1. Biomechanical Evidence: The complexity of human standing and its associated compensatory mechanisms Virginie Lafage PhD Frank Schwab MD Jean-Pierre Farcy MD NYU Hospital for Joint Diseases, New York, NY

2. Human Standing Position Balance state of equilibrium; equal distribution of weight Posture relative disposition of the body’s parts Stability state or quality of maintaining equilibrium, self-restoring http://pmgagey.club.fr/

8. Sagittal Plane Analysis

12. Clinical Group (Prospective IRB) Inclusion criteria 131 Adults No Previous spine surgery Cobb angle < 20  C7 Frontal Imbalance < 5cm Subdivision by SVA [Jackson, Spine 1994] 8.2 ± 4.8 0 ± 1.6 -4.8 ± 1.4 Mean SVA (cm) > 2.5cm -2.5 => 2.5cm < -2.5cm Inclusion criteria (SVA) 37 51 43 Number 68 ± 17 51 ± 18 35 ± 16 Mean age (year) Sagittal Forward (Sf) Neutral (N) Sagittal Backward (Sb)

13. Measurements 1. Frontal & Sagittal XRays Free standing position 2. Load distribution Simultaneous assessment of X-Rays and load distribution

14. 3. Vertebrae and FH location on XRays SpineView® 4. Gravity Line (GL) and heel line projected on X-rays => Distance between GL, Heel line and anatomical points Measurements

15. Pelvic Parameters PI = PT + SS S acral S lope P elvic T ilt P elvic I ncidence

16. Results

17. Spinal parameters With increasing SVA Kyphosis increases ( Sf group vs. the 2 others) Lordosis decreases 80 ± 50 0 ± 13 -45 ± 17 SVA (mm) 50 ± 13 57 ± 11 63 ± 12 Lordo (deg) -51 ± 20 -42 ± 15 -41 ± 11 Kypho (deg) Sagittal Forward (Sf) Neutral (N) Sagittal Backward (Sb)

18. With increasing SVA No significant differences in Sacral Slope Pelvic Tilt increases Higher Pelvic Incidence ( Sf group vs. the 2 others) => Differences in pelvic morphology and orientation Pelvic parameters 35 ± 10 36 ± 10 38 ± 9 Sacral Slope 21 ± 8 16 ± 6 10 ± 7 Pelvic Tilt 56 ± 11 52 ± 9 48 ± 10 Incidence Sagittal Forward (Sf) Neutral (N) Sagittal Backward (Sb)

19. S. Backward Neutral S. Forward GL vs. Heels = constant Pelvis shifts posteriorly Forceplate parameters Gravity Line Heels Line

20. Young Adult Gravity Line vs. Heel = Constant Required to keep our standing position Age and spinal deformity do not affect this constraint By definition, the whole body mass is equally distributed around the gravity line Conclusions

21. How to maintain balance if the trunk inclines forward ? => Body mass distribution should compensate trunk inclination to keep the GL within foot imprint What do we know ? Pelvis shifts posteriorly Pelvic tilt increases Feet are fixed in standing position Adaptation of lower extremities Hip flexion ? Knee flexion ? Ankle regulation ? Conclusions Young Adult