Biomechanical Evidence:Biomechanical Evidence:The complexity of human standing and its The complexity of human standing and its

associated compensatory mechanismsassociated compensatory mechanisms

Virginie Lafage PhDVirginie Lafage PhDFrank Schwab MDFrank Schwab MD

Jean-Pierre Farcy MDJean-Pierre Farcy MD

NYU Hospital for Joint Diseases, New York, NYNYU Hospital for Joint Diseases, New York, NY

Human Standing Position

BalanceBalancestate of

equilibrium; equal

distribution of weight

PosturePosturerelative

disposition of the body’s parts

StabilityStabilitystate or quality of

maintaining equilibrium, self-

restoring

http://pmgagey.club.fr/http://pmgagey.club.fr/

BalanceBalance

“[In a state of equilibrium ...] the body is positioned between two aligned, equal and opposite forces”.[Brisson MJ, 1803]

COMCOM

COPCOP

Human BodyHuman Body– Gravity Force => Gravity Force => Center of mass (COM) Center of mass (COM)– Ground Reaction => Ground Reaction => Center of Pressure (COP) Center of Pressure (COP)

Balance = COM & COP aligned on Balance = COM & COP aligned on the same verticalthe same vertical

Biospace / ENSAM

StabilityStability

COMCOM

COPCOP

Body = moving partsBody = moving parts– Perfect Alignment almost never reachedPerfect Alignment almost never reached– Sophisticated Control System (NCS)Sophisticated Control System (NCS)

MaintainMaintainStabilityStability

Two stabilization Strategies [Gagey]– COM Maneuver

• Body parts motion• Localize COM above COP

– COP Maneuver• Feet pressure distribution• Localize COP below COM

Biospace / ENSAM

Dynamic MeasurementsDynamic Measurements

• Stabilometry platformStabilometry platform– Ground reaction measurements (COP)Ground reaction measurements (COP)– Over time (sample rates)Over time (sample rates)– Travel over timeTravel over time

• Pressure data obtainedPressure data obtained• Gravity Line (GL) = COP in static condition Gravity Line (GL) = COP in static condition [Zatiorsky, 1998]

• Foot pressure distribution => foot locationFoot pressure distribution => foot location

Static MeasurementsStatic Measurements

Force Plate vs. X-rays Analysis

• X-raysX-rays• Global alignment Global alignment • Spinal parametersSpinal parameters• PelvisPelvis

• Forceplate technologyForceplate technology• LocationLocation

– anatomical structures vs anatomical structures vs GL and FeetGL and Feet• Compensatory mechanismsCompensatory mechanisms

– Pelvis (rotation / translation)Pelvis (rotation / translation)– Lower extremityLower extremity– feetfeet

Sagittal Plane AnalysisSagittal Plane Analysis

Adult Sagittal Imbalance– Degenerative– Pathologic– Iatrogenic

Remains poorly understood and challenging

Sagittal ImbalanceSagittal Imbalance

• Loss of global alignmentLoss of global alignment– Plumbline shift anteriorlyPlumbline shift anteriorly

=> Increasing disability=> Increasing disabilitySF-12, SRS-29, ODI (p<0.001)

=> Lumbar kyphosis marked disability=> Lumbar kyphosis marked disabilitySRS-29, ODI (p<0.05)

Glassman, Bridwell, Dimar, Horton, Berven, Schwab. SPINE 2005

Sagittal ImbalanceSagittal Imbalance

Lumbar lordosis modificationsLumbar lordosis modifications

Pelvic retroversionPelvic retroversion

Hip Hyper ExtensionHip Hyper Extension

Knee flexionKnee flexion

Compensatory MechanismsCompensatory Mechanisms

Inclusion criteria131 Adults

No Previous spine surgery

Cobb angle < 20°C7 Frontal Imbalance < 5cm

8.2 ± 4.80 ± 1.6-4.8 ± 1.4Mean SVA (cm)

> 2.5cm-2.5 => 2.5cm< -2.5cmInclusion criteria (SVA)

375143Number

68 ± 1751 ± 1835 ± 16Mean age (year)

Sagittal Forward (Sf)Neutral (N)

Sagittal Backward (Sb)

Subdivision by SVA [Jackson, Spine 1994]

Clinical Group (Prospective IRB)

1. Frontal & Sagittal XRaysFree standing positionFree standing position

2. Load distribution

Simultaneous assessment of X-Rays and loadSimultaneous assessment of X-Rays and load distributiondistribution

Measurements

3. Vertebrae and FH location on XRaysSpineView®

4. Gravity Line (GL) and heel line projected on X-rays

=> Distance between GL, Heel line => Distance between GL, Heel line and anatomical pointsand anatomical points

Measurements

Pelvic Parameters

SSacral acral SSlopelope PPelvic elvic TTiltilt PPelvic elvic IIncidencencidence

PI = PT + SSPI = PT + SS

ResultsResults

80 ± 500 ± 13-45 ± 17SVA (mm)

50 ± 1357 ± 1163 ± 12Lordo (deg)

-51 ± 20-42 ± 15-41 ± 11Kypho (deg)

Sagittal Forward (Sf)

Neutral (N)Sagittal

Backward (Sb)

With increasing SVA With increasing SVA

Kyphosis increases (Sf group vs. the 2 others) Lordosis decreases

Spinal parameters

35 ± 1036 ± 1038 ± 9Sacral Slope

21 ± 816 ± 610 ± 7Pelvic Tilt

56 ± 1152 ± 948 ± 10Incidence

Sagittal Forward (Sf)

Neutral (N)Sagittal

Backward (Sb)

With increasing SVAWith increasing SVA

No significant differences in Sacral SlopePelvic Tilt increases

Higher Pelvic Incidence (Sf group vs. the 2 others)

=> Differences in pelvic morphology and orientation

Pelvic parameters

S. BackwardS. Backward NeutralNeutral S. ForwardS. Forward

GL vs. Heels = constant

Pelvis shifts posteriorly

Forceplate parameters

Gravity LineGravity LineHeels LineHeels Line

Young Adult

Gravity Line vs. Heel = ConstantGravity 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

How to maintain balance if the trunk inclines How to maintain balance if the trunk inclines forward ?forward ?

=> Body mass distribution should compensate trunk inclination to keep the GL within foot imprint

What do we know ?What do we know ? Pelvis shifts posteriorly

Pelvic tilt increases Feet are fixed in standing position

Adaptation of lower Adaptation of lower extremitiesextremities Hip flexion ?

Knee flexion ? Ankle regulation ?

Conclusions

Young Adult

Permits essential quantification of foot positionPermits essential quantification of foot position– Relationship feet / pelvisRelationship feet / pelvis– Offset of Gravity Line (GL)Offset of Gravity Line (GL)

If GL-heel offset is fixed….then a balance formula exists

Further quantification of spino-pelvic relationship necessary

Pelvic Incidence ?

Force Plate Analysis

Clinical ImpactClinical Impact

X-rays offer only part of X-rays offer only part of the storythe story

Clinical Impact

S1 Fixed Heels Fixed

Jean Dubousset, MD