General Session: Biomechanics
Presented by: V. Lafage - View Audio/Video Presentation (Members Only)
V. Lafage(1), I. Obeid(2), R. Lafage(1), B. Liabaud(1), J. Varghese(1,3), H. Bao(1), J. Elysée(1), L. Day(3), D. Cruz(4), S. Ramchandran(4), S. Bess(4), T. Protopsaltis(4), P. Passias(1), A. Buckland(4), F. Schwab(1)
(1) Hospital for Special Surgery, Spine Service, New York, NY, United States
(2) Centre Hospitalier, Universitaire de Bordeaux, Bordeaux, France
(3) SUNY Downstate Medical Center, Department of Orthopaedic Surgery, Brooklyn, NY, United States
(4) NYU Langone Medical Center, Spine Division, Department of Orthopaedics, New York, NY, United States
Introduction: Previous study demonstrated an increase of lower limbs with progressive malalignment. In the setting of large sagittal deformity, patient compensate with the pelvic retroversion and an increase in knee flexion. In order to maintain standing position, the gravity line must rest directly over the feet. However, importance of pelvic and lower limb compensations to the position of the gravity line are. This study aims to investigate the role of lower limbs and pelvic retroversion on the gravity line position in the setting of sagittal malalignment.
Methods: Patients > 18yo with spinal pathology, full-body x-rays, and PI-LL 10-30° were included. Sagittal radiographic parameters were measured. Using validated normative data, a completely uncompensated model was simulated (UNCOMP) by removing pelvic tilt (PT) and lower limb compensations. A partially compensated model (PARTIAL) simulated the compensatory effect of PT only. The fully compensated alignment (COMP) represented actual alignment. Using anthropometric data and the center of gravity projection in standing posture, the distance between the gravity line and the feet (GL-Ft) was compared between groups.
Results: 825 cases were included (63.9yo, 65%F). Mean sagittal radiographic parameters for the cohort were PI=59°±15, PI-LL=18°±6, PT=27.4°±7.2, SVA=48mm±40. In the UNCOMP position, the GL-Ft distance was 91.2 ±27.4mm, with SVA 173mm, PT 14.8°, T1SPi 10.3°, and Knee Flexion (KA) 0.6°. With the addition of compensatory PT in PARTIAL position, the GL-Ft, SVA and T1SP1 were significantly reduced to GL-Ft=45.3±22.8mm, SVA=48mm, and T1SPi=-2.5°. With both pelvic and lower limb compensations included, COMP position had the smallest GL-FT distance=26.2 ±24.1mm (with SVA 48mm, PT 27.4°, T1SPi -2.2°, and KA 7.5°). The bending moment decreased dramatically from UNCOMP (0.889) to PARTIAL (0.439) to COMP (0.252). Moving from UNCOMP to PARTIAL leaded to a reduction of the GL-Ft distance by 50% and as a consequence a reduction of the bending moment by 50%. Addition of the lower limbs compensation reduced again the distance GL-Ft by 40%, giving a total reduction of the GL=Ft distance and bending moment by 70% between UNCOMP and COMP.
Conclusions: In patients with loss of lumbar lordosis, pelvic and lower limb compensatory mechanisms are essential in maintaining the position of the gravity line over the feet. While pelvic retroversion alone is responsible for correcting SVA, the additional recruitment of lower limb compensation is needed to completely restore the gravity line to the appropriate position. By unfurling the sagittal alignment and associated compensatory mechanisms, this study demonstrated that completely uncompensated alignment is not mechanically sustainable.
Gravity line at UNCOMP, PARTIAL and COMP