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Musculoskeletal Webinar
Musculoskeletal Simulation Webinar
David Wagner, PhDOzen EngineeringJuly 24, 2009
Please visit:http://www.ozeninc.com/default.asp?ii=273for upcoming webinars
Welcome to the WebinarWelcome to the Webinar. Please make sure
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or telephone
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2
Summary
Coupling musculoskeletal modeling and finite element analysis
Extracting and incorporating 3D geometry and material propertiesfrom tomographic medical image data
A Proposed workflow for incorporating musculoskeletal modeling
Modeling the human body – Musculoskeletal simulation of activitiesof daily living
Prevalent uses of simulation in the orthopedic industry
Uses of Simulation in the Orthopedic Industry
Replicating Physical Test Research (Internal/University)
Kim et al. 2008, SBC2008-193023
Li et al. 2008, SBC2008-192776
Design of Orthopedic Devices and ProstheticsASME Summer Bioengineering Conference (2008)
Finding out what went wrong
Finite-elementanalysis offailure of theCapital HipdesignsJanssen et al.2005
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Benefits of Simulation
The use of computational simulation can be beneficial if it:• accurately represents and replicates the physics of the system• increases the number of possible design iterations (within a fixed
time)• decreases the cost associated with each design iteration• improves the fidelity of analysis as related to making design
decisions• is integrated in the design process
Replicating Standardized Physical Tests
For example…ASTM F384 -06 Standard Specifications and Test Methods for Metallic Angled OrthopedicFracture Fixation Devices (no associated ISO standard)
• Methods for bending fatigue testing• Fatigue life over a range of maximum bending moment levels• Estimate the fatigue strength for a specified number of fatigue cycles• Not intended to define levels of performance of case-specific
ASTM F1264 Standard Specification and Test Methods for Intramedullary Fixation Devices• performance definitions• test methods and characteristics determined to be important to in-vivo performance
of the device (bending fatigue test, static torsion test, static four-point bend test)
• It is not the intention of this specification to define levels of performance or case-specific clinical performance of these devices, as insufficient knowledge to predictthe consequences of the use of any of these devices in individual patients forspecific activities of daily living is available
4
From Kojic 2008
Comparison of Fracture Fixation Devices
Fixed PlateInternal compressionresulting from screw +fixation plate geometry
Intramedullary nailBending stiffness:Kb = ExI
E, Young’s Modulus of ElasticityI, the second moment of inertia
for bending of the nail crosssection
Torsional stiffness:Kt = ExIt
G, Shear ModulusIt, the second moment of inertia
for torsion
From Kojic 2008
Example Analysis - Fixed Plate Boundary Conditions
FixedConstraint
~ approximatingof axial loadduring humanwalking (singlestance phase of70 kgindividual)
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From Kojic 2008
Example Analysis Results - Effective Stresses
No slipconditionmodeledbetweenscrews, plate,and bone =>i.e. bondedcontacts
MPa
From Kojic 2008
Example Analysis Results - Fixed Plate Stresses
Stainless steelused for plateand screws
E = 2.1x105 Mpa
Poissons ratio = 0.3
Maximum effective stressless than critical values forstainless steel. However,cyclic loading leading tomaterial fatigue must alsobe considered
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From Kojic 2008
Example Analysis - Intramedullary Nail
Same bone geometry,material properties, and
boundary conditions as inthe neutralization plateanalysis
From Kojic 2008
Example Analysis - Intramedullary Nail StressesEffective stress concentrations in the nail near the screw regions => However, stress valuesare significantly lower than the corresponding neutralization plate regions (~80 MPa).Implication is that risk of intramedullary nail failure is significantly lower when compared toneutralization plate.
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From Kojic 2008
Example Analysis - Intracapsular Fractures
Parallel Screws Dynamic Hip Implant
Comparison of implant designs for internal fixation of intracapsular fractures of thefemoral neck
From Kojic 2008
Example Analysis - Parallel Screws BCs
Positive correlationbetweenintraoperativestability and
femoral neckfractures that havehealed (versus didnot heal),Rehnberg et al.1989
Fixed BoundaryCondition
FR: Pelvis to femur head reaction force, 199 daNFA: Force generated by gluteal muscles, 137 daNBody weight: 70 daN
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Can we usesimulation in amore ‘pro-active’way to developbetter products?
Doing More with Simulation (one idea)
Summary
Coupling musculoskeletal modeling and finite element analysis
Extracting and incorporating 3D geometry and material propertiesfrom tomographic medical image data
A Proposed workflow for incorporating musculoskeletal modeling
Modeling the human body – Musculoskeletal simulation of activitiesof daily living
Prevalent uses of simulation in the orthopedic industry
9
• Help understand what is going on inside the human body
• We use simulation for many other engineering analyses,why not for the human body as well
• Design/redesign ‘safe’ working environments
• Teaching
• Functional assessments (neuromusculoskeletal system)
• Create/Mimic realistic movement
• Sometimes the only way to understand and learn moreabout complex systems (like people!)
Simulation for !Biomechanics" - Why?
• Musculoskeletal Analysis– AnyBody– LifeMod– Opensim/SIMM/SimTK– Madymo (TNO)– ESI Group– Marlbrook– Motek
• Digital Manikins– RAMSIS (Human
Solutions)– Jack (UGS/Siemens)– HumanBuilder/Delmia
(Dassault)– HumanCAD (NexGen)– SANTOS (U. Iowa)– Some others
• Motion Capture– BodyBuilder (Vicon)– Simi – Qualisys – SIMM (Motion Analysis)– XSENS– Many others
• CAE tools (FE/CAD)– ANSYS – LS-DYNA (ANSYS)– Abacus (Dassault)– AutoCAD (AutoDesk)– NASTRAN & ADAMS (MSC)– COMSOL
• Other tools– Matlab (Mathworks)– Mathematica
Simulation Software for !Biomechanics"
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The Holy Grail…
Task + Environment + Population
UniqueSimulation
from Parkinson and Reed (2008)
Working Within the Confines of the Current Technology
• Library of activities– Can’t rely (yet) on the musculoskeletal models to ‘adapt’ to new
task/environment conditions => particularly for novel (~non-cyclic)tasks
• Global Assessments vs. Better Products/Designs– Models that match measured results are great, but models that
exhibit realistic trends may be sufficient (and as useful)
• Better incorporation/understanding of variability– E.g. Within subject variability as indicator of model performance
• Will we ever be able to use Musculoskeletal Simulationwithout a corresponding validation study– Can’t ALWAYS be expected to conduct a validation study for a new activity– Must have confidence in the tools (e.g. Finite Element Models)
11
Expanding the Use of Activities of Daily Living with a
Library of Musculoskeletal Simulations
• Long-term stability of hip-implants have been
evaluated using normalwalking, sit to stand, stairclimbing, and combinationsof those activities.
• Traditionally used aspass/fail tests to identifywhether a particular designperforms to a set ofminimum specifications
• Significantly Underutilized
Musculoskeletal Models Used Here80
14.6
35
5.2
549
121
709
782804
17
121
121
(b)
Popular class of musculoskeletalmodels based on rigid bodydynamics:
• Bones and objects from theenvironment are rigid
• Muscles and ligaments aremass-less actuators
• Soft tissue – “wobbly“masses are not taken intoaccount (mass isconcentrated in bones)
• Phenomenological musclemodels
• ‘Easily’ scalable
Suited for simulating internal body forces (muscle,joint, ligament) for prescribed activities
Static 2D
Dynamic 3D (AnyBody
Modeling System)
12
},..,1{ ,0
],[ where,
)()(
)()(
MMi nif !"
==MRfffdCf
MuscleforcesJoint
reactions
Internalforces
Appliedforces
The matrix C is rectangular. This means that there areinfinitely many solutions to the system of equations.How to pick the right one?
Formulating Dynamic Equilibrium
Using Optimization to Get a Solution
!
Minimize
G(f (M))
Subject to
Cf = d
fi(M )
" 0, i # {1,..,n(M )}
Objective function. Differentchoices give different muscle
recruitment patterns.
What should be used for ?
!
G(f(M))
13
Musculoskeletal Models for Commercial Use
No ‘gold-standard’, just like with other pieces of engineeringsoftware
Commercially available (including open source) softwarepackages demand a knowledgeable user
Not traditionally incorporated in current design/engineeringmethodologies
Always room for improvement (I.e. improved validation, betteraccuracy, scaling to populations or patient specific, etc.)
Still must demonstrate where/how this arena of modeling canimprove specific processes (I.e. $$$)
Summary
Coupling musculoskeletal modeling and finite element analysis
Extracting and incorporating 3D geometry and material propertiesfrom tomographic medical image data
A Proposed workflow for incorporating musculoskeletal modeling
Modeling the human body – Musculoskeletal simulation of activitiesof daily living
Prevalent uses of simulation in the orthopedic industry
14
Bridging the Gap with Simulation
Physical Testing“Simulated”
Physical Testing“Simulated” In-
Vivo Performance
All the necessary pieces:GeometryMeshMaterial PropertiesBoundary ConditionsSolvePost-Processing
Setting up an FE Simulation Using Boundary Conditions Derived from aMusculoskeletal Model
15
Selected Arenas of Simulation (by Device)
The use of computational simulation can be beneficial if it:• accurately represents and replicates the physics of the system• increases the number of possible design iterations (within a fixed
time)• decreases the cost associated with each design iteration• improves the fidelity of analysis as related to making design
decisions
Starting with Geometry…
16
Incorporating Musculoskeletal Modeling
Implant Evaluation
17
Implant Optimization
Associated Software
18
Summary
Coupling musculoskeletal modeling and finite element analysis
Extracting and incorporating 3D geometry and material propertiesfrom tomographic medical image data
A Proposed workflow for incorporating musculoskeletal modeling
Modeling the human body – Musculoskeletal simulation of activitiesof daily living
Prevalent uses of simulation in the orthopedic industry
Geometry, Mesh, and Material Properties
Realistic geometries and material properties are practical ways toimprove the accuracy of the simulations
• A NIH (National Institute ofHealth) Project
• Goal is anatomically detailed,3D representation of thehuman body
• CT, MRI, Cryosection taken ofcadavers
• Male specimen released 1994• Female specimen in 1995• Publicly available with an
application to National Libraryof Medicine
CT
MRI
Cryosection
19
• Tool for working with segmentedmedical data
• Provides a GUI environment toapply various segmentationmethods
• Creates and exports advanced 3Dgeometries
• Can be used to export FiniteElement Mesh (if desired)
• Can be used to define iso-tropicmaterial definitions from apparentdensity relationships
Using Medical Data as Simulation Input
Deriving Material Properties From Scan Data
• In Ansys, the mesh can be changed by a number ofoperations, such as applying different boundary conditions orfor purposes of convergence
• Deferring the material property assignment until the simulationis fully set up ensures versatility
• Bonemat is a public domain program originally written byCinzia Zannoni et al. at The Rizzoli Institute*
• Uses a voxel data integration algorithm to determine materialproperties for finte elements regardless of relative voxel size
*Zannoni C, Mantovani R, Viceconti M. Material propertiesassignment to finite element models of bone structures: a newmethod. Med Eng Phys 1998;20(10):735–40.
20
Bonemat Workflow
• Bonemat takes 2 inputs:– A mesh in patran neutral file format (*.ntr)
– Volumetric CT data in a vtk file format(rectilinear grid or point cloud)
• Bonemat outputs:– An identical patran neutral mesh file with material properties assigned
– An informational frequency file on material property distributionSolution
Geometry Mimics
Commercially available software packages with
tomographic reconstruction capabilities
(Mimics, Analyze, Osiris) can also be used to
define material properties (isotropic) suitable for
FEA => using Hounsfield Units relationships
The material property of each
tetrahedral element was defined
using a procedure similar to that
used by Peng et al. (2006).
HU =
HU are normalized units associated with CT image
scans
- based on the linear attenuation coefficient (µ)
- based on scale -1000 (air) : + 1000 (bone), 0 (water)
Material properties from imaging data
21
Material properties from imaging data
The Hounsfield Units (HU) of each voxel in the CT scan indicates the radiodensity of the
material, distinguishing the different bone tissue types. There exist an approximate linear
relationship between apparent bone density and HU (Rho et al. 1995).
The maximum HU of the CT
scan, 1575, was defined to be
the hardest cortical bone of
density (2000 kg/m3) and the
HU value of 100 was defined to
be the minimum density of
cortical bone (100 kg/m3).
Density
100 kg/m3 2000 kg/m3
Material properties from imaging data
Elements were assigned elastic
moduli calculated from apparent
densities using axial loading
equations developed by Lotz et al.
(1991):
There exist an approximate power relationship between bone material properties and apparent
densities (Wirtz et al. 2000).
Elastic Moduli
A Poisson's ratio of 0.30 was
used for all materials.
HU >= 801, cortical bone (E = 2065!3.09 MPa)
HU <= 800, cancellous bone (E = 1904!1.64 MPa)
HU < 100, intramedullar tissue (E = 20 MPa)
22
Summary
Coupling musculoskeletal modeling and finite element analysis
Extracting and incorporating 3D geometry and material propertiesfrom tomographic medical image data
A Proposed workflow for incorporating musculoskeletal modeling
Modeling the human body – Musculoskeletal simulation of activitiesof daily living
Prevalent uses of simulation in the orthopedic industry
All the necessary pieces:
GeometryMeshMaterial PropertiesBoundary ConditionsSolvePost-Processing
Setting up the FE Simulation
23
Cycling Data
Cyclist Data
24
Musculoskeletal Simulation
Single RevolutionObserved Cadence of 62 rpm5 points of support (pelvis, feet,
hands)Anthropometry Matched to
SubjectSimulated Crank Torque =>
MechOutput = 170 (avg.mechanical output over acycle in Watts)
Musculoskeletal Simulation
Force and Moment ! Free Body Diagram"
1 revolution = 0.97 seconds
Cut Plane (vectorlengths correspond toforce magnitudes)
25
Muscle Force Boundary Conditions at a Single Time Step
FE Model in Dynamic Equilibrium- Matched mass and inertia
properties between rigid andflexible body simulations
- Matched points of forceapplication
- No ‘arbitrary’ constraints (i.e.nodal position fixed in space)
- Inertia loads applied
- Model supported by weak springs(~1e-3 Newtons), to prevent rigidbody motion
- Assumption of small deflections
26
Tested Fracture Fixation Plate (Distal Femur)
Geometry with 3 platethicknesses
3.25mm
4.0mm
4.75mm
Fatigue Life Results
Fatigue Life Minimum Cycles:3.25 mm => 178,000 cycles4.0 mm => 335,000 cycles4.75 mm => 14.7 million cycles
Plots are depicted at97% of cycle (t = 0.9704)
4.75 mm
3.25 mm
4.00 mm
Stress LifeFully Reversedt= 0.9409
Stress LifeFully Reversedt= 0.9409
Stress LifeFully Reversedt= 0.9409
27
Stress Contour Plots
Maximum Stress:3.25 mm => 855 MPa4.0 mm => 692 MPa4.75 mm => 584 MPa
Plots are depicted at97% of cycle (t = 0.9704)
4.75 mm
3.25 mm
4.00 mm
Yield Stress of Titanium Alloy => 930 MPa
Deformation Mode
Deformation from musculoskeletal forces @ 0.02 s, 18x scale
28
Equivalent Stresses for Three Plate Thicknesses
3.25mm
4.00mm
4.75mm
Summary of Simulation Capabilities
1. Replicating physical tests usingsimulation
2. Compare performance of newimplant design to current on themarket device
3. Replicate implant failureconditions associatedwith clinical and/or case-specificperformance criteria
4. Evaluate implant performancecriteria (i.e. total deformation,maximum stress, maximumstrain, and/or fatigue life)for physiologically realisticboundary conditions associatedwith a single or library ofactivities of daily living
29
Summary of Simulation Capabilities
5. Evaluate implant performance criteria(i.e. total deformation, maximumstress, maximum strain, and/or fatiguelife) for different populations (i.e. bonesize/geometry, bone quality/strength)performing relevant activities of dailyliving
6. Perform shape optimization ofparametrically defined implant tomaximize or satisfyone/multiple performance objectives orcriteria
7. Perform sensitivity analysis on screwplacement and/or implant variationswith respect to performance criteria
8. Evaluate internal bone stressesat/around implant-bone and bone-bone interfaces for laboratory andactivity of daily living criteria
Thank you for your attentionThank You For Your Attention
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