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