General Session: Adult Spinal Deformity

Presented by: R. Lafage - View Audio/Video Presentation (Members Only)

Author(s):

R. Lafage(1), B. Liabaud(1), B. Diebo(1), J. Oren(2), I. Gammal(2), S. Vira(2), M. Spiegel(2), E. Tanzi(2), T. Protopsaltis(2), T. Errico(2), F. Schwab(1), V. Lafage(1)

(1) Hospital for Special Surgery, New York, NY, United States
(2) NYU Hospital for Joint Diseases, Orthopedic Surgery, New York, NY, United States

Abstract

Hypothesis: The role of lower limbs is to permit larger recruitment of pelvic tilt (PT) and counteract anterior sagittal malalignment by maintaining center of gravity above ankles. Design: Retrospective review.

Introduction: While lower limbs compensatory mechanisms are an established response to progressive sagittal malalignment, their specific role and impact on surgical planning has not been evaluated. This study evaluates ability of knee flexion (KF) and pelvic shift (P.shift) to counteract high SVA and defines the relationship between lower limb compensatory apparatus and PT. Methodology: Single center retrospective review of full body xrays was performed in patients (pts) >20 yrs. Parameters were measured with dedicated software. Population was stratified by 50mm intervals of SVA and one-way ANOVA was performed to compare P.shift. (P.Shift=antero-posterior translation of the pelvis versus the feet) across SVA groups. Antero-posterior offset of each vertebra versus vertical line erected from distal tibial metaphysis (TM) was investigated. Linear regression was performed to predict PT using KF and P.Shift while controlling for pelvic incidence minus lumbar lordosis mismatch (PI-LL) and SVA.

Results: 2124 pt visits were included (PI 55.1±14.1°, PT:21.0±11°, PI-LL:6.3±17.3°, SVA:29±51mm). With progressively increased SVA, P.shift decreased from 30 to -100mm (all p< 0.005). Analysis of vertebrae offsets from the distal tibial metaphysis revealed that T9 was aligned with TM line across all SVA groups. (Fig) Prediction of PT based on PI-LL and SVA yielded R2=0.76 (p< 0.001); addition of KA and P.shift as independent parameters using hierarchical multiple regression led to significant improvement in R2, demonstrating the independent role of lower limbs parameters in PT prediction. KA and P.shift had a positive standardized coefficient (all p< 0.05).

Conclusion: Lower limb compensatory mechanisms increase with progressive sagittal malalignment. Antero-posterior translation of pelvis allows T9 vertebra to remain in line with the ankle ( “conus of economy”). Lower limb compensatory mechanisms are positive predictors of PT and do not need additional consideration for surgical realignment planning.

Summary: The role of the lower limbs in sagittal deformity surgical planning is not well understood. This study evaluated 2,124 full body xrays of patients presenting to a single spine center. The antero-posterior translation of the pelvis permits T9 to remain in line with the ankles, independent of Sagittal Vertical Axis (SVA) values. Lower limb compensatory mechanisms also permit an increase in recruitment of PT and therefore do not need additional consideration in the setting of surgical planning