Musculoskeletal Webinar. Welcome to the Webinar

Transcription

1 Musculoskeletal Webinar Musculoskeletal Simulation Webinar David Wagner, PhD Ozen Engineering July 24, 2009 Please visit: for upcoming webinars Welcome to the Webinar Welcome to the Webinar. Please make sure your audio is working Feel free to use computer speakers or telephone Type any questions you have here 1

2 Summary Prevalent uses of simulation in the orthopedic industry Modeling the human body Musculoskeletal simulation of activities of daily living A Proposed workflow for incorporating musculoskeletal modeling Extracting and incorporating 3D geometry and material properties from tomographic medical image data Coupling musculoskeletal modeling and finite element analysis Uses of Simulation in the Orthopedic Industry Replicating Physical Test Research (Internal/University) Kim et al. 2008, SBC Li et al. 2008, SBC Design of Orthopedic Devices and Prosthetics ASME Summer Bioengineering Conference (2008) Finding out what went wrong Finite-element analysis of failure of the Capital Hip designs Janssen et al

3 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 F Standard Specifications and Test Methods for Metallic Angled Orthopedic Fracture 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 casespecific clinical performance of these devices, as insufficient knowledge to predict the consequences of the use of any of these devices in individual patients for specific activities of daily living is available 3

4 Comparison of Fracture Fixation Devices Fixed Plate Internal compression resulting from screw + fixation plate geometry Intramedullary nail Bending stiffness: K b = ExI E, Young s Modulus of Elasticity I, the second moment of inertia for bending of the nail cross section Torsional stiffness: K t = ExI t G, Shear Modulus I t, the second moment of inertia for torsion From Kojic 2008 Example Analysis - Fixed Plate Boundary Conditions ~ approximating of axial load during human walking (single stance phase of 70 kg individual) Fixed Constraint From Kojic

5 Example Analysis Results - Effective Stresses No slip condition modeled between screws, plate, and bone => i.e. bonded contacts MPa From Kojic 2008 Example Analysis Results - Fixed Plate Stresses Stainless steel used for plate and screws E = 2.1x10 5 Mpa Poissons ratio = 0.3 Maximum effective stress less than critical values for stainless steel. However, cyclic loading leading to material fatigue must also be considered From Kojic

6 Example Analysis - Intramedullary Nail Same bone geometry, material properties, and boundary conditions as in the neutralization plate analysis From Kojic 2008 Example Analysis - Intramedullary Nail Stresses Effective stress concentrations in the nail near the screw regions => However, stress values are significantly lower than the corresponding neutralization plate regions (~80 MPa). Implication is that risk of intramedullary nail failure is significantly lower when compared to neutralization plate. From Kojic

7 Example Analysis - Intracapsular Fractures Comparison of implant designs for internal fixation of intracapsular fractures of the femoral neck Parallel Screws Dynamic Hip Implant From Kojic 2008 Example Analysis - Parallel Screws BCs Positive correlation between intraoperative stability and femoral neck fractures that have healed (versus did not heal), Rehnberg et al Fixed Boundary Condition F R : Pelvis to femur head reaction force, 199 dan F A : Force generated by gluteal muscles, 137 dan Body weight: 70 dan From Kojic

8 Doing More with Simulation (one idea) Can we use simulation in a more pro-active way to develop better products? Summary Prevalent uses of simulation in the orthopedic industry Modeling the human body Musculoskeletal simulation of activities of daily living A Proposed workflow for incorporating musculoskeletal modeling Extracting and incorporating 3D geometry and material properties from tomographic medical image data Coupling musculoskeletal modeling and finite element analysis 8

9 Simulation for!biomechanics" - Why? 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 more about complex systems (like people!) Simulation Software for!biomechanics" 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 9

10 The Holy Grail Task + Environment + Population Unique Simulation 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 Simulation without 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) 10

11 Expanding the Use of Activities of Daily Living with a Library of Musculoskeletal Simulations Long-term stability of hipimplants have been evaluated using normal walking, sit to stand, stair climbing, and combinations of those activities. Traditionally used as pass/fail tests to identify whether a particular design performs to a set of minimum specifications Significantly Underutilized Musculoskeletal Models Used Here Popular class of musculoskeletal models based on rigid body dynamics: Bones and objects from the environment are rigid Muscles and ligaments are mass-less actuators Soft tissue wobbly masses are not taken into account (mass is concentrated in bones) Phenomenological muscle models Easily scalable (b) Static 2D Suited for simulating internal body forces (muscle, joint, ligament) for prescribed activities Dynamic 3D (AnyBody Modeling System) 11

12 Formulating Dynamic Equilibrium Internal forces Applied forces Joint reactions Muscle forces Cf ( M ) i = d, where f = [ f ( R ) f " 0, i!{1,.., n, f ( M ) ( M) The matrix C is rectangular. This means that there are infinitely many solutions to the system of equations. How to pick the right one? } ] Using Optimization to Get a Solution Minimize Subject to G(f (M) ) Cf = d f i (M ) " 0, i # {1,..,n (M ) } What should be used for G(f (M) )? Objective function. Different choices give different muscle recruitment patterns. 12

13 Musculoskeletal Models for Commercial Use No gold-standard, just like with other pieces of engineering software Commercially available (including open source) software packages demand a knowledgeable user Not traditionally incorporated in current design/engineering methodologies Always room for improvement (I.e. improved validation, better accuracy, scaling to populations or patient specific, etc.) Still must demonstrate where/how this arena of modeling can improve specific processes (I.e. $$$) Summary Prevalent uses of simulation in the orthopedic industry Modeling the human body Musculoskeletal simulation of activities of daily living A Proposed workflow for incorporating musculoskeletal modeling Extracting and incorporating 3D geometry and material properties from tomographic medical image data Coupling musculoskeletal modeling and finite element analysis 13

14 Bridging the Gap with Simulation Physical Testing Simulated Physical Testing Simulated In- Vivo Performance Setting up an FE Simulation Using Boundary Conditions Derived from a Musculoskeletal Model All the necessary pieces: Geometry Mesh Material Properties Boundary Conditions Solve Post-Processing 14

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 15

16 Incorporating Musculoskeletal Modeling Implant Evaluation 16

17 Implant Optimization Associated Software 17

18 Summary Prevalent uses of simulation in the orthopedic industry Modeling the human body Musculoskeletal simulation of activities of daily living A Proposed workflow for incorporating musculoskeletal modeling Extracting and incorporating 3D geometry and material properties from tomographic medical image data Coupling musculoskeletal modeling and finite element analysis Geometry, Mesh, and Material Properties Realistic geometries and material properties are practical ways to improve the accuracy of the simulations A NIH (National Institute of Health) Project Goal is anatomically detailed, 3D representation of the human body CT, MRI, Cryosection taken of cadavers Male specimen released 1994 Female specimen in 1995 Publicly available with an application to National Library of Medicine Cryosection CT MRI 18

19 Using Medical Data as Simulation Input Tool for working with segmented medical data Provides a GUI environment to apply various segmentation methods Creates and exports advanced 3D geometries Can be used to export Finite Element Mesh (if desired) Can be used to define iso-tropic material definitions from apparent density relationships Deriving Material Properties From Scan Data In Ansys, the mesh can be changed by a number of operations, such as applying different boundary conditions or for purposes of convergence Deferring the material property assignment until the simulation is fully set up ensures versatility Bonemat is a public domain program originally written by Cinzia Zannoni et al. at The Rizzoli Institute* Uses a voxel data integration algorithm to determine material properties for finte elements regardless of relative voxel size *Zannoni C, Mantovani R, Viceconti M. Material properties assignment to finite element models of bone structures: a new method. Med Eng Phys 1998;20(10):

20 Bonemat Workflow Solution 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 distribution Geometry Mimics Material properties from imaging data 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 HU = HU are normalized units associated with CT image scans - based on the linear attenuation coefficient (µ) - based on scale (air) : (bone), 0 (water) The material property of each tetrahedral element was defined using a procedure similar to that used by Peng et al. (2006). 20

21 Material properties from imaging data Density 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). 100 kg/m kg/m 3 The maximum HU of the CT scan, 1575, was defined to be the hardest cortical bone of density (2000 kg/m 3 ) and the HU value of 100 was defined to be the minimum density of cortical bone (100 kg/m 3 ). Elastic Moduli Material properties from imaging data There exist an approximate power relationship between bone material properties and apparent densities (Wirtz et al. 2000). Elements were assigned elastic moduli calculated from apparent densities using axial loading equations developed by Lotz et al. (1991): 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) A Poisson's ratio of 0.30 was used for all materials. 21

22 Summary Prevalent uses of simulation in the orthopedic industry Modeling the human body Musculoskeletal simulation of activities of daily living A Proposed workflow for incorporating musculoskeletal modeling Extracting and incorporating 3D geometry and material properties from tomographic medical image data Coupling musculoskeletal modeling and finite element analysis Setting up the FE Simulation All the necessary pieces: Geometry Mesh Material Properties Boundary Conditions Solve Post-Processing 22

23 Cycling Data Cyclist Data 23

24 Musculoskeletal Simulation Musculoskeletal Simulation Single Revolution Observed Cadence of 62 rpm 5 points of support (pelvis, feet, hands) Anthropometry Matched to Subject Simulated Crank Torque => MechOutput = 170 (avg. mechanical output over a cycle in Watts) Force and Moment! Free Body Diagram" Cut Plane (vector lengths correspond to force magnitudes) 1 revolution = 0.97 seconds 24

25 Muscle Force Boundary Conditions at a Single Time Step FE Model in Dynamic Equilibrium - Matched mass and inertia properties between rigid and flexible body simulations - Matched points of force application - No arbitrary constraints (i.e. nodal position fixed in space) - Inertia loads applied - Model supported by weak springs (~1e-3 Newtons), to prevent rigid body motion - Assumption of small deflections 25

26 Tested Fracture Fixation Plate (Distal Femur) Geometry with 3 plate thicknesses 3.25mm 4.0mm 4.75mm Fatigue Life Results Fatigue Life Minimum Cycles: 3.25 mm => 178,000 cycles 4.0 mm => 335,000 cycles 4.75 mm => 14.7 million cycles Plots are depicted at 97% of cycle (t = ) Stress Life Fully Reversed t= mm Stress Life Fully Reversed t= Stress Life Fully Reversed t= mm 4.00 mm 26

27 Stress Contour Plots Maximum Stress: 3.25 mm => 855 MPa 4.0 mm => 692 MPa 4.75 mm => 584 MPa Plots are depicted at 97% of cycle (t = ) 3.25 mm 4.75 mm 4.00 mm Yield Stress of Titanium Alloy => 930 MPa Deformation Mode Deformation from musculoskeletal 0.02 s, 18x scale 27

28 Equivalent Stresses for Three Plate Thicknesses 3.25mm 4.00mm 4.75mm Summary of Simulation Capabilities 1. Replicating physical tests using simulation 2. Compare performance of new implant design to current on the market device 3. Replicate implant failure conditions associated with clinical and/or case-specific performance criteria 4. Evaluate implant performance criteria (i.e. total deformation, maximum stress, maximum strain, and/or fatigue life) for physiologically realistic boundary conditions associated with a single or library of activities of daily living 28

29 Summary of Simulation Capabilities 5. Evaluate implant performance criteria (i.e. total deformation, maximum stress, maximum strain, and/or fatigue life) for different populations (i.e. bone size/geometry, bone quality/strength) performing relevant activities of daily living 6. Perform shape optimization of parametrically defined implant to maximize or satisfy one/multiple performance objectives or criteria 7. Perform sensitivity analysis on screw placement and/or implant variations with respect to performance criteria 8. Evaluate internal bone stresses at/around implant-bone and bonebone interfaces for laboratory and activity of daily living criteria Thank you for your attention Thank You For Your Attention V1)-2)$1)'$&2$W+"U$*%$X"&$(-Y)$-+X$Z&)2."+2$"+$20)3*[3$'"0*32$0#)2)+')\$()#) N,&23&1"2W)1)'-1S"T)+*+3=3",O ])^*+-#2$#)1-')\$'"$,&23&1"2W)1)'-1$2*,&1-."+$)Y)#X$,"+'( N(_04``UUU="T)+*+3=3",`\)%-&1'=-20a**b?GPO!"#$%&#'()#$*+%"#,-."+/$01)-2)$3"+'-3'4 5&#$c&2*+)224 <"11-^"#-."+$N*=)=$d#-+'2/$'#-*+*+dO <"+2&1.+d D"eU-#) 5678$789:877;:89/$:8<= *+%"S"T)+*+3=3", UUU="T)+*+3=3", 29

30 30