louisville, KY, United States
(7) San Francisco Medical Center, University of California, Department of Neurosurgery, San Francisco, CA, United States
(8) Scripps Clinic Torrey Pines, La Jolla, CA, United States
(9) NYU Langone Medical Center, Spine Division, Department of Orthopaedics, New York, NY, United States
(10) Washington University School of Medicine, Department of Orthopaedic Surgery, St. Louis, MO, United States


Introduction: Although radiographic proximal junctional kyphosis (PJK) is frequently observed in adult spinal deformity surgery (ASD), the impact on health-related quality of life is inconsistent. Despite occurring in up to 40% of ASD cases, the etiology is not well understood. Using biomechanical modeling, this study aims to compare mechanical loading sustained at the UIV in patients with and without PJK.

Methods: Operative ASD patients with min 2-yr follow-up (FU) were included. Based on anthropometric data the center of gravity projection in standing posture and associated mass of the upper body (above UIV) were modeled. The resulting bending moments at the UIV junction were calculated, validated on an external full-body database and standardized for 1kg. Spino-pelvic parameters and bending moments at UIV were compared between cohorts. The relationship between PJK magnitude and sagittal alignment was investigated using bi-variate correlations. Similar analysis was conduct on a stratified cohort based on location of UIV (UT: T1-T7; LT: T8-L1).

Results: 349/450 pts (57yo, 82% F) were evaluated. Radiographic PJK rate at 2Y FU was 44%. There were no significant differences in postop spino-pelvic parameters between +PJK and -PJK pts. PJK patients had a larger bending moment at the UIV (0.099 vs 0.14 N, p< 0.001). While PJK angle correlated very weakly with PI-LL (r=0.111), correlation was moderate with the UIV bending moment (r=-0.457). The LT group had a larger rate of PJK (50.7% vs 39.7%, p=0.047). Correlation analysis within each group confirmed moderate to strong correlations between PJK angle and bending moments at the UIV (r=-0.578 for UT group, r=-0.651 for LT group).

Conclusion: Evaluation of the mechanical loading at the interface between instrumented and non-instrumented spine segments revealed correlations for presence and severity of PJK. PJK magnitude correlated moderately (r>0.45) with the moment at UIV, emphasizing the relationship between PJK and mechanical loading. Further studies should investigate progressive PJK using the moment at the UIV.

Schematic of the moment at UIV