2009-07-24 musculoskeletal simulation webinar

1Musculoskeletal Webinar

Musculoskeletal Simulation Webinar

David Wagner, PhDOzen EngineeringJuly 24, 2009

Please visit:http://www.ozeninc.com/default.asp?ii=273for upcoming webinars

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2Summary

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

3Benefits 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 exampleASTM 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

4From Kojic 2008

Comparison of Fracture Fixation Devices

Fixed PlateInternal compressionresulting from screw +fixation plate geometry

Intramedullary nailBending stiffness:Kb = ExI

E, Youngs 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)

5From 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 MpaPoissons ratio = 0.3

Maximum effective stressless than critical values forstainless steel. However,cyclic loading leading tomaterial fatigue must alsobe considered

6From 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.

7From 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

8Can we usesimulation in amore pro-activeway to developbetter products?

Doing More with Simulation (one idea)

Summary






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"

10

The Holy Grail

Task + Environment + Population

UniqueSimulation

from Parkinson and Reed (2008)

Working Within the Confines of the Current Technology

Library of activities Cant 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 Cant 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 wobblymasses 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






14

Bridging the Gap with Simulation

Physical TestingSimulated

Physical TestingSimulated 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






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):73540.

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 ()

- based on scale -1000 (air) : + 1000 (bone), 0 (water)

Material properties from imaging data

21


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


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

22

Summary






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



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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2009-07-24 musculoskeletal simulation webinar

Documents

design of orthopedic

levels of performance

static torsion test

fatigue strength

available4from kojic

bondedcontactsmpafrom

fidelity of analysis

design processreplicating